M.Sc. (Previous) Chemistry
Paper – I : INORGANIC CHEMISTRY
BLOCK – II
UNIT – 4 : Magnetic properties of Transition Metal
UNIT – 5 : Metal π Complexes
UNIT – 6 : Reaction Mechanism of Transition Metal
Author – Dr. Purushottam B. Chakrawarti
Edtor – Dr. M.P. Agnihotri
UNIT-4
MAGNETIC PROPERTIES OF TRANSITION METAL
COMPLEXE
Structure
4.0
Introduction
4.1
Objectives
4.2
Magnetic Moments
4.3
4.2.1
Number of Unpaired Electrons
4.2.2
Spin Only Formula
Anomalous Magnetic Moments
4.3.1
Orbital Contribution in Magnetic Moments
4.3.2
Curie's Law
4.4
Magnetic Exchange Coupling
4.5
Let Us Sum Up
4.6
Check Your Progress: The Key
4.0
INTRODUCTION
Substances were first classified as diamagnetic or paramagnetic by
M.Faraday (1845). But it was not untill many years later that these
phenomenon came to be understand in terms of electronic structures. When
any substance is placed in an external magnetic field, there is an induced
circulation of electrons producing a net magnetic moment aligned in
opposition to the applied field. This is the diamagnetic effect and it arises
from paired electrons within a sample. Paramagnetism is produced by
unpaired electrons in a sample. The spin and Orbital motion of these
electrons give rise to permanent molecular moments that tend to alignt
themselves with an applied field.
Magnetic properties and electronic spectra are closely connected.
Magnetic susceptibility measurements are used to decide between different
electronic configurations. It may be mentioned, although the electronic
spectra is a powerful method for investigating transition metal complexes,
additional and complementary information can be provided by magnetic
measurements.
In this unit we shall discuss how net magnetic moments of transition
metal complexes can be worked out; and in what conditions anomalous
magnetic moments are obtained.
However, it will be advantageous if you recall what you have already
studied earlier about the basic concepts of magnetic moments of atoms.
4.1
OBJECTIVES
The main aim of this unit is to study magnetic properties of transition
metal complexes and to establish their correlation with their spectral
properties. After going through this unit you should be able to:
 calculate magnetic moments and number of unpaired electrons in a
transition metal complex;
 describe under what conditions spin-only formula will be useful to
calculate µ of the complexes;
 discuss under which conditions orbital contributions will be important
to calculate µ of the complexes; and
 explain magnetic exchange coupling and spin crossover to describe
anomalous magnetic moments of some complexes.
4.2
MAGNTIC MOMENTS
When a substance is subjected to a magnetic field, H, a magnetization,
I, is induced. The ratio I/H is called the "volume susceptibility", K, and can
be measured by a variety of techniques, including the Gouy balance method,
the Faraday method, and an nmr method. The volume susceptibility is
simply related to the "gram susceptibility," x, and the "molar susceptibility",
xm
x
K
d
xM 
K
M
d
where d and M are the density and molecular weight of the substance,
respectively. For most substances, K, x and xM have negative values; such
substances are weakly repelled by a magnetic field and are called
"diamagnetic". For substances having unpaired electrons that do not strongly
interact with one another, K, x and xM have relatively large positive values;
these substances are attracted into a magnetic field and are called
'paramagnetic."
When a paramagnetic substance is placed in a magnetic field, the
moments of the paramagnetic molecules or ions tend to align with the field;
however, thermal agitation tends to randomize the orientations of the
individual moments. Theoretical analysis of the situation leads to the
relations;
x Mcorr 
N 2
3kT
where X Mcorr is the molar susceptibility which has been corrected both
for the diamagnetic contribution to the susceptibility (due to the non
paramagnetic atoms in the sample) and for any small temperatureindependent paramagnetism arising from paramagnetic excited states of the
system. N is Avogadro's number, k is the Boltzmann constant, µ is the
"magnetic moment" of the molecule, and T is the absolute temperature. By
substituting numerical values for N and k, we obtain;
X Mcorr 
0.125 2
T
or   2.83 X Mcorr T
4.2.1 Number Of Unpaired Electrons
Once an experimental value of xM has been obtained for a
paramagnetic substance, it can be used to determine how many unpaired
electrons there are per-molecule or ion. In order to translate the experimental
result into the number of unpaired spins, it must be recognized that a
measured susceptibility will include contributions from both paramagnetism
and diamagnetism in the sample. Even though the latter will be small, it is
not always valid to consider it negligible. The most common procedure is to
correct a measured susceptibility for the diamagnetic contribution.
Compilations of data from susceptibility measurements on a number of
diamagnetic materials make it possible to estimate the appropriate correction
factors. The diamagnetic susceptibility for a particular substance can be
obtained as a sum of contributions from its constituent unit: atoms, ions,
bonds, etc. The basic assumption underlying such a procedure, namely, that
the diamagnetism associatiated with an individual atom or other unit in
independent of environment, has been shown to be valid.
The next step is to connect the macroscopic susceptibility to
individual molecular moment and finally to the number of unpaired
electrons. From classical theory, the corrected or paramagnetic molar
susceptibility is related to the permanent paramagnetic moment of a
molecule µ, by:
xM 
N 2 2
3RT
where N is Avogadro's number, R is the ideal gas constant, T is the
absolute temperature, and µ, is expressed in Bhor magnetrons (BM) (1 BM =
eh/4  m). Solving this expression for the magnetic moment gives:
 3RTX m 
 

2
 N

1/ 2
 2.84( x M T )1 / 2
As we know, this paramagnetic moment in the spins and orbital
motions of the unpaired electrons in the substance. There are three possible
modes of coupling between these components spin-spin, orbital-orbital, and
spin-orbital. For some complexes, particularly those of the lanthanides, we
must consider all three types of coupling. The theoretical paramagnetic
moment for such a complex is given by:
  g[ J ( J  1)]1 / 2
where J is the total angular momentum quantum number and g is the
Lande splitting factor for the electron, defined as:
g 1
J ( J  1)  S ( S  1)  L( L  1)
2 J ( J  1)
The value of J depends on the total orbital angular momentum
quantum number, L, and total spin angular momentum quantum number.
4.2.2 Spin Only Formula
For complexes in which spin-orbit coupling is nonexistent or
negligible but spin and orbital contributions are both significant, the
predicted expression for µ is;
  [4S (S  1)  L( L  1)]1 / 2
This equation describes a condition that is never fully realized in
complexes because the actual orbital contribution is always somewhat less
than the ideal value. This occurs because the orbital angular momentum is
reduced from what it would be in the free metal ion by presence of ligands.
In the extreme case, where general situation in complexes having A or E
ground states, which would include octahedral d3, d4 (high spin), d6 (low
spin), d7 (low spin) and d8 cases. Furthermore, when a complex involves a
first-row transition element, even if the ground state is T, the orbital
contribution generally may be ignored. For the L=O condition, the above Eq.
reduces to;
  [4S (S  1)1 / 2  2[S (S  1)]1 / 2
which is known as the spin only formula for magnetic moment. By
recognizing that S will be related to the number of unpaired electrons (n) by
S = n/2, the expression may be further simplified to;
  [n(n  2)]1 / 2
Check Your Progress - 1
Notes :(i) Write your answers in the space given below .
(ii) Compare your answers with those given at the end of the
unit.
1. Molar susceptibility xM, is given by the relation;
xM = ..........................................
2. Magnetic moment, µ is given by the relation;
µ = ..........................................
3. The spin only formula is;
µ = ..........................................
4. Magnetic moment, µ and number of un-paired electrons, n, are
related as;
µ = ..........................................
4.3
ANOMALOUS MAGNETIC MOMENTS
Table 4.1 indicates that the values of magnetic moment calculated
using the spin only formula in number of cases differ from the values
obtained from theoretical considerations. This difference is supposed to be
due to two reasons, firstly due to the contribution of orbital magnetic
moment; and secondly due to dependence of magnetic properties on the
experimental temperature (Curie's Law).
Table 4.1: Magnetic properties of some complexes of the first-row
transition metals
Central
metal
Ti3+
High spin complexes
Low spin complexes
No. of d
No. of
No. of
µ(expt) µ(calc)b
µ(expt) µ(calc)b
electrons unpaired
unpaired
BM
BM
BM
BM
electrons
electrons
1
1
1.73
1.73
-
V4+
1
1
V3+
2
2
V2+
3
3
Cr3+
3
3
Mn4+
3
3
Cr2+
4
4
Mn3+
4
4
Mn2+
5
5
Fe3+
5
5
Fe2+
6
4
Co3+
6
4
Co2+
7
3
Ni3+
7
3
Ni2+
8
2
Cu2+
9
1
1.681.78
2.752.85
3.80390
3.703.90
3.8-4.0
1.73
-
-
-
2.83
-
-
-
3.88
-
-
-
3.88
-
-
-
3.88
-
-
-
4.754.90
4.905.00
5.656.10
5.706.0
5.105.70
-
4.90
2
2.83
4.90
2
3.203.30
3.18
5.92
1
1.73
5.92
1
1.802.10
2.0-2.5
4.90
0
-
-
4.90
0
-
-
4.305.20
-
3.88
1
1.8
1.73
3.88
1
1.8-2.0
1.73
2.803.50
1.702.20
2.83
-
-
-
1.73
-
-
-
2.83
1.73
4.3.1 ORBITAL CONTRIBUTION IN MAGNETIC MOMENTS
In an octahedral ligand field, only the t2g orbitals remain degenerate
and rotationally related. The eg orbitals get separated by 10Dq. Hence,
orbital momentum due to the dx2-y2 orbital electron gets quenched, and the
spin-only formula should apply.
It can be seen that the orbital angular momentum formula should be
important for the high spin d1, d2, d6 and d7 ion complexes and low spin d4,
d5 ions in octahedral field. In the tetrahedral field, the high spin d 3, d4, d8 and
d9 ion should have a significant contribution from the orbital angular
momentum. The magnetic moment of [CoCl4]2- (4.4 BM) and that of
[Co(H2O)6]2+ (5.0) confirm the above statements, the orbital moment
contributes for the high spin octahedral, but not for the tetrahedral
complexes.
Even for the other ions, where no orbital moment is expected, the
observed values significantly depart from the spin-only formula (though the
differences are small). This is attributed to the spin-orbitals interactions
which oppose the quenching of the orbital moments by mixing the orbitals.
This explains the generally, lower µeff values obtained for Cr2+, Cr3+, V3+ and
V+ and higher values for spin free Fe2+, Co2+, Ni2+ and Cu2+ complexes.
Greatest deviations occur for the Co2+ and Fe2+ complexes, for which
unquenched orbital moments contribute significantly.
For the 4d and 5d ions, diamagnetism results for even numbered
electrons, and paramagnetism to the extent of one unpaired electron only is
observed for the old numbered electrons, indicating that spin pairing takes
place for these ions as far as possible. This may be due to (i) reduced
enterelectronic repulsions in larger sized ions reducing the electron pairing
energies, (ii) higher LF or MO splittings. The µ at room temperature is
generally lower than µs and cannot be used to determine the unpaired
electrons due to (iii) high spin-orbit coupling constants which align L and S
vectors in opposite directions destroying the paramagnetism.
Further, (iv) the Curie or Curie-Weiss law does not hold, the
variation of µ with L is complex and depends upon the number of the
electrons present.
Some ions like MnO4-, CrO42- and low spin Co3+ complexes show
temperature-independent paramagnetism (TIP) even though they do not have
any unpaired electron. This is due to the spin-orbit coupling of the ground
state to a paramagnetic excited state under the influence of the magnetic
field. The degree of mixing is independent of temperature but depends on
the applied magnetic field, as the excited state is well separated from the
ground state, whose population does not change with temperature.
4.3.2 CURIE'S LAW
The observed magnetic moments for the metals in t2g ground state
are temperature dependent and usually depart from the µs value due to
probably the t2g electron delocalization and lower symmetry ligand field
components.
Pierre Curie established in 1895 that paramagnetic susceptibility is
inversely proportional to the absolute temperature.
xM = C/T
This expression, which is known as Curie's Law, is actually a
restatement of magnetic moment. The Curie law is obeyed fairly well by
paramagnetic substances that are magnetically dilute, i.e. those in which the
paramagnetic centers are well separated from each other by diamagnetic
atoms. In materials that are not magnetically dilute, unpaired spin on
neighboring atom may couple with each other, a phenomenon referred to as
magnetic exchange. Materials that display exchange behavior can usually be
treated with a modification the Curie-Weiss Law;
xM 
C
(T   )
where ø is a constant with units of temperature. If the interacting magnetic
dipoles on neighboring atoms tend to assume a parallel alignment, the
substance is said to be ferromagnetic (Fig. 4.1(b)). If, on the other hand, the
tendency is for an anti-parallel arrangement of the coupled spins, the
substance is anti-ferromagnetic.(Fig. 4.1(c)) In any material that exhibits
magnetic exchange, the tendency towards spin alignment will complete with
the thermal tendency favoring spin randomness. In all cases, there will be
same temperature below which magnetic exchange dominates, this
temperature is called the Curie temperature (TC) if the type of exchange
displayed is ferromagnetic and the Neel temperature (TN) if it is antiferromagnetic. The change in susceptibility as the temperature is decreased
below either TC or TN may be quite dramatic.
Paramagnetism
Ferromagnetism
Antiferromagnetism
(a)
(b)
(c)
Fig.4.1 Schematic representations of magnetic dipole arrangements
in (a) paramagnetic, (b) ferromagnetic, and (c) anti-ferromagnetic
materials.
Fe(Phen)2(CNS)2 is an example which shows significant variation in
magnetic moment with temperature. (Fig. 4.2)
Fig.4.2 The magnetic moment of Fe(phen)2(NCS)2
as a functions of temperature.
4.4
MAGNETIC EXCHANGE COUPLING
As we know, a number of transition metal ions form both high and
low spin complexes, and we have now seen that magnetic susceptibility
allow us to experimentally distinguish one from the other. Within ligand
field theory, these two spin configurations in octahedral complexes are
explained in terms of relative magnitudes of  and pairing energy (P): We
associate high spin complexes with the condition    P and low spin
complexes with    P . For complexes in which the energy difference
between  and P is relatively small, an intermediate field situation, it is
possible for the two spin states to coexist in equilibrium with each other.
Consider the Fe2+ ion. At the two extremes, it forms high spin paramagnetic
[Fe(H2O)]2+ (S=2) and low spin diamagnetic [Fe(CN)6]4- (S=0).
Octahedral complexes with 4, 5, 6 or 7 d electrons can be either
high-spin or low-spin, depending on the magnitude of the ligand-field
splitting,  . When the ligand-field splitting has an intermediate value such
that the two states of the complex have similar energies, the two states can
coexist in measurable amounts at equilibrium. Many "crossover" systems of
this type have been studied.
4.5
SPIN CROSSOVER
With the change in field strength, change in the magnetic moment
i.e., the change from high-spin to low-spin can be explained in terms of
splitting of electronic states with the field strength, e.g. the Tanabe-Sugano
diagram to these d6 complexes show that near the crossover point between
weak and strong field the difference in energy between the spin-free (5T2g)
and spin-paired (1A1g) ground states becomes very small (Fig. 4.3) within
this region, it is reasonable to expect that both spin state may be present
simultaneously and that the degree to which each is represented will depend
on the temperature (  - P = kT). A complex illustrating these effects is
[Fe(phen)2(NCS)2] (Fig. 4.2). At high temperature a moment consistent with
four unpaired electrons is observed, but as the temperature is decreased, a
sharp drop in magnitude is observed at 175K where the low-spin form
becomes dominant. Usually spin transitions occur somewhat more gradually
than in the case shown here, and reasons for the abruptness observed for this
complex, as well as some residual paranagnetism seen at low temperature
have been discussed extensively.
5
T2g
1
A1g
E

Fig.4.3 Variation in energies of 5T2g and 1A1g terms with increasing
 for d6 octahedral complexes. At weak field (high spin complexes) the
ground term is 5T2g, while at strong fields (low spin complexes) it is 1A1g
Note that in the region immediately on each side of the spin crossover
point, the energy difference between the two terms is small; thus high
and low spin complexes coexist.
In solutions, these systems are fairly straightforward; the change in
magnetic susceptibility with temperature can be interpreted in terms of the
heat of conversion of one isomer to another. However, treatment of the
system as an equilibrium between two spins yields  H=3.85 kcal mol-1 and
 S = 11.4 for the high spin  low spin conversion. On the other hand, spin
crossover in solids is a complex phenomenon because of cooperative
structural changes and changes in the energy separation of the high-spin and
low-spin states with temperature. Thus the magnetism of Fe(phen)2(NCS)2
change sharply at 174K, as shown in Fig. 4.2
Check Your Progress - 2
Notes :(i) Write your answers in the space given below .
(ii) Compare your answers with those given at the end of the
unit.
(A)(i) Orbital Contribution in magnetic moment is important for high
spin............................ions complexes; and low spin ...................
ions in octahedral field.
(ii)
The
greatest
deviation
in
magnetic
moment
occurs
for...............complexes.
(B) Curie's Law state that.....................................................................
............................................................................i.e. Xm =............
(C) In octahedral complexes for dn configurations (n=...................) the
two states (low-spin and high-spin) of complexes can coexist in
measurable amount's at equilibrium at ligand field splitting has.
(D) The change from high spin to low spin can be explained in terms
of...................................................................................................
(E) The crossover point is reached when the difference in energy
between..........................................................states become very
small.
4.6 LET UP SUM UP
 Substance may be diamagnetic or paramagnetic when any substance is
placed in an external magnetic field, there is an induced circulation of
electrons producing a net magnetic moment in opposition to the
applied field. This is the diamagnetic effect and it arises from paired
electrons within a sample.
 Paramagnetism is produced by unpaired electrons in a sample. The
spin and orbital motion of these electrons give rise to permanent
molecular moments that tends to align themselves with an applied
field.
 When a substance is subjected to a magnetic field, H, a magnetization
I, is induced. The ratio of I/H is called volume susceptibility, k. The
volume susceptibility is simply related to the 'gram susceptibility', x
and the molar susceptibility, XM asX
K
d
or X M 
K
M
d
where d and M are the density and molecular weight of the substance,
respectively.
 xM is the molar susceptibility which has been corrected both for the
diamagnetic contribution to the susceptibility and for any smll
temperature-independent paramagnetism from paramagnetism excited
states of the system, and may be given as,
X Mcorr 
0.125 2
or   2.83 X Mcorr T
T
when values of Avogadro's number, A, Boltzman constant K,
the magnetic moment of the substance, µ and absolute temperature are
substituted.
 Once an experimental value of XM has been obtained for a
paramagnetic substance, it can be used to determine how many
unpaired electrons there are per molecule or ion.
 For complexes in which spin-orbit coupling is nonexistent or
negligible but spin and orbital contributions are both significant µ is
given by;
μ  [4 S(S 1)  L(L  1)]1/ 2
 When a complex involves a first row transition element, even if the
ground state is T, the orbital contribution generally may be ignored,
and we get L=O and µ is given by spin only formula;
μ  [4 S(S 1)1/ 2  2[S(S 1)]1/ 2
 By recognizing that S will be related to the number of unpaired
electrons (n) by S = n/2, the above expression is simplified to;
μ  [n(n  2)]1 / 2
 Magnetic moment calculated using the spin only formula in number of
cases differ from the values obtained from theoretical considerations.
This deviation may due to be either the contribution of orbital
magnetic moment, or due to dependence of magnetic properties on the
experimental temperature, (Curie's Law).
 The orbital angular momentum formula may be important for the high
spin d1, d2, d6 and d7 ion complexes, and low spin d4, d5 ions in
octahedral field.
 The observed magnetic moments for the metals in t 2g ground state are
temperature dependent and usually depart from the µs values due to
probably the t2g electron delocalization and lower symmetry ligand
field components.
 The Curie's Law states that paramagnetic susceptibility is inversely
proportional to the absolute temperature;
xM = C/T
 If the interacting magnetic dipoles on neighboring atoms tend to
assume a parallel alignment, the substance is said to ferromagnetic,
and if, on the other hand, the tendency is for an anti-parallel
arrangement of the coupled spins, the substance is anti-ferromagnetic.
 Fe(phen)2(CNS)2 is an example which shows significant variation in
magnetic moment with temperature. A number of transition metal ions
form both high and low spin complexes. These two spin
configurations in octahedral complexes are explained in terms of
relative magnitudes of  and pairing energy (P). High spin complexes
are formed when  < P and low spin when  > P.
 For complexes in which the energy difference between  and P is
relatively small an intermediate field situation, it is possible for the
two spin sates to co-exist in equilibrium with each other.
 Variation in energies of 5T2g and 1A1g terms with increasing  for d6
octahedral complexes show, at weak fields (high spin complexes), the
ground term is 5T2g, while at strong fields (low spin complexes) on
each side of the spin crossover point, the energy difference between
the two terms is small; thus high and low complexes may coexist.
4.7
1.
CHECK YOUR PROGRESS: THE KEY
(i)
XM 
KM
d
(ii)  = 2.84 (XMT)½
(iii)  = [4S (S+1) ]½ = 2 (S(S+1)]½
(iv)  = [n (n+2)]½
2.A (i) High spin d1, d2, d6 and d7 ion complexes, and low spin d4 and d5
ions.
(ii) For Fe2+ and Co2+ complexes.
B.
Curie's Law states that paramagnetic susceptibility is inversely
proportional to the absolute temperature, i.e. XM= C/T
C.
For dn configurations (n = 4, 5, 6, 7) the ligand field splitting has
an intermediate value.
D.
In terms of splitting of electronic states with the fields strength.
E. Between spin free 5T2g and spin paired 1A1g ground states become very
small.
5
METAL
π
COMPLEXES
Structure
5.0
Introduction
5.1
Objectives
5.2
Metal Carbonyls
5.3
5.4
5.2.1
Classification
5.2.2
Isolobal Concept
5.2.3
Methods of Preparation and Properties
5.2.4
Structure
5.2.5
Vibrational Spectra
Metal Nitrosyls
5.3.1
Neutral NO and NO- Complexes
5.3.2
Complexes of NO+
5.3.3
Pure Nitrosyl Complexes
5.3.4
Nitrosyl Carbonyl Complexes
5.3.5
Nitrosyl Halide Complexes
5.3.6
Nitroso Cyanide Complexes
Dinitrogen Complexes
5.4.1
5.5
Fixation of Nitrogen
Dioxygen Complexes
5.5.1
Heme Proteins and Transportation of O2
5.5.2
Haemoglobin
5.6
Tertiary Phosphine as Ligand
5.7
Let Us Sum Up
5.8
Check Your Progress: The Key
5.0 INTRODUCTION
π-bonding in complexes was proposed for the first time, by Pauling

(1924), in the form of back-bonding (M 
L) to account for electro-
neutrality of metal to ligand bond. According to him, if the ligand, linked
with the metal ion through LM, ϭ-bond, has vacant π-orbitals, it can
accept lone pair of electrons from metal-ion (if present) to form ML, πbonds. This also accounts for the extra stability of metal complexes with
unsaturated ligands. However the latest and the most successful theory of
bonding for metal-complexes Ligand Field Theory (LFT), explained
quantitatively while Mπ-bonding stabilizes, the complex, LM
π-
bonding destabilize it. This also explains positions of CN- and F- ligands in
the spectrochemical series.
Most transition metals form complexes with a wide variety of
unsaturated molecules such as carbon monoxide, nitric oxide, dinitrogen,
dioxygen etc. In many of these, the metal is in zero or another low oxidation
state and, as we have already mentioned, π-bonding between the metal and
the ligands is believed to play an important part in stabilizing these
complexes. In this regard metal carbonyls are important as they involve both
metal carbon ϭ and π-bonds. In this unit we shall consider the metal
carbonyls, anions derived from them, some of their substitution product, and
complexes formed by a few other ligands.
5.1 OBJECTIVES
The main aim of this unit is to study π-complexes of transition metals,
with special reference to bonding and their structures. After going through
this unit you should be able to:
 describe metal carbonyls, their classification, methods of preparation
and reactions; with special reference to their structures,
 discuss how these complexes, almost without exception, conform to
the effective atomic number rule and isoloble concept,
 explain bonding in these complexes in terms of IR spectra;
 describe preparation, properties and structures of metal nitrosyls,
 discuss dinitrogen complexes and their importance in the fixation of
nitrogen;
 explain formation of dioxygen complexes with special reference to
transportation of oxygen by heme proteins; and
 describe the nature of complexes with tertiary phosphine as a ligand.
5.2 METAL CARBONYLS
The compounds formed by the combination of CO molecules with
transition metals are known as metallic carbonyls.
Carbon mono-oxide posses a unique property of unsaturation by
virtue of which it may combine with a large number of metals under suitable
conditions. Such compounds of CO with metals are termed as metallic
carbonyls.
In carbonyls, a metal atom is directly linked to the carbon atom of a
carbonyl group. Since the electrons forming OCM bond are supplied
solely by CO molecule, metal atom in carbonyls is said to be in zero
oxidation state. In metal carbonyls CO molecules act as neutral ligands.
Metal carbonyls vary considerable in their properties ranging from volatile
nonpolar to the nonvolatile electrovalent carbonyls. For example-nickel
forms volatile nonpolar carbonyls, where as alkali and alkaline earth metals
from non-volatile electrovalent carbonyls.
The general formula of the carbonyls may be given as Mx(CO)y
where M is a metal capable of forming carbonyl. Metal carbonyls may be
regarded as parents of number of related compounds such as metal nitrosyl
carbonyl, M (NO)y (CO)x, and metal carbonyl hydrides HxM (CO)y.
5.2.1 Classification
Carbonyls are classified into two distinct groups:
a.
Monocular carbonyls: These carbonyls have the general
formula Mx(CO)y which contain more than one metal atom
per molecule.
b.
However the carbonyls having 2 metal atoms are called
binocular carbonyls, and
c.
those having more than two metal atoms as ploynuclear
carbonyls. Polynuclear carbonyls may be homonuclear e.g.
[Fe3(CO)12 or heteronuclear e.g. MnCo(CO)9, MnRe(CO)10]
(Table 5.1)
They have following characteristics:
i. These are almost insoluble in organic solvents.
ii. Many polynuclear carbonyls decompose at or below the
melting point.
5.2.3 Preparation And Properties Of Carbonyls
a. Direct synthesis from metals and carbon mono-oxide, for
example:
1.
Nickel reacts with CO at room temperature and normal
pressure;
C
Ni  4CO 40


 Ni(CO) 4
2.
When CO is passed over reduced iron at 108o-220o and
pressure of 50 to 200 atom pressure. Fe(CO)5 is formed;
Fe - + 5CO
Fe(CO)5
Rhenium, osmium and iridium carbonyls could not be prepared by
direct reactions.
Table 5.1 The binary carbonyls
Electrons needed
13
to attain noble
gas
configuration
First transition V(CO)6
series
12
Cr(CO)6
11
10
Mn2(CO)10
9
Fe(CO)5
CO2(CO)8
blue solid white solid yellow solid yellow liquid
(sublimes)
(m.p.154o)
(b.p.103o) Fe2 (m.p.51o)
liquid
(CO)12 black
CO6(CO)16
(b.p.43o)
solid
black solid
Second
Mo(CO)6
Te2(CO)10
Ru(CO)5
transition series
white
white solid
colourless
Rh2(CO)8*
solid(subli
liquid
mes)
22o)
orange solid
Ru2(CO)9*
Rh6(CO)16
Ru3(CO)12
black solid
orange
(m.p.- Rh4(CO)12
solid
(m.p.-154o)
W(CO)6
Re2(CO)10
series
white
white
solid(subli
(m.p.177o)
Ni(CO)4
orange solid colourless
(sublimes)
Third transition
8
Os(CO)5
solid colourless
liquid
Ir2(CO)8
yellow solid
Ir4(CO)12
mes)
(m.p.15o)
yellow solid
Os2(CO)9
Ir6(CO)16 red
Orange
solid solid
Os2(CO)12
Yellow
solid
(m.p.224o)
b. Indirect synthesis involving the Gringed reagent: job prepared
chromium hexacarbonyl by the action of CO on a mixture of grignard
reagent and anhydrous chromium chloride in ether solution.
According to Hiber the primary reaction is as follows:
C6H5MgBr + CrCl3 + CO  Cr(CO)2(C6H5) + MgBrCl + MgBr2
The unstable intermediate compound is composed with acid to
yield the hexacarbonyl:
3Cr(CO)2 (C6H5)4 + 6H  Cr(CO)6 + 2Cr3+ + 12C6H-5 + 3H2
The reactions gives low yield which can be improved by using
high carbon mono-oxide pressure.
c. Indirect synthesis involving metal compounds: Metal carbonyls can
be prepared by the reaction of CO with certain metal compounds for
example:
i. Nickel tetracarbonyl may be prepared by passing CO into a
suspension of nickel cyanide, sulphide or mercaptide suspended in
NaOH solution.
2NiX4 + 2nCO  2Ni(CO)nX + X2
Ni(CO)nX + (4-2n)CO  Ni(CO)4 + NiX2
ii.Ruthenium pentacarbonyl may be prepared by the action of CO and
Rul3 in the presence of an iodine acceptor:
CO ]
CO ]
RuI 3 [
 Ru (CO) 4 I 2 [
 Ru (CO) 5
Similarly [Ir(CO)4]7 may be prepared.
d. Synthesis by carbonylating the metallic salts with CO in the presence
of reducing agent. When salts like R4I3, CrCl3, VCl3 are made to treat
with CO in presence of a suitable reducing agent like Mg, Ag, Cu, Na,
H2 etc.
CrCl3  CO  LiAlH y 115

 Cr(CO)6 + LiCl + AlCl3
o
C 250atmpressure


 2Ru(CO)5 + 6Agl
2Rul3 + 10CO + 6Ag 175
210atmpressure
C
   2Mn2(CO)10+ 2Mgl
2Mnl3 + 10CO + 2Mg 25
e. Synthesis from other carbonyls: when iron pentacarbonyl is exposed
to UV light it loses CO and forms Fe2(CO)9. This compound
undergoes thermal decomposition to yield iron pentacarbonyl and
trimeric tetracarbonyl.
.V .

 Fe2(CO)4 + CO
2Fe(CO)5 U

 Fe(CO)5 + [Fe(CO)4]3 + CO
2Fe2(CO)9 heat
f. Synthesis from Carbonyl hydrides: when iron carbonyl hydride is
oxidised by MnO2 or H2O2,[Fe(CO)4]3 is formed.
g. By treatment of oxide of metals with CO under pressure:
Carbonyls of osmium and rhenium are prepared by the reaction of CO
with their oxides under pressure.
OsO4 + 9CO
C
100


Os(CO)5 + 4CO2
50 atm pressure
C

 Re2(CO)10 + 7CO2
Re2O7 + 17CO 75
atm
200
h. Preparation of Mo (CO)6 and W(CO)6 from Fe(CO)5
MoCl6 + 3Fe(CO)5  Mo(CO)6 + 3FeCl2 + 9CO
i. Preparation of Fe2(CO)9 and Os2(CO)9 from Fe(CO)5 and Os(CO)5-9
with cooled solution of Fe(CO)5 and Os(CO)5 in glacial CH3COOH is
irradiated with u.v. light, Fe(CO)9 and Os2(CO)9 are obtained
respectively.
light
 Fe2(CO)9 + CO
2Fe(CO)5 U.V.
.V .light

 Os2(CO)9 + CO
2Os(CO)5 U
Properties of Carbonyls
i. The metal carbonyls are crystalline solids, except for nickel carbonyl
and the pentacarbonyls of iron, ruthenium and osmium which are
liquids.
ii. Many are coloured for example: Crystals of cobalt carbonyl are
orange and iron pentacarbonyls is yellow oil and nicked carbonyl is
colourless.
iii. Due to their covalent nature renders them insoluble in water, most of
them are soluble in solvents like CCl4.
iv. Excepting V(CO)6 all the carbonyls are diamagnetic. V(CO)6 is
paramagnetic and its paramagnetic property corresponds to the
presence of one unpaired electron. The metal in carbonyls are in zero
oxidation state.
Table 5.2 Colour And Melting Points Of Some Carbonyls
Carbonyl
V(CO)6
Melting Point, (oC)
Colour and shape
Black crystals
Decomposes
vacuum
at
70oC,
Sublime
in
Carbonyl
Melting Point, (oC)
Colour and shape
Cr(CO)6
Colourless crystals
Sublime in vacuum
Mo(CO)6
Colourless crystals
Sublime in vacuum
W(CO)6
Colourless crystals
Sublime in vacuum
Mn2(CO)10
Golden crystals
154o-155o
Re2(CO)10
Colourless crystals
Sublime at 140o and decompose at 177oC
Fe(CO)5
Yellow Liquid
B.P. 103oC
Fe2(CO)9
Bronze Mica-like
Decomposes at 100oC
platelets
Fe3(CO)12
Dark green crystals
Decomposes at 140oC
CO2(CO)8
Orange crystals
51oC
Ni(CO)4
Colourless Liquid
B.P. 43oC
Chemical Properties
1. Substitution Reactions: Some or all CO groups present in carbonyls
can be replaced by monodentate ligands such as alkyl or aryl
isocyanide (CNR) PR3, PCl3, Py, CH3OH etc.
Ni(CO)4 + 4CNR  Ni(CNR)4 + 4CO
Ni(CO)4 + 4PCl3  Ni(PCl3)4 + 4CO
Fe(CO)5 + 2CNR Fe(CO)3 (CNR)2 + 2CO
2. Action of NaOH or Na metal: Formation of carbonylate ion:
Aqueous alcoholic solution of NaOH reacts with Fe(CO)5 to form
carbonylate anion [Fe(CO)4]-.
Fe(CO)5 + 3NaOH  Na+[H+Fe2-(CO)4]-Na2Co3 + H2O
H-atom in [H+Fe2-(CO)4]- ion is acidic which implies that Fe
atom in this ion is in -2 oxidation state.
Na-metal in liquid NH3 is able to convert Fe2(CO)9. Co2(CO)8,
Fe3(CO)12, Cr (CO)6,.Mn2(CO)10 etc, into carbonylate anions and in
this conversion these carbonyls are reduced.
Fe2(CO)9 + 4Na  2Na 2 [Fe2-(CO)4]2- + CO
Co2(CO)8 + 2Na  2Na+[Co-(CO)4]4
3. Action of halogens: Most of the carbonyls react with halogens to
yield carbonyl halides. For example:
Fe(CO)5 + X2  Fe(CO)4X2 + CO
Mo(CO)6 + Cl2  Mo(CO)4Cl2 + 2CO
Mn2(CO)10 + X2(X = Br, I)  2Mn(CO)5X
Both Co2(CO)6 and Ni(CO)4 are decomposed into metallic
halides and CO when treated with halogens.
Co2(CO)8 + 2X2  2CoX2 + 8CO
Ni(CO)4 + Br2  NiBr2 + 4CO
4. Action of NO: many carbonyls react with nitric oxide (NO) to form
metal carbonyls nitrosyls. For example:
C
 Fe(CO)2(NO)2 + 3CO
Fe(CO)5 + 2NO 95
3Fe3(CO)9+4NO  2Fe(CO)2(NO)2+ Fe(CO)5+Fe3(CO)12+6CO
5. Action of H2: Formation of carbonyl hydrides (reduction): when
Mn2(CO)10 and Co2(CO)8 react with H2, they get reduced to carbonyl
hydrides, Mn(CO)5H and Co(CO)4H respectively.
C , 200atmpressure


 2[Mn-(CO)5H+]0
Mn2(CO)10 + H2 200
C , 200atmpressure


 2[Co-(CO)4H+]
Co2(CO)8 + H2 165
6. Action of heat: Different carbonyls yields different products when
heated for example:
C

 Fe + 5CO
Fe(CO)5 250
C


 3Fe(CO)5 + Fe3(CO)12
3Fe2(CO)9 70
C

 3Fe + 12CO
Fe3(CO)12 140
Metal Carbonyls of Different Groups:
1.
Carbonyls of Sixth B Metals
These form carbonyls of one type only M(CO)6 where M =
Cr,Mo, Or W, but chromium also forms Cr(CO)5+
A. Chromium Hexacarbonyl Cr(CO)6.
Preparation:
i.
It is prepared by job's method by passing CO at 50 atm. pressure
and at room temperature into a suspension of chromic chloride in
ether. Which has been treated with phenyl magnesium bromide at 70oC.
ii.
Chromium hexacarbonyl can be prepared by treating a solution of
a chromic salt dissolved in ether with Al(C2H5)3 and carbon monooxide at a high temperature and pressure.
iii. It may also be prepared by carbonylating CrCl3 with CO in the
presence of a reducing agent like LiAlH4.
C , 200atmpressure


 Cr.(CO)6 + LiCl + AlCl3
CrCl3 + CO + LiAlH4 175
Properties:
1.
Chromium hexacarbonyl exists in colourless rhombic crystals
which sublime without decomposition and dissolve in either,
chloroform, CCl4 and benzene.
2.
It is attacked by air, bromine, cold aqueous alkali, dilute acids
conc. HCl and Conc.H2SO4. It is however decomposed by chlorine
or by conc. nitric acid.
3.
Decompositions: It gets decomposed by F2 at -75oC to form CrF6.
4.
Action of Na-Metal or NaBH4: Cr(CO)6 when is treated with Na
metal or NaBH4 in liq. NH3 carbonylate anion is formed. In these
reactions the carbonyls are reduced.
Liq.NH

 Na 2 [Cr2-(CO)5]2- + CO
Cr(CO)6 + 2Na 
3
NaBH / Liq . NH

 Na 2 [Cr2-(CO)10]2- + 2CO
Cr(CO)6 
4
5.
3
Substitution reactions: Some CO groups present in Cr (CO) 6 can be
replaced by pyridine to get a number of products.
Py
Py
py



Cr (CO) 3 ( Py) 3
Cr(CO)6 
Cr(CO)4(Py)2 
Cr2(CO)7(Py)5 
Yellow brown solid Yellow red solid Bright red solid
B.
Molybdenum Hexacarbonyl and Tungsten Hexacarbonyl.
Preparation:
1.
Both these carbonyls may be prepared by job's method which
involve the reaction of either MoCl6 or WCl6 with CO in the
presence of phenyl magnesium bromide.
2.
Both may also be prepared by the action of CO at 225o and 200
atm. pressure on metallic molybdenum or tungsten reduced in the
presence of copper or iron.
Properties:
1.
They are colourless, Mo(CO)6 sublimes at 40oC and boils at
156.4o, whereas W(CO)6 sublimes at 50oC and boils at 175oC.
2.
They are stable in air and dissolve in organic solvents like ether,
chloroform, CCl4 and benzene.
3.
Mo(CO)6 do not react with air, cold aqueous alkali, acids, except
conc. nitric acid or with thiols or nitric oxide.
4.
Bromine and chlorine can decompose Mo(CO)6 and W(CO)6.
5.
With pyridine, phenanthroline and ethylene diamine, the CO group
in Mo(CO)6 and W(CO)6 is replaced.
  M(CO)5Pyr2  M2(CO)7PYr5  M(CO)3PYr3
M(CO)6 Pyridine
2.
Carbonyls or VII Group:
These form volatile carbonyls of the formula M2(CO)12 where
M = Mn, Te and Re.
Manganese carbonyl, Mn2(CO)10.
Preparation:
1.
This is prepared by treating manganese iodide and magnesium
with CO in ether under high pressure. In this reaction, magnesium
acts as a reducing agent.
C , 210atmpressure


 Mn2 (CO)10
2MnCl2+10CO+2Mg (in diethyl ehter) 250
+ 2MgI2
2.
By carbonylating MnCl2 with CO in presence of (C8H5)2CONa
C140atm.

 Mn2(CO)10+
2MnCl2+10CO+4(C8H5)2CONa 165
4(C6H5)2CO+4NaCl
Properties:
1.
Manganese carbonyl forms volatile, golden yellow, crystalline,
solid which melts at 155oC in a sealed tube. It is soluble in organic
solvents. It is slowly oxidised in air, especially in solution.
2.
Action of halogens: Mn2(CO)10 reacts with halogens to form
carbonyl halides. Mn2(CO)10 + X2 (X=Br2I)  2Mn(CO)5X
3.
Action of Na-Metal: Na-metal in liquid NH3 converts Mn2(CO)10
into carbonylate anion. In this reaction the oxidation state of Mn
decreases from zero to -1;
Liq.NH

 2Na  [Mn-(CO)5]Mn2(CO)6 + 2Na 
3
4.
Action of H2: Mn2(CO)10 gives carbonyl hydride, Mn(CO)5H; in
the formation of this compound the oxidation state of Mn
decreases from zero to -1.
200C , 200atmpressure
    2[Mn-(CO)5H+]0
Mn2(CO)10 + H2 
5.
Substitution Reaction: Mn2(CO)10 reacts with PR3 to form
Mn(CO)4(PR3):
Mn2(CO)10 + PR3  2Mn(CO)4(PR3) + 2CO
6. Diamagnetic nature: Mn2(CO)10 is a diamagnetic substance,
diamagnetic character is confirmed by the fact that all the electrons
in Mn2(CO)10 are paired and Mn-Mn bonds is present in it.
3.
Carbonyls of VIII Group Metals
A. Carbonyls of Iron: Three carbonyls of iron are known, these are:
a. Iron Pentacarbonyl, Fe (CO)5
Preparation:
i.
It can be prepared by the action of CO on iron powder at 200 oC
and 200 atm. pressures.
Fe + 5CO  Fe (CO)5
ii.
Recently it has been prepared by the action of CO on Ferrous
iodide in the pressure of Cu which acts as a halogen acceptor.
200C
 Fe (CO)5I2
FeI2 + 4CO 
iii. It may also be prepared by the action of CO on FeS at 200 oC and
200 atm. pressure in the presence of copper.
200C , 200atmpressure
    2Fe(CO)5 + Cu2S
2FeS + 10CO + 2Cu 
Properties:
i.
Fe(CO)5 is a yellow liquid which is soluble in methyl alcohol,
ether, acetone and C6H6. It is insoluble in H2O.
ii.
Decomposition: M thermal decomposition at 250oC it yields pure
Fe.
250C
 Fe + 5 Co
2Fe(CO)5 
iii. Action of u.v. light: When cooled solution of Fe(CO)5 in glacial
CH3 COOH is irradiated with u.v. light, Fe(CO)9 is formed. The
above reaction is reversed in darkness.
iv. Hydrolysis: Fe(CO)5 gets hydrolysed by H2O and acids
Fe (CO)5 + H2SO4  FeSO4 + 5CO + H2
v.
Action of alkali: Fe(CO)5 + 4NaOH  Na+[Fe2-(CO)4H+]- +
Na2CO3 + H2O
vi. Action of NH3: with NH3 it yields Fe(CO)4H2
Fe(CO)5 + H2O + NH3  Fe (CO)4H2 + NH2COOH
vii. Reaction with halogen:
Fe(CO)5 + X2  Fe (CO)4X2 + CO
The velocities of these reactions have been found to follow the
order CI < Br < I.
b.
Iron Enneacarbonyl, Fe2(CO)9
When iron pentacarbonyl is dissolved in glacial acetic acid and
is exposed to u.v. light for 6 hours, Fe2(CO)9 is formed which dissolves
in acetic acid, on cooling with water, golden crystals of the
enneacarbonyl are precipitated and are filtered off.
2Fe(CO)5  Fe2(CO)9 + CO
Properties
i. Fe2(CO)9 forms golden triclinic crystal, it is diamagnetic and nonvolatile. It is insoluble in water but soluble in toluene and pyridine.
When heated to 50oC decomposes to form Fe2(CO)12.
3Fe2(CO)9  3Fe (CO)5 + Fe3(CO)12
ii. Action of heat: when heating is done at 100oC, Fe2(CO)9
decomposes to form iron, CO and some Fe (CO)12.
4Fe2(CO)9  Fe + Fe(CO)5 + Fe3(CO)12 + CO
iii. Action of NO: With NO it gives Fe(CO)2 (NO)2 together with
Fe(CO)5 and Fe2(CO)12.
3Fe2(CO)9+4NO2Fe(CO)2(NO)2+Fe(CO)5 + Fe3(CO)12 + 6CO
c. Iron Dodecarbonyl, Fe3(CO)12.
Preparation
It can be prepared by heating Fe2(CO)9 dissolved in toluene at 70oC.
3Fe2(CO)9  3Fe(CO)5 + Fe3(CO)12
Properties
i. Fe3(CO)12 forms deep crystals which are soluble in organic
solvents like toluene, alcohol, ether and pyridine.
ii. Action of Heat: When heated to 140oC Fe3(CO)12 decomposes to
give metallic iron and CO.
C

 3Fe + 12CO
Fe3(CO)12 140
iii. Reaction with Na: Carbylate axion is formed when Fe3(CO)12
reacts with Na metal in Liq. NH3.
Liq.NH

 3Na2+[Fe2-(CO)4]2
Fe3(CO)12 + 6Na 
3
iv. Substitution Reactions: This reaction takes place with pyridine
and methyl alcohol.
Fe3(CO)12 + 3Py  Fe3(CO)9(Py)3 + 3Fe(CO)5
B.
Carbonyls of Cobalt
It forms two carbonyls
i. Cobalt Octacarbonyl, CO2(CO)8
Preparation
i. It is prepared by the action of CO and the reduced metallic cobalt
at 220oC and 250 atm.
2Co + 8CO Co2(Co)8
ii. When a solution of cobalt carbonyl hydride is treated by an acid,
hydrogen is evolved and Co2(CO)8 remains
2Co(CO)4H  Co2(CO)8 + H2
Properties
1. Cobalt octacarbonyl forms orange transparent crystals.
2. It is insoluble in water but is soluble to some extent in alcohol,
ether, CS, etc.
3. Action of air: On exposure to air, dicobalt octacarbonyl is
converted into deep violet basic carbonate of cobalt.
4. Action of Na-metal in liq. NH3: When CO2(CO)8 reacts with Nametal in liq. NH3, it gets reduced to carbonylate anion.
NH
 2Na[Co(CO)4]
Co2(CO)8 + 2Na lig
3
5. Action of NO: Co2(CO)8 reacts with NO at 40oC to form cobalit
carbonyl nitrosyl, [Co-(CO)3(NO)]0. Thus in this reaction the
oxidation state of cobalt decreases from 0 to -1.
Co2(CO)8 + 2X2  2Co2X2 + 8CO
6. Dispropotination Reaction
a. Strong bases cause disproportination into Co(+2) and Co(-1)
2Co(CO)8 + 12NH3  2[Co(NH3)6][Co(CO)4]2 + 8CO
b. With isocyanides it gives penta-co-ordinate cobalt(I) cation.
Co2(CO)8 + 5CNR  [Co(CNR)5][Co(CO)4]+4CO
(ii) Dodecarbonyltetra Cobalt, [CO4(CO)12]
Preparation
1. It is prepared by heating Co2(CO)8 at 60oC.
2. It may also be obtained by oxidizing cobalt carbonyl hydride
below -26oC.
Properties:
i. It is black crystalline solid.
ii. It is very unstable easily oxidized by air and can be
recrystallized from hot benzene.
C. Carbonyls Of Nickel.
(i) Nickel Tetracarbonyl, Ni(CO)4
Preparation
i. Ni(CO)4 can be prepared by the action of CO on reduced nickel at
30-50oC.
Ni + 4CO  Ni(CO)4
ii. When Nickel iodide is heated with CO in the presence of a halogen
acceptor, nickel carbonyl is formed.
NiI2 + 4CO  Ni(CO)4 + I2
Properties
i. It is colourless liquid, m.p. = -23oC, b.p. = 43oC
ii. It has no solubility in water but dissolved in organic solvents.
iii. It decomposes at 180o-200oC in to nickel and CO.
C

 Ni + 4CO
Ni(CO)4 180
3
iv. It reacts with H2SO4 and form NiSO4
Ni(CO)4 + H2SO4  NiSO4 + H2 + 4CO
v. It reacts with Ba(OH)2 and gives BaCO3
Ni(CO)4 + Ba(OH)2  H2Ni(CO)3 + BaCO3
D. Carbonyls of Ruthenium
It forms three carbonyls :
a. Ruthenium Pentacarbonyl, Ru(CO)5
Preparation
i. It is prepared by the action of CO and reduced ruthenium at
200oC and 200 atm. pressures
Ru + 5CO  Ru(CO)5
Properties
i. It is colourless soluble liquid having m.p. = -22oC
ii. It has no solubility in water but is soluble in alcohol, benzene
and CHCl3.
iii. It undergoes decomposition to give Ru2(CO)9 and Ru3(CO)12.
iv. It reacts with halogen to yield Ru(CO)Br and CO.
v. It is photosensitive and yields ruthenium enneacarbonyl.
b. Ruthenium Enneacarbonyl, Ru2(CO)9
It is prepared by exposing pentacarbonyl to u.v. radiation. It
forms yellow monoclinic crystals. It is volatile; it is less stable
towards heat, with iodine. It yield, Ru (CO)2I2.
c. Ru3(CO)12
It is prepared in small quantities along with Ru 2(CO)9 when
Ru(CO)5 is heated at 50oC or by exposing Ru(CO)5 to u.v. light. It
is a green crystalline solid.
E.
Carbonyl Of Osmium
It forms two carbonyls:
a. Osmium pentacarbonyl. Os2(CO)5
It is a colourless having m.p.-15oC. It is obtained.
i. by the action of CO on OsI3 at 120oC and 200 atm. pressure in
the presence of copper.
ii. by the action of CO on OsO4 at 100oC and 50 atm. pressure.
OsO4 + 9CO  Os(CO)5 + 4CO2
b. Osmium eneacarbonyl, Os2(CO)9
It is a yellow crystalline solid. It is prepared by the reaction of
OsI3 with CO in the presence of copper, it is more stable towards
heat than Ru4(CO)9. It melts at 224oC and sublimes without
decomposition.
F.
Carbonyl Of Iridium
It forms 2 carbonyls:
a. Iridium Octacarbonyl, Ir2(CO)8
It is prepared by the reaction of either KIr2Br6 or KIr2Br6 or
KIr2I6 with CO at 200oC and 200 atm. pressures.
It is yellow crystalline solid having m.p. 160oC.
b. Iridium Dodecarbonyl, Ir4(CO)12
It
forms
orange
yellow
rhombohedra
crystals
which
decomposes at 200oC. It is prepared by treating Irl3 with CO under
pressure.
G. Carbonyl of Platinum
Preparation
i.
When CO is passed over PtCl2 at 250oC, PtCl2(CO) and
2PtCl23CO are obtained on heating these yield PtCl2(CO)2.
3PtCl2 + 5CO  PtCl2.2CO + 2PtCl2.3C0
Properties
These carbonyls are decomposed by water and HCl.
PtCl2.CO + H2O  Pt + 2HCl + CO2
PtCl2.CO + H2O  Pt + 2HCl + CO2 + CO
PtCl2.CO + HCl  H[PtCl3.CO]
PtCl2.CO + HCl  H[PtCl3.CO] + CO
5.2.4 Structure of Metal carbonyls
1. Effective Atomic Number Rule:
The structure of CO is :C:O:
It is probable that the lone pair of electrons on the carbon atom
can be used by forming a dative bond with certain metals (MC  O,
Thus (MC  O) types of bonds were assumed to be present in metal
carbonyls. In the formation of MC  O bonds, the electrons are
supplied by the molecules of CO and the metal atom is thus said to
have zero-valency. The Number of molecules of carbon mono-oxide
which can unite with one atom of the metal is controlled by the
tendency of the metal atom to acquire the E.A.N. of the next inert gas.
For the stable nonnumeric carbonyl.
E.A.N. = m + 2y = G
Where
M = Atomic number of the metal M
Y = No. of CO molecules
G = At. No. of next inert gas
Carbonyls
Cr(CO)6
Atomic
Number of
the metal
24
Number of electron E.A.N. Succeeding
contributed by CO
inert gas
groups
12
36
Kr(36)
Fe(CO)5
26
10
36
Kr(36)
Ni(CO)4
28
8
36
Kr(36)
Mo(CO)6
42
12
54
Xe(54)
Ru(CO)5
44
10
54
Xe(54)
W(CO)6
74
12
86
Rn(86)
Os(CO)5
76
10
86
Rn(86)
on the basis of E.A.N. rule it can be explained why Ni atom fails to
form a hexacarbonyl Ni(CO))6 because EAN or Ni atom in Ni(Co)6
would be equal to 28 + 2 x 6 = 40. Which is not the atomic number of
any of the noble gases. Mononuclear carbonyls having the metallic
atom with odd At. No. V(CO)6 and Mn(CO)5 & Co(CO)4 are the
example of such carbonyls. They do not obey EAN rule
V = 23e6CO = 12 eV(CO)6= 35e-
Mn = 25 e5CO = 10 eMn(CO)5= 35e-
Co = 27 e4CO= 8 eCo(CO)4= 35e-
Therefore the metals with odd atomic number cannot form
monocular carbonyls but forms polynuclear carbonyls for example,
Mn(25) and Co(27) form polynuclear carbonyls.
2. Polynuclear Carbonyls:
Sidgwick and Bailey gave the general formula for polynuclear
carbonyls.
G
Where
X m  2y
 X 1
X
G = The At. No. of next Inert Gas.
M = The At. No. of metal atom.
Y = The No. of CO molecules in one
molecule of the carbonyl.
Mn2(CO)10, CO2(CO)8 etc. obey the E.A.N. rule, their E.A.N. per
atom of metal is 36.
For example: E.A.N. of Mn2(CO)10 may be calculated as:
Electrons from 2Mn Atom
=
25 x 2 = 50
Electron from 10CO molecules =
10 x 2 = 20
Electrons from one Mn-Mn Bond=
1x2=2
Total =
E.A.N. for one Mn atom = 72/2 = 36
72
The formation of binuclear carbonyls having metal atoms with
odd atomic number can also be explained on the basis of 18-electron
rule as shown below for Co2(CO)8.
Co2(CO)8
2Co = 2 x 9e-
=
8Co = 2 x 8e-
18e=
16e-
Co-Co bond = 1x 2e- = 2eCo2(CO)8 = 36e
Electrons on one Co atom = 18eDrawback
X-Ray diffraction method shows that the bonds are
intermediate between the M-C = 0 and M=C=0 states, i.e. there is some
double bond character in M-CO. The EAN rule does not explain double
bond character. This is explained by both MOT and VBT.
3. Molecular Orbital Approach
According to the M.O.T. carbon and Oxygen atom undergo
overlapping to form bonds in CO as follows
i.
2 sp hybrid orbital of carbon and 2px of oxygen overlap to form
a localised   bond.
ii.
2py of carbon and 2py of oxygen overlap to form a π-bond.
iii. 2pz of carbon and 2 pz of Oxygen overlap to form another πbond.
iv. There will be 2 non-bonding electrons in the 2sp hybrid orbital
of carbon.
v.
There will be 2 non-bonding electrons in 2s atomic orbital of
oxygen.
vi. There will be no electron in the anit-bonding molecular orbitals,
formed as result of  anti π overlapping.
As the total No. of bonding electrons is six and that of
antibonding electrons nil, bond order of the molecule is three. Hence,
the No. of bonds between carbon and oxygen atoms in CO molecules
is 3, one  and two π.
The lone pair of electrons on carbon could be expected to form
a strong dative bond (  ) due to the electron density remaining close
to the nucleus of the carbon atom. As metal atom-carbon mono-oxide
bonds are readily formed in metal carbonyls. It is expected that there
is some additional bonding mechanism in the formation of metalcarbon monooxide bonds in the metal carbonyls.
Mechanism
1.
Firstly, there is a dative overlapping of filled carbon  -orbital
i.e. 2sp hybrid orbital with an empty metal  -orbital (MCO) as
in the figure 5.1.
-
m
+
+ C = 0 :
-
M
+
C = O:
Fig. 5.1 L - M  bonding
2.
Secondly, there is a dative overlapping of a filled d-orbitals of
metal with empty antibonding p-orbital of the carbon atom
(MCO), resulting in the formation of a dative π bond. The
shaded portion in figure, indicate the filled orbitals, whereas
empty portions indicate vacant orbitals. i.e. having no electrons.
As there is a drift of metal electrons into CO (MCO)
orbitals will tend to make the CO as a whole negative and at the
same time there is a drift of electrons from CO to the metal
(MCO) to make CO positive. Thus enhancing the acceptor
strength of the π bond formation and vice versa.
Fig. 5.2(a) dπ - Pπ back bonding
Fig. 5.2(b) M - CO  and π bonding
3. Valence Bond Method: Monocular Carbonyls
In this method, the molecule may be represented by resonance
structures.
M -+ C – O  M = C = O
with a large amount of the double bond character, it is this structure
that account for their stability. From either the molecular orbital or the
valency bond view point, back donation is seen in both.
Structure of Ni(C)4
1.
The vapour density of nickel carbonyl and the freezing points of
its solution in benzene indicate the molecular formula to be
Ni(CO)4.
2.
Electron diffraction studies shows that Ni (CO)4 molecule has
tetrahedral shape with Ni-C-O linear units. Figure shows that the
Ni-C bond length in this molecule is 1.50Ao which is shorter by
0.32Ao in comparison in Ni-C single bond length (=1.82Ao)
found in carbonyls. The C-O bond length in this carbonyl has
been found to equal to 1.15 Ao. Which is larger that the C-O bond
length in CO molecule (=1.128Ao) (Fig. 5.3)
3.
Fig. 5.3 L Tetrahedral structure of Ni (CO)4 molecule
Raman Spectra shows that nickel atom in the nicle carbonyl must
be tetrahedrally hybridised as in the figure 5.3.
Titrahedral shape of Ni (CO)4 arises due to Sp3 hybridisation of Ni-
atom. Which is diamagnetic, all the ten electrons present in the valence
shell of Ni atom are paired in 3d orbitals. Thus the valence shell
configuration of Ni atom in Ni (CO)4 molecule becomes 3d10 4SO CO 
Ni bond is caused by the overlap between the empty sp3 hybrid orbital on
Ni-atom and doubly filled sp hybrid orbital on C atom in CO molecule,
as in the figure 5.3 (b)
Because of the formation of 4 OC  M bonds, a large negative
charge gets accumulated on central Ni atom. Pauling suggested that the
double bonding occurs with the back donation of d-electron from Ni
atom to CO ligands to such an extent that electroneutrality principle is
obeyed. According to which the electron pair is not shared equally
between Ni and C-atoms of CO ligand but gets attracted more strongly by
C-atom which prevents the accumulation of negative charge on Ni-atom,
in keeping with the greater electronegativity of C-atom compared to Ni
atom.
Evidences:
1.
The above structure (Fig. 5.3) is supported by the following
reactions:
i.
When an alcoholic solution of the carbonyl is treated with
orthophenanthroline to yield a stable ruby-red compound
Ni(CO)2 phen. It confirms that two C=O groups of Ni(CO)4 are
replaced by one molecule of phenanthroline.
ii.
Similarly the reactions of Ni(CO)4 with diarsine indicates the two
C=O groups are replaced and remaining two are retained.
2.
i.
Structure of Fe(CO)5: The various evidences are:
The vapour density and the freezing points of benzene solution
shows that its molecular formula is Fe(CO)5.
ii.
Electron diffraction, Raman and I.R. spectra shows that it has
trigonal bipyramidal shape and Fe-C axial bond and Fe-C basal
bond lengths are equal to 1.797Ao and 1.842Ao respectively. It
has dsp3 hybridisation of Fe atom (Fig. 5.4(c).
III. Molecule is Diamagnetic and the distance Fe-C is 1.84Ao(Fig.
5.4).
Fig. 5.4 (a) : Structure of Fe(CO)5
Structure of Cr(CO)6
It has octahedral configuration. The internuclear bond lengths
are:
Cr-C
Cr-O
C-O
1.92
3.08
1.16Ao
According to old concept when chromium forms Cr(CO)6 one
electron of 4s orbital missing and three 4p orbitals become empty which
are hybridised to form six d2sp3-hybrid orbitals six molecules of CO
donate a lone pair of electrons each to six vacant hybrid orbitals to form
six CrCO  -bonds as shown in the Figure 5.5. Therefore Cr(CO)6
molecule is diamagnetic in nature and octahedral in geometry.
When chromium atoms form chromium carbonyl [Cr(CO)6] the
metal atom exhibits d2sp3 hybridisation. Out of 6 d2sp3 hybrids three
hybrid orbitals are half filled and three hybrid orbitals are empty. Three
electrons remain in 3d orbitals as shown in the figure. 5.5(a) and (b).
The Bond Structure of Cr(CO)6 Shows 2 kinds of bonds between Cr
and Co.

(a) Simple Covalent Bonds Cr-C  0 (b) Double bonds Cr 
c=O
In the resultant resonance structure all Cr-C bonds have been
identical, each of the 6 CO groups get linked to the metal atom by a 
bond are constructed from the d-orbitals of the metal atom. CO (groups
I) are bound to the metal atoms by simple ionic bonds.
Fig. 5.5 Structure of Cr(CO)6
Hence these CO groups are replaceable by any other molecule
capable of donating lone pair of electrons to the metal atom where as
CO (groups II) are not replaceable.
In the same way structure of MO(CO)6 and W(CO)6 can be
explained. Various internuclear bond lengths of these carbonyls are as
under:
TABLE : INTERNUCLEAR BOND LENGTHS
Metal
M-C(A)
M-O(A)
C-O(A)
Cr
1.916
3.98
1.171
Mo
2.063
3.23
1.145
W
2.06
3.19
1.148
Thus, the bond structure of Cr(CO)6 shows 2 kinds of bonds
between Cr and CO.
i.
Simple covalent bonds
ii.
Double bond
Cr-C  0
(I)
Cr = 0
(II)
Structure of Polynuclear Carbonyls
These crabonyls obeys EAN rule, if two electrons from each
metal metal bond present in these carbonysls are included in calculating
the electrons per metal atom; eg metal-metal bonding is evident in
Mn2(CO)10 as in Figure. 5.6
Structure of Dinuclear Carbonyls
(a) (i) Mn2(CO)10 : Its structrue is shown in Fig 5.6.
Fig. 5.6
(ii) Structure of Fe2(CO)8
I.R. and X-Ray study show that in this molecule each Fe atom
is directly linked with the other Fe atom by a S-bond (Fe-Fe S-bond) to
three bridging carbonyl gropus (>C = 0) by a  bond (Fe-C  bond) and
to three terminal carbonyl gropus (-C  = 0) by a co-ordinate bond
(FeC co-ordinate bond). The presence of Fe-Fe bond is supported by
the diamagnetic character of Fe2(CO)9 molecule. Fe-Fe bond distance has
been found to be equal to 2.46Ao. The terminal C-O bond distances from
the structure given in figure. (5.7)
The co-ordination number of each Fe atom is not equal to 6 but
equal to 7.
Fig. 5.7: Structure of Fe2(CO)8
Similarly structures of Co2(CO)8 can be represented as in Fig. 5.8
Fig. 5.8: Structure of Fe2(CO)8
(b) Structure of Trinuclear Carbonyls
Os3(CO)12 and Ru3(CO)12 possess similar structure (Fig. 5.9a)
where as Fe3(CO)12 has a different structure 5.9(b). Os and Ru molecules
do not have any bridging CO group (Fig. 5.9 (a)). In Fe3(CO)12 each of
the two Fe atoms is linked with three terminal CO groups, two bridging
CO groups and third Fe atom is linked with four terminal CO groups and
to each of two Fe atoms.(Fig. 5.9 (b). It is also shown by a structure
similar to Fig. 5.9.
Structure of Fe3(CO)12
According to old concept each iron atoms gets hybridized
trigonal bipyramidally (dsp3). The three trigonal bipyramides get
arranged in such a manner so that the carbonyl groups at two of the
equatorial apices of each bipyramid and held in common by two
bipyramides dxz and dyz orbitals are available to form Fe-Fe bonds. It is
solid. The three Fe atom get situated at the corner of an isosceles triangle
and the twelve CO arranged at the twelve CO arranged at the vertices of
an icosahedra. Two Fe-Fe bond lengths are 2.698 Ao and one Fe-Fe bond
length is 2.56Ao. (Fig. 5.9)
Fig. 5.9 Fe3(CO)12 Complex
Fig. 5.9 (a) M3 (CO)12 Structure
Fig. 5.9 (b) Structure of Fe3 (CO)12
(c) Tetra and Hexanuclear Carbonyls
On the similar grounds structures of tetranuclear carbonyls, such as
M4(CO)12 [M = Co, Rh, Ir] and hexanuclear carbonyls, M6(CO)16 eg.
Rh6(CO)16 can be represented as in Figs. 5.10 and 5.11 respectively.
Some heteronuclear carbonyls are also known e.g. Mn2Fe(CO)14 is shown
in Fig. 5.12.
(a)
(b)
Fig. 5.10 (a) Structure of Ir4(CO)12
(b) M4(CO)12; M = Co or Rh
Fig. 5.11 Structure of Rh6(CO)16
Fig. 5.12 Structure of Mn2Fe(CO)14
5.2.5 Vibrational Spectra
IR spectra give important information regarding nature of carbonyl
groups present in metal carbonyl complexes. We can differentiate between
the terminal carbonyl e.g. in Mn2(CO)10 and bridging carbonyl groups, as in
Co2(CO)8.
Metal-carbon distances in Fe2(CO)9 and Co2(CO)8 fall into two
groups, metal-bridging carbonyl distances being about 0-1 A longer than
metal-terminal carbonyl distances. Such a difference is compatible with the
concept of two-electron donation by terminal carbonyls, and one-electron
donation (to each of two metal atoms) by bridging carbonyls, through the
possible existence of  bonds of different strengths makes quantitative
interpretation impossible. That the extent of  bonding to terminal and
bridging carbonyls is different is clearly shown by carbonyl stretching
frequencies. Carbon monoxide itself has stretching frequency of 2143 cm-1;
neutral metal carbonyls known to have no bridging carbonyl groups have
stretching frequencies in the range 2125-2000 cm-1; and Fe2(CO)9 and
Co2(CO)8, in addition to showing bands in this region, also show carbonyl
absorption at 1830 and 1860 cm-1 respectively. In general, carbonyl
absorption in the 1900-1800 cm-1 region is indicative of the presence of
bridging carbonyl groups in uncharged species, though the presence of other
groups may result in the lowering of the stretching frequencies of terminal
carbonyl groups into this region (in carbonylate anions such as [Co(CO)4]and [Fe(CO)4]2- very low carbonyl stretching frequencies of 1883 and 1788
cm-1 respectively result from the strong metal-carbon   bonding which
stabilizes the low oxidation state of the metal. In a few neutral species
believed to contain carbonyl groups bonded to three metal atoms, stretching
frequencies of 1800 cm-1 or less are found.
Thus, in summary the terminal carbonyl absorption is obtained in the
range of 2125-2000 cm-1, while bridging carbonyl frequency is obtained in
the 1900-1800 cm-1 region. While, the strong metal carbon  bonding is
indicated by very low carbonyl structing frequencies of 1883 and 1788 cm-1
respectively.
Check Your Progress-1
Notes :(i) Write your answers in the space given below .
(ii) Compare your answers with those given at the end of the
unit.
(a) Generally metal carbonls involve...............................  -and .........
.....................  -bonding between CO and metal atom.
(b)
Metal
atoms
with
even
number
of
electrons
easily
form............carbonyl, but the metal ions with odd number of electrons
give.......................or...............................
(c) I.R.
spectra
of
carbonyls
at..........................cm-1
for
show
the
C-O
terminal
stretching
carbonyl
frequency
group
and
at................cm-1 for the bridging carbonyl group. The M-CO π bond is
indicated
by
the
absorption
at...................and.............Cm-1
respectively.
(d) While
Mn2(CO)10
has........................bridging
carbonyl
group,
Fe2(CO)9 has................................................................................
5.3
METAL NITROSYLS
Nitrosyls are the compounds in which the nitrogen of the nitrosyl
group is directly bonded to the atoms or ions, or the compounds containing
nitric oxide group are called nitrosyl compound. NO molecule is an odd
electron molecule having an unpaired electron, it readily unites with other
elements by direct addition to form nitrosyl compounds. Nitric oxide form
nitrosyl compound by the following 3 ways:
A positive ion, NO+ is formed due to the loss of an electron which
i.
then combine with atom or molecule (:N:::O)+ or (:N  O:)+
A negative ion NO- is formed due to the gain of an electron from
ii.
some electropositive metal and it has structure as below:
(:N:: O)- or (:N=O:)iii. NO may act as a co-ordinating group through the donation of an
electron pair, such behaviour involve neutral molecule or NO + or
NO- group.
The electronic configuration of NO group is so flexible that it is rather
impossible to write its any one configuration in metal nitrosyls. However in
all nitrosyls nitrogen atom is linked with the metal possible modes are given
below:
I


:N
:N
:O:
Links
:O:
Links
II
III

with


bonds

(neutral)
bonds
D
:N

V
D
:N

:O:
with Links
and
IV
:N

:O:
with Links
:O:with Accepts
dative bonds
dative bonds
electron
(cationic)
(cationic)
from metal
(cationic)
(anionic)
Mode (I) is rarely seen, while (II), (III) and (IV) modes are formed
after transferring one electron to the metal atom. Out of these modes (II) and
(III) are similar to the carbonyl group linked with a metal atom. Mode (V) is
seen only in a few complexes only, e.g. [Co(CN)5NO].
5.3.1 Neutral NO and NO- Complexes
As has been pointed out metal complexes of neutral NO and its anion,
NO- are very rare. Fe(NO)2(CO)2 is supposed to be the important example of
metal complex with neutral nitric oxide molecule. This is prepared by the
action of nitric oxide on Fe(CO)5. During the reaction, NO replaces neutral
CO, hence it is supposed to be a complex of neutral NO. However, the
experimental evidences are not supportive.
The important examples of anionic NO- are the metal complexes,
formed by the action of nitric oxide with ammonical solution of Cobalt (II)
salts, with the general formula [Co)NH3)5NO]X2. Two series of isomeric
complexes are formed one having black colour, while the other one has red
colour.
The black series contains the monomeric cation [Co(NH3)3NO]2+, in
which a very low N-O stretching frequency of 1170 cm-1 and a long N-O
bond (variously reported as 1.26 or 1.41A) suggests the presence of NO-.
The red series are derivatives of hyponitrite, the structure of the diametric
cation being.
Similarly [Co(CN)5NO]3- anion is also supposed to be a complex of
NO- anion, since it gives NO- stretching frequency at 1150 cm-1.
5.3.2 Complexes of NO+
Most complexes of nitric oxide and transition metals are best
considered to be those of the NO+ ion, three electrons being transferred to
the metal atom: M-N back π-bonding then takes place in exactly the same
way as for carbon monoxide. Because of its positive charge, however,
coordinated NO is a better π-acceptor than coordinated CO, and the N-O
stretching frequency in complexes of NO+ is some 300-500 cm-1 lower than
that in salts such as NO+BF4-.
Two NO+ derivatives of iron may be mentioned briefly here. The
species formed in the brown-ring test for nitrate is [Fe(H2O)5NO]2+. The
equilibrium
[Fe(H2O)6]2+ + NO  [Fe(H2O)5NO]2+ + H2O
is reversible, and the brown complex may be destroyed by blowing
nitrogen through the solution to remove nitric oxide. In this species the N-O
stretching frequency is 1745 cm-1, and the magnetic moment is 3.9 B.M.,
corresponding to the presence of three unpaired electrons; formally,
therefore, the ion is a high-spin d7 complex of Fe1 and NO+, but the N-O
stretching frequency indicates very strong π-bonding and the intense brown
colour strongly suggests Fe1-NO+ charge transfer.
5.3.3 Pure Nitrosyl Complexes
Pure nitrosyl complexes of M(NO)4 formula have been reported.
Important complexes in this series are Fe(NO)4, Ru(NO)4 and Co(NO)4. In
addition to this trinitrosyl cobalt, Co(NO)3 has also been reported.
Fe(NO)4 is prepared by the action of nitric oxide under pressure and
below 45oC temperature on Fe(CO)5. While M(NO)4 nitrosyls of Ru and Co
are prepared by the same method using Ru2(CO)9 and Co2(CO)8
respectively.
Fe(NO)4 is a black crystalline substance which decomposes in to
Fe(NO) and Fe(NO)2. The structure of tetranirtrosyl iron, Fe(NO)4 has been
shown tetrahedral, while that of trinitrosyl cobalt, Co(NO) 3 pyramidal. Nitric
oxide links with iron, following II mode, as a three electron donor and
results in a strong ML back π-bonding (Fig. 5.13).
5.3.4 Nitrosyl Carbonyl Complexes
Mononuclear nitrosyl carbonyls are restricted to the following
compounds; Co(NO)(CO)3, Fe(NO)2(CO)2, Mn(NO)3CO and Co(NO)3
(isoelectronic with Ni(CO)4; Mn(NO)(CO)4 (isoelectronic with Fe(CO)3; and
V(NO)(CO)5 (isoelectronic with Cr(CO)6). In addition a binuclear species
Mn2(NO)2(CO)7 (isoelectronic with Fe2(CO)9 and a number of nitrosyl
complexes containing organic groups or triphenylphophine as substituents
have been prepared. Nitric oxide displaces carbon monoxide from V(CO) 6,
(Ph3P)2Mn2(CO)8,
Fe2(CO)9
and
Co2(CO)8
to
give
V(NO)(CO)5,
Mn(NO)(CO)4, Fe(NO)2(CO)2 and Co(No)(CO)3 respectively; the further
action of nitric oxide on the manganese and cobalt compounds yields
Mn(NO)3(CO) and Co(NO)3. All of these substances are solids of low
melting point or liquids which are thermally rather unstable and are
decomposed by air and by water. In the reaction of Fe(NO) 2(CO)2 with
alkali in methanol, [Fe(NO)(CO)3]- is formed, but under comparable
conditions Co(NO)(CO)3 gives [Co)CO)4]-, Co(OH)2 and other cobalt-free
products.
The limited evidence available is consistent with tetrahedral structures
for Fe(NO)2(CO)2 (Fig. 5.14) and Co(NO)(CO)3 and a trigonal bipyamidal
structure (with NO in the equatorial plane) for Mn(NO)(CO)4 (Fig. 5.15);
(Ph3P)2Mn(NO)(CO)2 also has a trigonal bipyramidal structure, the two
triphenylphosphine molecules occupying the apical positions. Since
Co(NO)3 shows two N-O stretching frequencies in the infrared, it must be
pyramidal rather than planar, but the detailed structure is not known.
5.3.5 Nirtosyl Halide Complexes
Volatile diamagnetic nitrosyl halides of formula Fe(NO)3X are formed
by the action of nitric oxide on iron carbonyl halides in the presence of
finely divided iron as a halogen-acceptor. These readily lose NO to give
[Fe(NO)2X]2, in which the halogen atoms act as bridges. Analogous
compounds of cobalt and nickel may be formed by reactions similar to those
involved in the high pressure synthesis of carbonyls; for example,
CoX2 + Co + 4NO  2Co(NO)2X
4NiI2 + 2Zn + 8NO  2[Ni(NO)I]4 + 2ZnI2
The ease of formation of these compounds increases in the sequences
Ni < Co < Fe and X = Cl < Br < I. Nitrosyl chloride and nickel carbonyl in
liquid hydrogen chloride, on the other hand, give Ni(NO) 2Cl2, which is
probably monomeric and tetrahedral. Nitrosyl halides are also formed by
some metals which, so far as is known, do not form nitrosyls or nitrosyl
carbonyls. Thus molybdenum and tungsten (but not chromium) carbonyls
react with nitrosyl chloride:
M(CO)6 + 2NOCl
20
M(NO)2Cl2 + 6CO
CH 2 Cl2
Palladium (II) chloride in methanolic solution yields Pd(NO)2Cl2, and
nitrosyl halide molecules or anions are formed also by several other
transition metals.
5.3.6 Nirtoso Cyanide Complexes
Sodium nitropursside is also a complex resulted from the coordination
of NO+.
Sodium nitroprusside [nitrosopentacyano-ferrate (II)] is prepared by
the action of nitric acid or sodium nitrite on the hexacyanoferrate (II). In the
former process the overall reactions is
[Fe(CN)6]4- + 4H+ + NO3-  [Fe(CN)5NO]2- + CO2 + NH4+
In the latter process, two successive equilibria are involved:
[Fe(CN)6]4- + NO2-  [Fe(CN)5NO2]4- + CN[Fe(CN)5NO2]4- + H2O  [Fe(CN)5NO]2- + 2OHThese are driven to completion by adding barium chloride to the
reaction mixture and blowing a current of carbon dioxide through the hot
solution to remove the hydrogen cyanide liberated by the reaction
2[Fe(CN)6]4-+2NO2-+3Ba2++3CO2+ H2O  2[Fe(CN)5NO]2-+
2HCN + 3BaCO3
The formulation of the complex anion as a NO+ derivative of iron (II)
is supported by its diamagnetism, a N-O stretching frequency of 1939 Cm-1
and a N-O distance of 1.13A. The purple colour obtained from
nitrosopentacyanoferrate
(II)
and
sulphide
is
[Fe(CN)5(NOS)]4- analogous to [Fe(CN)5NO2]4-.
Structure of Nitrosyl Co-ordination Compounds:
due
to
the
ion
If we compare the electronic structure of NO with CO, it is observed
that NO has an additional electron in antibonding π M.O., which may be
readily lost to form the nitrosonium ion, NO+.
The additional electron present in π molecular orbital of NO can be
supplied to metal atom thus increasing its effective number by one unit and
neutral No is itself converted into NO+ ion. Then, this NO+ is co-ordinated
through nitrogen with the metal atom by donating its lone pair to the metal.
1.
Cobalt atom may increase its E.A.N. from 27 to 28 by accepting an
additional electron from a neutral molecule of NO:
Co + NO  Co- + NO+ cobalt ion may then combine with one
NO+ group and 3 CO molecules to form stable compounds Co - + NO+
+ 3CO  Co(NO)(CO)3 In this compound the E.A.N. of Co is,
27+1+2+6 = 36 of stable Krypton.
2.
Similarly the formation of Fe(NO)2(CO)2 can be explained. Sidgwick
gave the electronic structure of metallic nitrosyls as belowM+  (:N: ::O:+) or M2- - N+  O+
The accumulation of charge on the central atom favours strong
π-bond formation with the attached groups.
Thus the most of metal nitrosyls are formed by donation from
the (NO)+ to the metal atom with the M-O back bonding in a manner
analogous to M-C bond in carbonyl it is known as three electron
donor
M + NO  M- + NO+ M2- - N+ = 0+
In terms of M.O.T. the hybrid orbital on N atom having a lone
pair [(sp)2N lone pair] overlaps suitable vacant hybrid orbital on M
ion (sp3 in tetrahedral or d2sp3 in octahedral) to form ON+  M-  bond and the empty π2* or π1* M.O. will overlap with the filled dorbitals to form M- NO+ π bond. This type of overlap transfers some
charge from M- ion to NO+ ion. The molecule of NO is a resonance
structure of the following forms:
π
π
N 
O:  N – O  -N 
O+:
On this basis resonance structures of NO, the metallic nitrosyls
may be represented as:
π
π
π
π
M--N 
O: M- N 
O M 
N 
O: M--N-O
Nitric oxide is a paramagnetic molecule with an electron in an
anti-bonding orbital. This electron is relatively easily lost with
formation of the NO+ ion and an increase in the N-O stretching
frequency from 1878 cm-1 in NO to 2200-2400 cm-1 in nitrosonium
salts.
Structure of various groups of nitrosyl complexes are shown in
Fig. 5.13 to 5.20
Fig. 5.13 Structure of Fe (NO)4
Fig. 5.14 Structure of Fe(CO)2 (NO)2
Fig. 5.15: Structure of [Mn(NO) (CO)4]
Fig. 5.16: Structure of [Fe(NO)2I]2
Fig. 5.17: Structure of Fe(NO)3Cl
Fig. 5.18: Structure of [Ni (NO)I]4
Fig. 5.19: Red salt of Diethyl Ester of [FeS2(NO)4]2-
Fig. 5.20: Anion of Red salt of [Fe4(NO)7S3]
5.4
DINITROGEN COMPLEXES
In
1965,
Allen
and
Senoff
obtained
salts
containing
the
[Ru(NH3)5N2]2+ cation by the action of hydrazine hydrate on various
compounds of tri- and tetrapositive ruthenium, amongst them ruthenium
trichloride and ammonium hexachlororuthenate (IV).
Thus, these substances (often called nitrogenyl or dinitrogen
complexes, to distinguish them from those containing the nitride ion) have
been known for only a few years.
Many other complexes containing one or two (but not, so far, more)
molecules of coordinated nitrogen have now been prepared, and it is clear
that N2 acts as a  -donor and π-acceptor in the same way as isoelectromic
CO, though the complexes formed are much less stable than carbonyls.
Much of the interest in this field centres on the possibility of developing new
methods for nitrogen fixation; up to the present time, however, no method
has been found for the reduction of nitrogen in the complexes described here
(though this has been achieved by systems involving an organ titanium
complex under powerfully reducing conditions).
Most, though not all, nitrogenyl complexes have triphenylphosphine
and halide or hydride as other ligands in the complex. The following
examples illustrate methods for their preparation.
(a) The action of nitrogen on a metal complex: for example,
CoCl2 + Ph3P
NaBH 4
N
(Ph3P)3CoH3 
(Ph3P)3CoH(N2)
EtOH
3
N
[Ru(NH3)5H2O]2+ 
[Ru(NH3)5(N2)]2+
3
(b) Another method of preparation of dinitrogen complex is the
reaction of coordinated azide:
[Ru(NH3)5Cl]2+ N3-
MeSO3 H
[Ru(NH3)5N2]2+
NH 3
(c) Similarly reaction of (Ph3P)2Ir(CO)Cl with RCON3:
(Ph3P)2Ir(CO)Cl + RCON3  Ir(PPh3)2(CO)(Cl)(N2.NCOR)
CHCl3 / EtOH
(Ph3P)2IrCl(N2)
(d) Reaction of coordinated NH3 with HNO2:
[Os(NH3)5(N2)]2+ + HNO2  [Os(NH3)4(N2)2]2+ + 2H2O
The most stable dinitrogen complexes are those of heavier members
of iron and cobalt groups. Some are unaffected by dry air and can be heated
to 100-200oC without decomposition. Most are rapidly oxidised by air and
decompose on heating gently.
The orange solid (Ph3P)3CoH(N2+) shows reversible displacement
with hydrogen, ethylene or ammonia. Some of the reactions of
(Ph3P)2IrCl(N2) (yellow solid) are as follows:
(Ph3P)2IrCl(N2) + Ph3P  (Ph3P)3IrCl + N2
(Ph3P)2IrCl(N2) + HCl  (Ph3P)3IrHCl2 + N2
(Ph3P)2IrCl(N2) + CO  (Ph3P)2Ir(CO)Cl + N2
Dinitrogen complexes show an asymmetric IR N  N stretching
frequency in the range 2230-1920 Cm-1(Raman stretching frequency in N2 is
2331 cm-1).
In metal complexes dinitrogen either has a terminal position or as a
bridge:
N
M–N–N
N
M–N–N–N M
M
N
Terminal
Structures
of
the
M
N
Bridging
two
important
dinitrogen
complexes
[Ru(NH3)5N2Ru(NH3)5]4+ and [Sm(N5C5Me5)2I2(N2)] are shown in Fig. 5.21
and 5.22 respectively.
Fig. 5.21: Structure of [Ru(NH3)5(N2)Ru(NH3)5]
Fig. 5.22: Structure of [Sm(N5C5Me5)2I2(N2)]
5.4.1 Fixation of Nirtrogen
Dinatrogen complexes while show possibility of developing new
methods for nitrogen fixation, they also help in the understanding of the
probable mechanism of biological fixation of nitrogen.
An important enzyme-system is related with the atmospheric fixation
of nitrogen; which involves an important step in nitrogen-cycle and is
responsible for supply of nitrogen to the plants growth (e.g. Blue-green
algae, symbiotic bacteria legume)
The active enzyme in fixation nitrogen is nitrogenase. In this enzyme
two proteins take part in the reaction. Small protein has molecular weight of
57000-73000 and contains Fe4S4 group; while the large protein is a tetramer
of molecular weight 220000-240000. It has 2 molybdenum atoms, nearly 30
iron atoms and nearly 30 mobile sulphide ions. Fe-S group probably
functions as redox centre, and the active site for dinitrogen binding is
probably molybdenum atom. (Fig. 5.23)
Fig. 5.23: Fixation of Nitrogen
5.5
DIOXYGEN COMPLEXES
Amongst all the donor atoms oxygen is most important. The donor
ability of oxygen is related with its partial charge; higher is the negative
charge, higher will be the donar ability. Large number of coordination
compounds are available in which oxygen uses one of its two lone pairs of
electrons.
The most important example of dioxygen complexation
is
transportation of oxygen in aerobic-organisms through heme and hemocynin
mechanism. Although, hemoglobin and hemocynin are known since long
time for their specific ability of absorption and release of oxygen; but now a
number of synthetic compounds have this property, e.g. Bis (Salicylic)
ethylenedimmine cobalt (II).
Heme, protein is the most important group of metallic porphyrin,
which functions as a oxygen carrier in aerobic organism. In the centre of its
porphyrin ring is iron (Fe2+), which is linked with the protein part of
haemoglobin. Heme is very much sensitive for reaction with oxygen and the
reactive oxygen complex, forming an intermediate product, is converted into
Fe(II) porphyrin or Hemin.
As has been shown earlier, heme protein functions as the oxygen
carrier during respiration of aerobic organisms. In this process, vertebrates
use two heme-proteirs: hemoglobin and myoglobin. Hemoglobin takes
dioxygen from lungs or gills and passes it to the tissues. Where it is stored in
myoglobin. The cytochromes present in tissues, which functions as electron
carrier, reacts with dioxygen and reduces it. The oxidation power of
dioxygen is thus used in burring of the food. In this way during
transportation storing and use of dioxygen three heme proteins play
important part; these are hemoglobin, myoglobin and cytochrome.
5.5.2 Hemoglobin
Hemoglobin is the red pigment of blood. It has two parts: (a) 96% part
of it is a simple, specific protein called globin and (b) 4% remaining part is
the prosthetic group hence:
Globin
Hemoglobin
Heme
Protoporphyrin
Fe(II)
It is a globular protein, which is made up of polypeptide chins. These
chains are arranged in a regular tetrahedral form and are linked with the four
rings of pyrole. Molecular weight of hemoglobin is nearly 64500.
Hemoglobin molecule can coordinate with dioxygen without
oxidation of iron. The bonding of iron with dioxygen is so strong that
oxyhemoglobin does not decompose during its transportation in the body.
Still it is so weak that its contact with oxidase decomposes it readily. The
various steps during oxidation of hemoglobin are:
Ist step: Bonding with dioxyen:
IInd step: Bonded dioxygen links with other heme (  -peroxo complex is
formed) :
IIIrd step: Decomposition of  per oxo complex into ferryl complex.
IVth step: Reaction of ferryl Complex with heme to give Hematin:
In living being steps I and IV do not take place, otherwise total heme
would have precipitated as hematin. Apart from other reactions, steps III and
IV are checked by sterric hinderance. Thus dioxygen is carried away by
oxyhemoglobin and either stored in oxymyoglobin or given to cytochromes
for use.
In lungs or gills of vertebrates, the following reactions take place:
Hb + 4O2  Hb (O2)4
Hemoglobin Oxyhemoglobin
While in tissues, the reaction that takes place is:
Hb(O2)4 + 4Mb  4Mb(O2) + Hb
Myoglobin
5.6
TERTIARY PHOSPHINE AS LIGAND
Large number of triphenylphosphine and similar substituted metal
carbonyls are known, e.g. Ni(CO).(Ph3P)2. This compound is of great
importance as a catalyst for the polymerisation of olefins and acetylenes e.g.
butadiene to cyclooctadiene and acetylene to benzene and styrene.
Analogous compounds can be obtained by the action of triphenyl
phosphine on iron pentacarbonyl. Similarly dicobalt octacarbonyl gives two
products with Ph3P in 1:1 ratio of Co and Ph3P. One compound is
[Co2(CO)6(PPh3)2] and the other is the salt [Co(CO)3(PPh3)2][Co(CO)4] in
which the cation has the expected trigonal pyramidal structure.
A platinum complex, Pt (CO)2(PPh3)2 can be obtained by the action of
CO on Pt(PPh3)4. As a matter of fact, substitution of triphenylphosphine for
some of the carbonyl groups greatly enhances the stability of the compound;
thus although Co(CO)4 I is unstable, (Ph3P)Co(CO)3I can be made by the
remarkable reaction.
2 CF I
 (Ph3P)Co(CO)3I + I- + C2F6
[(Ph3P)Co(CO)3]- 
3
Triphenylphosphine carbonyl halides of rhodium and iridium may be
prepared by interaction of the metal halide (or a complex halide) and
triphenylphosphine in a variety of organic solvents, the solvent serving as
the source of the carbonyl group:
Ph P
 (Ph3P)2Ir(CO)CI
(NH4)2IrCl6 
3
Ph P
 (Ph3P)2Ir(CO)CI
IrCl3.3H2O 
3
The product of these reactions- (Vaska's compound) is a highly
reactive complex.
Vaska's compound is a carbonyl halide; and many triphenylphosphine complexes containing rhodium and iridium show similar
reactivity and catalytic activity.
The iridium compounds is remarkable for its reversible uptake of H 2,
O2 and SO2 to give crystalline 1:1 adducts which can be decomposed by
lowering the pressure; for example,
O
(Ph3P)2Ir(CO)Cl 
O2Ir(PPh3)2(CO)Cl
2
In the oxygen adduct, oxygen atoms occupy cis octahedral positions;
the O-O distance of 1.30 Ao suggests that Oxygen is present as O2- rather
than O22-. Some of the many other reactions of Vaska's compounds are
shown in Fig. 5.24
Fig. 5.24
Check Your Progress-2
Notes :(i) Write your answers in the space given below .
(ii) Compare your answers with those given at the end of the unit.
A.(i) Most of the metal nitrosyls are formed with..................ion.
However, most of the pure nitrosyl complexes have the general
formula............... (M = ...........................................)
(ii) The various modes of linking of NO are:
(a) ......................................
(b) ......................................
(c) ......................................
(d) ......................................
B.
Fixation of nitrogen involves enzyme..........................., which has
two
proteins.
The
small
contains........................group;
protein,
while
mol-weight.....................,
the
large
protein,
mole
weight......................... contains................Mo atoms...................Fe
atoms and............mobile sulphide ions.
C. (i)
Hemoglobin
binds
dioxygen
to
give..........................:
(reaction)................................................................................
(ii)
The product is given to...............................for storage and (iii) is
used by...........................for burning of food.
D. Vaska compound has general formula....................................
5.7
LET US SUM UP

Most transition metals form complexes with a wide
variety of unsaturated molecules, such as CO, NO, O2, N2
etc., using ML π-bonding, which stabilize these
complexes.

CO molecules combine with transition metal atoms
(generally in zero oxidation state) to give series of
carbonyls, varying from mononuclear, di-nuclear, trinuclear, tetra-nuclear to hexa-nuclear carbonyls. In which
EAN rule is strictly followed.

Metals with even number of electrons give stable
mononuclear carbonyls; but the metals possessing odd
number of electrons do not form stable mononuclear
carbonyls. The shortage of one electron is compensated
by linking with H or Cl or by dimmer formation, e.g.
V(CO)6 forms H[V(CO)6, Na[V(CO)6], [V(CO)6]Cl or
V2(CO)12.

IR spectra give important information regarding the
nature of CO group in the complex. Thus the terminal
CO group indicate by the stretching frequency at 21251850cm-1 (or 2125-2000cm-1). While the bridging COgroup is indicated by the stretching frequency at 19001800 cm-1 region. Frequency at 1883 and 1788cm-1
respectively are indicative of strong π-bonding (ML).

Ni(CO)4 is tetrahedral, Fe(CO)5 is TBP, Cr(CO)6 is
octahedral while the di-nuclear, trinuclear, tetranuclear
and hexanuclear carbonyls have structures derived from
linking of octahedral in respective numbers of sharing
corners or side or a face.

Nitric oxides combine with transition metals to form
coordination compounds. The general modes of linking
may be-

(a)

(b)
(c)
(d)
D
:N
D
:N
:N
:N
:O:
Links with 
bonds
(neutral)
:O:
:O:
:O:
Links with  Links with Links with
and  bonds dative bonds dative bonds
(cationic)
(cationic)
(cationic)





Most of the nitrosyl complexes are derived from linking
of NO+ (nitrosonium ion).

Pure nitrosyls have general formula M(NO)4 with M =
Fe, CO, Ru. However Co(NO)3 has also been reported.

NO displaces CO from V(CO)6, (Ph3P)2Mn(CO)8,
Fe2(CO)9
and
Mn(NO)(CO)4,
Co2(CO)8
to
Fe(NO)2(CO)2
give
V(NO)(CO)5,
and
Co(NO)(CO)3
nitrosylcarbonyl complexes respectively. In addition to
these, binuclear species such as Mn2(NO)2(CO)7 are also
formed.

Many dinitrogen complexes have been reported e.g.
[Ru(NH3)5N2],
[(NH3)5RuN2Ru(NH3)5],
[(Ph3P)2IrCl(N2)], [Os(NH3)4(N2)2] etc.

Dinitrogen
complexes
while
show
possibility
of
developing new methods for nitrogen fixation, they also
help in the understanding of the probable mechanism of
biological fixation of nitrogen.

The active enzyme in fixation of nitrogen is nitrogenase.
The enzyme has two proteins one small (mol wt. 5700073000) protein contains Fe4S4 groups; while the large
protein (mol wt. 220000-240000) is a  2  2 tetramer,
which has 2 Mo atoms, nearly 30 Fe atoms and nearly 30
mobile sulphide ions. Fe-S group functions as a redox
centre and the active site for dinitrogen binding is
molybdenum atom.

Amongst all the donar atoms oxygen is most important.
This ability is related with its partial charge.

The most important example of dioxygen complexation
is transportation of oxygen in aerobic organisms, through
heme and hemocynin mechanism.

During respiration of aerobic organisms two heme
proteins,
hemoglobin
and
myoglobin,
are
used.
Hemoglobin takes dioxygen from lungs or gills and
passes it to the tissue where it is stored in myoglobin.
The cytochromes present in tissues use the oxidation
power of dioxygen in burring of food.

Hb + 4O2  Hb(O2)4
Hemoglobin Oxyhemoglobin
Hb(O2)4 + 4Mb  4Mb(O2) + Hb
Myglobin Oxyhemoglobin

Large numbers of triphenyl phosphine and similar
substituted
metal
carbonyls
are
known
e.g.
Ni(CO)(Ph3P)2. This compound is of great important as a
catalyst for polymerisation of olefins and acetylenes.

Substitution of triphenyl phosphine for some of the
carbonyl groups greatly enhances the stability of the
compound.

Most widely studied compound is 'Vaska compound'
(Ph3P)2Ir(CO)Cl, which is used for the preparation of
large
number
of
triphenyl
phosphine
containing
complexes.
5.8
CHECK YOUR PROGRESS: THE KEY
1.(a) Involve CO  M  and M  CO  -bonding.
(b) Form mononuclear carbonyl....................give dimers or mononuclear
carbonyls linked with H or Cl.
(c)
At 2125-2000 cm-1
and at 1900-1800 cm-1
at 1883 and 1788 cm-1 respectively
(d) Has no bridging group, Fe2(CO)9 has three bridging groups.
2(A) (i) With NO+ ion
formula M(NO)4 (M = Fe, CO and Ru).
(ii)

(a)

(b)
:N
:N
:O:
Links with 
bonds
(neutral)
:O:
Links with 
and  bonds
(cationic)

(c)

D
:N

(d)
D
:N

:O:
:O:
Links with
Links with
dative bonds dative bonds
(cationic)
(cationic)
B. Enzyme nitrogenase, small protein mol. wt 57000-73000
contains Fe4S4 group
Large protein mol. wt. 220000-240000 contains 2 Mo atoms
30 Fe atoms and 30 mobile sulphide ions.
C.(i) To give Oxohemoglobin
Hb + 4O2  Hb(O2)4
(ii) Myoglobin for stroage
and (iii) by cytochromes
D. Vaska compound has general formula:
(Ph3P)2Ir(CO)Cl
Unit - 6
REACTION MECHANISM OF TRANSITION METAL
COMPLEXES-I
Structure
6.0
Introduction.
6.1
Objectives.
6.2
Energy Profile of a Reaction.
6.3
6.2.1
Reactivity of metal Complex - Inert and Labile Complexes.
6.2.2
Valence Bond and Crystal Field applications.
Kinetics of Octahedral Substitution
6.3.1
Nucleophilic Substitution
6.3.2
Hydrolysis Reactions
6.3.3
Factors affecting Acid Hydrolysis
6.3.4
Base- Hydrolysis-Conjugate Base Mechanism
6.3.5
Anation Reaction
6.3.6
Reactions without Metal-Ligand Bond-Cleavage
6.4
Let Us Sum Up
6.5
Check Your Progress: The Key
6.0
INTRODUCTION
Metal complexes are generally classified as 'Labile" and 'Inert' with
reference to their reactivity. The ability of a complex to engage itself in
reactions involving the replacement of one or more ligands in its
coordination sphere by other ligand is called lability of the complex. The
complexes that undergo rapid substitution are termed labile. Where as those
with low rates of substitution are called inert. However, the degree of lability
or inertness of a transition metal complex can be correlated with the delectron configuration of the metal ion. Nearly half of all reactions of
transition metal complexes may be considered substitution reactions, while
the remaining half are redox-reactions.
Equilibrium and kinetics play important and central part, for
determining the outcome of inorganic reactions. It is often helpful to
understand the mechanism of the reactions. A chemist who wishes to
synthesise an octahedral complex, as an example, must have some idea of
lability of the complex in order to choose appropriate experimental
conditions for synthesis.
Because the mechanism is rarely known finally and completely, the
nature of the evidence for a mechanism should always be kept in mind in
order to recognise what other possibilities might also be consistent with it. In
the first part of this unit we describe how reaction mechanisms are classified,
and distinguish between the steps by which the reaction takes place and the
details of the formation of the activated complex. Then these concepts are
used to describe the currently accepted mechanisms for the substitution
reactions of complexes.
However, you may recall what you have already studied about the
basic concept of kinetics and of current views on the nature of substitution at
a saturated carbon atom, as inevitably organic chemical thinking has
expected a great influence on the interpretation of the kinetics of inorganic
reactions.
6.1
OBJECTIVES
The main aim of this unit is to look in detail at the evidence and
experiments that are used in the analysis of reaction pathways and develop a
deeper understanding of the mechanism of substitution reactions of d-block
metal complexes. After going through this unit you should be able to:

discuss the energy profile of a reaction and explain lability in terms
of VBT and CFT principles;

describe nuelcophilic substitution reactions in octahedral complexes
in terms of SN1 and SN2 reaction mechanisms, and the evidences
supporting them;

explain acid- and base-hydrolysis reactions and their mechanisms;

explain water exchange (Anation) is a binuclear reaction and the rate
of this reaction depends upon the nature of the metal ion; and

6.2
describe the catalysts form octahedral substitution reactions;
ENERGY PROFILE OF A REACTION.
Why does a chemical reaction take place? What happens in a
chemical reaction? Answer of these and similar other questions are
important for a chemist; so that he can have control over a chemical reaction
and can either complete it or stop it, according to the need.
In order to convert reactants into product, it is necessary that the
groups or the atoms, linked in what ever manner, in the reactant molecules
should separate (may be partially) and then reunite (re-link) in the form of
the products. Unless this takes place using a suitable mechanism, the
reaction will not take place. On the thermodynamic basis, the possibility of
conversion of reactants, into products is only when the state of disorder and
the bond-energies in the products are relatively high. Both of these, affect
the direction of a chemical change and on the effect of these depends the
important thermodynamic functions Gibb's free energy, G. For a chemical
reaction, free energy is related with the heat content or the useful energy,
H , and the disorder, S , according to the following relation:
G = H - T S
That is, a chemical reaction will go in the direction in which there is
decrease in free energy, i.e. G should be negative.
In order to a chemical reaction takes place, (i) the total bond energies
in the product are stronger then that in the reactants and the total disorder
(entropy) of the products is high or (ii) the total bond forces in the products
are stronger than that in the reactants and the products is less, but T S is
greater than H or (iii) the total bond forces in the products are weaker as
compared to that in the reactants, but the increase in entropy is so high that it
compensates the energy absorbed.
6.2.1 Reactivity of Metal Complex - Inert And Labile Complexes
Almost all the reactions of transition metal complexes may be divided
into two categories; (a) Substitution reactions and (b) Redox-reactions. In
coordination chemistry rate of a reaction is equally important, as the reaction
equilibrium.
The ability of a complex to engage itself in reactions involving the
replacement of one or more ligands in its coordination sphere by other
ligands is called the lability of the complex. The complexes that undergo
rapid substitution (half time period or T1/2 or reaction rate k is used to denote
the speed of the reactions) are termed labile, whereas those with low rates of
substitution are called inert.
The inertness of the complex has nothing to do with the stability as
determined
thermodynamically.
Thus,
[Ni(CN)4]2-,[Mn(CN)6]3-
and
[Cr(CN)6]3- all have high stability constants. Yet the rate of exchange of CNby the labelled
14
CN- gives half time period as 30 S, 1 h and 24 days,
respectively. Therefore [Ni(CN)4]2- is labile, while [Cr(CN)6]3- is inert.
Similarly [Fe(H2O)6]3+ (bond energy = 690 KJ mol-1) is labile while
[Cr(H2O)6]3+ is inert.
[Ni(CN)4]2- is thermodynamically stable but kinetically labile but
[Co(NH3)6]3+ is kinetically inert but thermodynamically unstable.
6.2.2 Valence Bond (VBT) And Crystal Field (CFT) Applications
(a)
VBT Application
According to VBT Octahedral Complex are of two types:
i. Outer-Orbital Complexes which involve sp3d2 hybridisation.
ii. Inner-Orbital Complexes which involve d2sp3 hybridisation.
The two d-orbitals involved in sp3d2 and d2sp3 hybridisation are dx2-y2
and dz2 eg set orbitals.
1. Outer-Orbital Octahedral Complexes
Outer-orbital octahedral complexes (sp3d2 hybridisation) are generally
labile for example the octahedral complexes of Mn 2+(3d5)Fe2+,
Fe3+(3d5) Co2+ (3d7) Ni2+ (3d8) Cu2+ (3d9) and Cr2+ (3d4) exchange
ligands rapidly and hence are labile. This is because, the use of outer
d-orbitals does not make effective overlap between metal and ligand
orbitals resulting in weaker bonds.
2. Inner-Orbital Octahedral Complexes
The six d2sp3 hybrid orbitals are filled with the six electron pairs
denoted by the 6 ligands. dn electrons of the central metal will occupy
dxy, dyz and dxz orbitals. These complexes are inert as the use of
inner d-orbitals results in an effective overlap between metal and
ligand orbitals giving stronger bonds. Inner orbital octahedral
complexes are given in the Table 6.1 which explain the following
observations:
a. In the labile inner-orbital octahedral complexes there is at least one
d-orbital of t2g set empty, so that this empty d-orbital may be used
to accept the electron pair from the incoming ligand in forming the
transition state with coordination number seven(unstable), which
finally stabilise in to an octahedral complex (Coordn. No. 6),
removing one ligand (Fig 6.1).
Fig. 6.1
b. In the inert-orbital octahedral complexes every d-orbital of t2g set
(i.e. dxy, dyz and dxz) contains at least one electron, and have no
vacant orbtial to link an extra ligand.
Table 6.1 Distribution of dn-electrons in various t2g orbitals for
labile and inert inner-orbital octahedral complexes (according
to VBT)
n
n
Type of the
d
Distribution of d electron (shown by
Example of central
complex
configu arrows) in t2g orbitals. Electrons shown by
metal ions
ration crosses in eg orbitals have been donated by
six ligands to enter d2sp3 hybrids and are
in opposite spins.
d
s
p
t2g
eg
2 2
xy yz zx x -y z2
px py pz
0
inner
d
xx xx xx xx xx xx Sc(+3), Y(+3), rare
orbital
earth (+3), Te(+4),
labile
Zr(+4), Hf(+4), Ce(+4),
octahedral
Th(+4), Nb(+5), Ta(+5),
complexes
Mo(+6), W(+6)

d1
xx
xx xx xx xx xx
Ti(+3), V(+4), Mo (+5),
W Re(+6)
d2


xx
xx xx xx xx xx
Ti(+2), V(+3), Nb (+3),
Ta(+3), W(+4), Re(+5),
Ru(+6)
inner
orbital
inert
octahedral
complexes
d3



xx
xx xx xx xx xx
V(+2), Cr(+3), Mo(+3),
W(+3), Mn(+4), Re(+4)
d4
 

xx
xx xx xx xx xx
[Cr(CN)6]4-, Mn(CN)6]1
Re(+3), Os(+3), Ir(+4)
Type of the
dn
Distribution of dn electron (shown by
complex
configu arrows) in t2g orbitals. Electrons shown by
ration crosses in eg orbitals have been donated by
six ligands to enter d2sp3 hybrids and are
in opposite spins.
d
s
p
t2g
eg
2 2
xy yz zx x -y z2
px py pz
d5



xx
Example of central
metal ions
xx xx xx xx xx [Mn(CN)6]4-, Re(+2),
Fe(CN)6]3- Ru(+3),
Os(+3), Ir(+4)
d6



xx
xx xx xx xx xx [Fe(CN)6]4-, Ru(+2),
Os(+2), Co(+3)
(except Co Fe34Rh (+3), Ir (+3)
Table 6.2 Loss in CFSE, E  (in the units of Dq) in the formation of a
pentagonal bipyramidal intermediate in octahedral substitution reactions on
the basis of SN2 associated mechanism
SN2 association mechanism Octahedral (oct.)Pentagonal bipyramidal
(pent.bipy.)
(C.N. = 6)
dn ion
(C.N. = 7)
Strong ligand field (spin-paired or
Weak ligand field (spin-paired or
low-spin complexes)
low-spin complexes)
Oct.
pent.bipy.
(C.N.= 6)
(C.N.= 7)
d0
0 Dq
0 Dq
d1
4
5.28
E
E
Oct.
pent.bipy.
(C.N.= 6)
(C.N.= 7)
0 Dq
0 Dq
0 Dq
0 Dq
0
4
5.28
0
SN2 association mechanism Octahedral (oct.)Pentagonal bipyramidal
(pent.bipy.)
(C.N. = 6)
dn ion
(C.N. = 7)
Strong ligand field (spin-paired or
Weak ligand field (spin-paired or
low-spin complexes)
low-spin complexes)
Oct.
pent.bipy.
(C.N.= 6)
(C.N.= 7)
d2
8
10.56
d3
12
d4
E
E
Oct.
pent.bipy.
(C.N.= 6)
(C.N.= 7)
0
8
10.56
0
17.74
-4.26
12
7.74
-4.26
16
13.02
-2.98
6
4.93
-2.07
d5
20
18.30
-1.70
0
00
0
d6
24
15.48
-8.52
4
5.28
0
d7
18
12.66
-5.34
8
10.56
0
d8
12
7.74
-4.26
12
7.74
-4.26
d9
6
4.93
-1.07
6
4.93
-1.07
d10
0
0.00
0
0
0.00
0
Table 6.3 Loss in CFSE, E  (in the units of Dq) in the formation of a
pentagonal bipyramidal intermediate in octahedral substitution reactions on
the basis of SN1 associated mechanism
dn ion
SN1 association mechanism
Octahedral (oct.) Syware Pyramidal (Squ. pyi)
(C.N. = 6)
(C.N. = 5)
Strong ligand field (spin-paired
Weak ligand field (spin-paired
or low-spin complexes)
or low-spin complexes)
d0
Oct.
(C.N.= 6)
0 Dq
pent.bipy.
(C.N.=5)
0 Dq
d1
4
d2
E
E
0 Dq
Oct.
(C.N.= 6)
0 Dq
pent.bipy.
(C.N.= 5)
0 Dq
0 Dq
4.57
0
4
4.57
0
8
9.14
0
8
9.14
0
d3
12
10.00
-2-00
12
10.00
-2
d4
16
14.57
-1.43
6
9.14
0
d5
20
19.14
-0.86
0
00
0
d6
24
20.00
-4.00
4
4.57
0
d7
18
19.14
0
8
19.14
0
d8
12
10.00
-2.00
12
10.00
-2
d9
6
9.14
0
6
9.14
0
d10
0
0.00
0
0
0.00
0
The value of CFSE mentioned are in the units of Dq and have been
given for both the fields viz. strong field and weak field and for both the
mechanism (SN1, and SN2).
Negative values of E denotes a loss of CFSE when octahedral
complex is changed into an activated complex which may be square
pyramidal or pentagonal bipyramidal. If the CFSE of the activated complex
is greater than that of octahedral complex. E  has been given zero value
which shows that these complexes do not loose CFSE when they are
changed into activated complexes.
The octahedral complexes formed by the ions for which there is large
loss in CFSE are least labile i.e. such complexes are inert.
On the other hand octahedral complexes given by ions for which there
is little or no loss in CFSE are labile i.e. such complexes react rapidly. Thus
we see:
i Both high spin and low spin octahedral complexes of d 0, d1 and d2
ions will react rapidly, i.e. these are labile complexes, in which there
is no loss in CFSE.
ii. According to VBT inner-orbital octahedral complexes of d3, d4, d5,
and d6 ion are inert while these are called low spin or spin paired
complexes according to CFT.
CFT predicts that low spin complexes of these ions are also inert
whether the mechanism is assumed to be SN1 or SN2 in which CFSE
values decreases.
The ion with maximum loss of CFSE will form the most inert
complex. Thus the order of inertness of low spin complexes formed by d 3,
d4, d5 and d6 ions is:
Order of inertness
:
d6 > d6 > d4 > d5
Loss of CFSE for SN1 mechanism
:
-4.00>-2.00>-1.43>-0.86
Loss of CFSE for SN2 mechanism
:
-8.52-4.26-2.98-1.70
The order of reactivity will be reverse of the above i.e. the order of
reactivity will be d6 > d3> d4 > d5 it is supported by the following facts:
i. High spin octahedral complexes of d3 ion will react slowly, i.e. these
are inert complexes because for this ion there is substantial loss in
CFSE whether the substitution mechanism is SN1 or SN2.
ii. High spin octahedral complexes of d5 ion react rapidly i.e. these are
labile complexes, since there is no loss in CFSE.
iii. Both high spin and low spin octahedral complexes of d8 ion are inert.
According to VBT d8 ion [3dxy2, 3yz2, 3dxz2, 3d(x2-y2), 3dz2]
will form outer orbital complexes and will be labile. Therefore in case
d8 ion VBT & CFT gives different predictions.
iv. Both high spin and low spin octahedral complexes of d10 ion are labile.
Factors Affecting the Liability of Complex
1. Charge of the metal ion: For the isoelectronics complexes there is a
decrease in lability with the increase of the charge of the central metal
ion.
i. The order of lability of the complex is as follows:
Lability order : [AlF6]3- > [SiF6]2 > [PF6]- > [SF6]0
Cationic charge : +3
<
+4
<
+5
<
+6
ii. The rate of water exchange represented by:
[M(H2O)6]n + 6H2O*  [M(H2O*)6]n+ + 6H2O
decreases with the increase of cationic charge in the series
Rate of water exchange:
[Na(H2O)6]+ > [Mg(H2O)n]2+ > [Al(H2O)6]3+
Cationic Charge +1
<
+2
<
+3
2. Radii of the Central ion : Complexes having central atoms with
small ionic radii react more slowly than those having larger central
ions i.e. the lability increase with the increase of ionic radius e.g.
Order of liability: [Mg(H2O)6]2+<[Ca(H2O)6]2+< [Sr(H2O)6]2+
Cationic Size (A)
0.65 <
0.99 <
1.13
3. Charge to Radius Ratio Values: Octahedral complexes having the
central metal ion with the largest charges to radius ratio will react
slowest (Fig. 6.2).
i. The first row transition elements [Ni(H2O)6]2+ (a d8 system) has
the largest value of half life i.e. it reacts slowest. The hydrated
M2+ ions [M(H2O)x]2+ of the first row transition elements are all
high spin complexes.
ii. [Cu(H2O)6]2+ reacts most rapidly because the 2 water molecules
above and below the square plane of the tetragonal distorted
octahedral shape of [Cu(H2O)6]2+ are exchanged. The remaining
four H2O molecules lying in the square plane react slowly.
Fig. 6.2: Half-lives (in sec) at 25oC for the exchange
of water by some hydrated metal ions.
4. Geometry of the Complex: Four co-ordinated complexes react
more rapidly than analogues 6-co-ordinated complexes e.g. the very
stable [Ni(CN)4]2+ undergoes rapid exchange with 14CN-,
[Ni(CN)4]2+ + 414CN-  [Ni(14CH)4]2- + 4CN
while 6-co-ordinated complexes like [Mn(CN)6]4- and [Co(CN)6]3have the same stability as [Ni(CN)4]2+. The greater reactivity of 4co-ordinated complexes may be due to the fact there is enough room
round the central ion for the entry of a 5th group into the coordination sphere to form on activated complex.
Check Your Progress-1
Notes : (i) Write your answers in the space given below .
(ii) Compare your answers with those given at the end of the
unit.
A.(i)For a reaction to go in the forward direction G should
be................................
(ii)According to the thermodynamic relation G = .........................
That is for conversion of the reactants into the products, the
bond energies and the state of disorder should be....................i.e. the
value of the  H should be.....................and that of T  S should
be...............................
B.(i) According to VBT, generally labile complexes are........................
complexes, while the inert complexes are.................complexes.
(ii) Inner orbital complexes may be labile, if they have at least..............
in.........................set is vacant, e.g. in..................
(iii) According to CFT inert complexes have............................values of
................................
6.3
KINETICS OF OCTAHEDRAL SUBSTITUTION
Substitution reactions involve the activated complex which is most
unstable changes to give the product x-y and z. Thus the various steps
responsible for the reaction are
X + Y - Z  X.......... Y..........Z  X - Y + Z
Reactants
Activated complex
(Transition State)
Unstable
Products
The difference in energy between the reactants and the activated
complex is called activation energy.
These reaction involves two process (1) SN1 and SN2
1. In SN1 process the rate-determining slow step is a metal-ligand bond
breaking step, since the co-ordination No. of the complex MX5Y (=6)
is decreased to 5 which is the co-ordination number of the
intermediate MX5.
For a ligand replacement reaction of the general type
[LnMX] + Y = [LnMY] + X
(For simplicity all charges are omitted), the mechanism analogous to
unimolecular nucleophilic substitution (sN1) at a carbon atom would
be:
slow
 [LnM] + X
[LnMX] 
fast
 [LnMY] + X
[LnM] + y 
The rate of SN1 mechanism is first order with respect to MX5Y,
i.e. the rate-determining step in this mechanism is unimolecular.
On the other hand the rate determining step for SN2 mechanism
is bimolecular, i.e. its rate of reaction is second order first order with
respect to MX5Y and first order with respect to Z. Thus
for SN1 mechanism rate = K [MX5Y],
and
for SN2 mechanism rate = K [MX5Y][Z]
Here, it may be mentioned that the kinetic data would be
equally compatible with ion-pair formation (if both reactants are ions)
followed by a unimolecular reaction of the ion-pair:
k1
[Ln MX] + Y 
[Ln MX] Y
slow
 [Ln MY] + X
[Ln MX] y 
This leads to
k k [ L MX ][Y ]
d
[Ln MY] = 1 2 n
dt
k 1  k 2
= k[Ln MX][Y]
where
k=
k1k 2
k 1  k 2
Detailed investigation of such a reaction can lead to a value for k 1/k-1,
the equilibrium constant for ion-pair formation.
6.3.1 Nucleophilic Substitution
As has been motioned, nucleophile substitution reactions in octahedral
complexes follow either of the two mechanisms, the dissociation mechanism
or the SN-1 mechanism and the association mechanism or the SN-2
mechanisms. The rate determining step in association or dissociation, may
be worked out by analysing the rate-laws of the reactions taking place and
the specific conditions under which the reactions take place. The difference
in these two mechanisms depends on, whether the rate determining steps is
the formation of a new Y...............M bond or the dissociation of an old
M...................X bond.
(a) SN-1 or Dissociation Mechanism
The nucleophilic substitution unimolecular reaction actually
proceeds in two steps. In the first, slow and rate determining step, one
ligand Y is lost and a five coordinated intermediate is formed.
In
the
second
step
the
short-lived
penta-coordinated
intermediate of very limited stability is attacked rapidly by the
nucleophilic reagent, Z to give the complex, MX5Z.
There two steps are diagrammatically shown in Fig. 6.3
Fig. 6.3: SN1 or dissociation mechanism for the substitution
reaction MX5Y + Z  MX5Z + Y
For the SN1 mechanism, the following points are important:
(1) The trans effect of the ligands would not be operative due to the
dissociation of the ligand completely from the octahedral complex.
(2) The rates of SN1 substitution (k1) should be inversely proportional to
the strength of the Co-L bond, and depend on the charge, steric
factors, and chelating effects of the leaving group L.
(3) Increase in the electron density on Co atom by the electron donors in
SNn should assist the M-L bond breaking.
(4) k1 is independent of the nature of E as well as its concentration
except for the OH- group for which the reaction is of the second
order.
(5) Cis effect. Ligands having another pair of electrons like CNS - or
OH- increase the rate of hydrolysis of the complexes about ten fold
when present cis to L, as compared to the rate when they are present
trans to L. This is due to the stabilization of the square pyramidal
complex by the electron pair donation by OH- or CNS- along the Cis
position through p-d-  bonding (Fig. 6.4). No rearrangement takes
place and the product is 100 percent Cis isomer. The ligands that do
not show the eis effects are those that do not have an extra pair of
electrons (NH3) or are themselves  acceptors (NO2-, CO, NO, etc.).
Fig. 6.4: Cis-effect
From Table 6.4, it can be seen that for the formation of the 5coordinate intermediate, high energy changes are required for the
low spin d3, d6 and d8 and high spin d3 and d8 ions. Hence, these
complexes do not favour the SN1 mechanism for the substitution.
Table 6.4 Changes in LFSE (in Dq) for Changing a 6coordinate Complex to a 5-Coordinate (SP) or a 7-Coordinate
(Pentagonal Bipyramid) species.
System
do, d10
High spin
CN = 5
CN = 7
0.00
0.00
Low spin
CN = 5
CN = 7
0.00
0.00
d1
0.57
1.28
0.57
1.28
d2
1.14
2.56
1.14
2.56
d3
-2.00
-4.26
-2.00
-4.26
d4
3.14
-1.07
-1.43
-2.98
d5
0
0
-0.86
-1.70
d6
0.57
1.28
-4.00
-3.52
d7
1.14
2.56
1.14
-5.34
d8
-2.00
-4.26
-2.00
-4.26
d9
3.14
-1.07
3.14
-1.07
+ value indicate gain in CFSE while - values indicate loss in CFSE.
(b) SN-2 or Association Mechanism
SN-2 or the nucleophilic bimolecular substitution reaction also
proceeds through two steps:
The first step is slow step and involves the attachment of the
incoming nuclepohile, Z to MX5Y to form a seven-coordinate unstable
intermediate (perhaps transition state) which is probably pentogonal
bipyramidal in shape. Obviously it is a metal-ligand bond-making step.
( Z )


MX5Y Slow
(C.N.=6)
MX5YZ
Unstable seven-coordinatee
Intermediate (C.N.=7)
This reaction is rate-determining and bimolecular because two
reactants viz MX5Y and Z are involved in this step. Thus the rate of this
rate-determining reaction is of second order: first order with respect to
the complex, MX5Y and first order with respect to the entering ligand,
Z, i.e.,
Rate of reaction = K[MX5Y][Z]
In the second step either at the same as Z adds to MX5Y or
shortly thereafter, Y leaves MX5YZ rapidly to give MX5Z. This is a fast
step.
MX5YZ
Unstable seven-
Fast


MX5Z
-Y
(C.N.=6)
coordinatee Intermediate
(C.N.=7)
Both these steps are shown diagrammatically in Fig. 6.1
This mechanism is similar to Eigen-Wilkins Mechanism, which
presents formation of the association complex [L-MX5-Z] in the preequilibrium step: Thus the following equilibrium will be established:
LMX5 + Z  MX5. Z; K =
[ LMX 5 .Z ]
[ LMX 5 ][ Z ]
The value of the equilibrium constant, K, for the association
complex, may be obtained using Fuoss-Eagan equation,
K=
where,
4
 a3 NAe-v/RT
3
a = Nearest reach-distance
v = Coulomb potential energy at a-distance
NA = Avogadro number = (Z1Z2e2/4  ea)
As in the octahedral complexes, the six ligands are already
present along the three C4 axes along which the eg orbitals are
concentrated, the t2g orbitals that lie along the C2 axes most probably
have to be approached by the seventh ligand to form the associated
complexes in the SN2 process.
Hence, if the t2g orbitals are filled (Co2+ in low spin octahedral
complexes), the higher activation energy required to empty one of the
t2g orbitals will make the complex inert.
Table 6.4 also shows that due to the loss of the CFSE energies,
the d3 and low spine d6 ions require highest activation energies,
followed by d7, d8 (Ni2+complexes are labile due to the expulsion of
ligand by the eg orbitals) and high spin d3 and d8 ions.
Thus, SN-1 and SN-2 reactions differ in the following points:
(i) In SN1 process the rate-determing slow step is a metal-ligand bond
breaking step, since the coordination number of the complex, MX 5Y
(=6) is decreased to 5 which is the coordination number of the
intermediate, MX5. On the other hand in SN2 process the ratedetermining step involves a metal-ligand bond making step, since
C.N.=6 is increased to 7.
(ii) The rate of SN1 mechanism is first order with respect to MX5Y, i.e.,
the rate-determining step in this mechanism is unimolecular. On the
other hand the rate-determining step for SN2 mechanism is
bimolecular, i.e. its rate of reaction is second order: first order with
respect to MX5Y and first order with respect to Z. Thus:
for SN1 mechanism rate = K[MX5Y]
and for SN2 mechanism rate = K[MX5Y][Z]
6.3.2 Hydrolysis Reactions
The substitution reactions in which a ligand is replaced by a H 2O
molecule or by OH- groups are called hydrolysis reactions. They are of two
types (a) when an aqua complex is formed by the replacement of a ligand by
H2O molecules are called acid hydrolysis or equation reactions, while (b) the
reactions, in which a hydroxo complex is formed by the replacement of a
ligand by OH- group are called base hydrolysis reactions.
Acid hydrolysis reactions occur in neutral and acidic solutions (pH <3)
while base hydrolysis reactions occur in basic solution (pH  10).
Examples are:[CoIII(NH3)5Cl]2+ + H2O[CoIII(NH3)5(H2O)]3+ + Cl- Acid hydrolysis
[CoIII(en)2ACl]+ + H2O  [CoIII(en)2A(H2O)]2+ + Cl- reaction
[A = OH-, Cl-, NC-, NO2-]
[Co(NH3)5Cl]2++OH-[Co(NH3)5(OH)]2++Cl- (Base hydrolysis reaction)
(a) Acid Hydrolysis or Aquation :
When NH3 or ammines like ethylene diamine or its derivatives
co-ordinated Co(III) are replaced very slowly by H2O molecules and
hence in acid hydrolysis only the replacement of ligands other than
amines is usually considered.
The rate of hydrolysis of the reaction is of first order.
[Co(NH3)5X]2+ + H2O[Co(NH3)5(H2O)]3+ + XThe rate of hydrolysis reaction is of first order.
In aqueous solution the concentration of water is always
constant, the effect of changes in water concentration on the rate of the
reaction cannot be determined.
The rate law K = K1[Co(NH3)5X]2+[55.5] does not indicate
whether these reactions proceed by an SN2 displacement of X by H2O
or by an SN1 dissociation followed by the addition of H2O.
The rate law for acid hydrolysis at low pH thus becomes
-
d
[Co(NH3)5X] = kA[Co(NH3)5X]
dt
(If X is the anion of a weak acid, a term kH+[Co(NH3)5X][H+] is
added.) As we have shown previously, such a rate law is compatible
with either a slow dissociation of the complex into [Co(NH 3)5]3+ and X
or replacement of X by H2O as the rate-determining step. In order to try
to decides between these alternatives, the rates of hydrolysis of a series
of complexes of formula [Co(AA)2Cl2]+, where AA is a substituted
ethylendiamine, were examined. For replacement of a single chloride
ion at pH 1 the order found for values of kA was
CH2NH3
CH3CHNH2
<
CH2NH3
CH3CHNH3
<
CH2NH2
(CH3)2CNH2
<
CH3CHNH2
(CH3)2 CNH2
Such an acceleration of substitution by bulky ligands suggests
that the dissociative mechanism is operative; although introduction of
methyl groups must have some inductive effect, the variation in base
strengths among the diamines is very much less than the variation in
rate constants for the hydrolysis of their cobalt (III) complexes, and it
seems reasonable to attribute the kinetic effect mainly to steric factors.
Now since steric factors favour SN1 reactions, this is evidence for the
dissociative mechanism. Further evidence for this mechanism is
provided by:
(a) a general inverse correlation between the rate of replacement of X
in [Co(NH3)5X] and the formation constant of the [Co(NH3)5X]
complex from [Co(NH3)5(H2O)]3+ and X, and
(b) the decrease in the rate of the exchange reaction
[Co(NH3)5(H2O)]3+ H218O = [Co(NH3)5(H218O)]3+ + H2O at high
pressures.
6.3.3 Factors Affecting Acid Hydrolysis
(i)
Effect of Charge on the Complex:
The value of rates of acid hydrolysis of some Co(III) complexes at
pH=1 shows that the divalent monochloro complexes react about 100 times
slower than the monovalent dichloro complexes.
As the charge of the complex increases, a decrease in rate is observed
and the acid hydrolysis of the divalent complexes like [Co(NH 3)4(H2O)Cl2]2+
occurs in two steps:
[Co(NH3)4 (H2O)Cl]2+ +
slow

 Cl
6-co-ordinate complex
[Co(NH3)4(H2O)]3+ +
[Co(NH3)4(H2O)]3+ + Cl5-co-ordinate Intermediate
fast

 H 2O
[Co(NH3)4(H2O)2]3+
The acid hydrolysis represented by equation (1) would proceed more
rapidly than that represented by equation (2) because the separation of a
negative charge in the form of Cl ion from a complex ion with higher charge
is more difficult.
(ii)
Effect of Chelation
When NH3 molecules in [Co(NH3)5Cl]2+ complex ion are replaced
partially or completely by polyamines like en, trien, diene, tetraene etc, the
rates of the reaction of the divalent complex ions shows that as the number
of -CH2-CH2 or -(CH2)2-chelated links increases the rate values decreases.
The replacement of NH3 molecules by polyamines increases the size
of the complex i.e. the chelated complex has larger size. The larger the size
of the ion less its solvation energy will be and hence less easily it will be
formed. Thus the stability of the transition state in which the Cl ion is only
partially lost and in which the solvation is less efficient will be reduced. The
rate of equation is slowed down by chelation because of reduced stability of
the transition state due to less efficient solvation.
(iii)
Effect of Substitution on ethylene diamine
When H atoms on carbon atom or on nitrogen atom of en groups of
trans [Co(en)2Cl2]+ are replaced by the alkyl groups like CH3,C2H5 etc. the
ligand becomes more bulky, if the strained complex having bulky ligand
reacts by SN1, dissociative mechanism and co-ordination number 6 is
reduced into 5 co-ordinated intermediate, on the other hand if the strained
complex reacts by SN2 displacement process, the crowding on the complex
is increased as it is converted into a transition state of co-ordination number
seven. The rate of hydrolysis of trans [Co(AA2 Cl2)]+ at 25oC and pH=1
corresponding to the replacement of only one Cl- ion by H2O molecule are
given. Here AA is the diamine.
(iv)
Effect of Leaving Group
The rate of reaction of [Co(NH3)5X]2+ corresponding to the
replacement of X with H2O molecule depends on the nature of X because the
bond breaking step is important in rate determining step. The reactivity of Xgroups decreases in the order.
HCO3->NO3->I->Br->Cl->SO4-->F->CH3COO->SCN-<NO2
6.3.4 Base- Hydrolysis-Conjugate Base Mechanism
The base hydrolysis reaction represented by following equation:
[Co(NH3)5Cl]2++ OH-  [Co(NH3)5(OH)]2++ ClIt involves following two mechanisms.
1.
SN-2, Displacement Mechanism
The reaction proceeds as:
(  OH )
fast

 [Co(NH3)5(OH)Cl]+ 
 [Co(NH3)5(OH)]2++ Cl[Co(NH3)5Cl]2+ slow
(C.N.=6)
(C.N.=7)
Rate of Reaction =
=
The rate law is 2.
(C.N.=6)
K[Complex][base]
K[Co(NH3)5Cl][OH-]
d
[Co(NH3)5Cl] = KB[Co(NH3)5Cl][OH-]
dt
SN-1 Displacement Mechanism:
The complex which acts as a Bronsted acid is converted into its
conjugate base (CB), [Co(NH3)4(NH2)Cl]
+
which is obtained by
removing a proton H+ from the amino group present in the complex.
CB is an amido complex, since it contains an amido group.
SN-1 mechanism fails to explain quite a few observations:
(1) 7-coordinate complexes are not very stable.
(2) The value of kn is nearly 104 times higher than kA. Why should
hydroxyl ions posses the exceptionally high nucleophilic
activity as compared to the similar anions?
(3) If a proton cannot be abstracted from N5 ligands (e.g., [Co(py)4Cl2]+ or [Co(CN)5Cl]3-), reaction rate for hydrolysis is very low.
To overcome the above difficulties, an alternative
mechanism is proposed by Garrick (1987). In this case the OHions abstract a proton form a ligand in N5 group giving CB of
the ligand. This undergoes the dissociative mechanism as
shown below:
fast
 [(NH3)4Co(NH2)Cl]++H2O (6.1)
[(NH3)5CoCl]2++OH- 

 [(NH3)4Co(NH2)]2+ + Cl- (6.2)
[(NH3)4Co(NH2)Cl]+ slow
fast
 [(NH3)5Co(OH)]2+
[(NH3)4Co(NH2)]2++H2O 
(6.3)
The rate determining step is the dissociation of the amido
complex given in Eq.(6.2) whose concentration would depend
upon the concentration of hydroxyl ions present. This is the
SN1CB process.
The rate law will bed
= [Co(NH3)5OH] =
dt
K1K2[Co( NH 3 )5 Cl ][OH ]
K1[ H 2O] 2 K2[ H 2O]
= K[Co(NH3)5Cl][OH]
where,
K=
K1 K 2
K 1[ H 2O]2  K 2 [ H 2O]
Though it seems very unlikely that reduction in one
positive charge form [Co(NH3)6]3+ to [Co(NH3)5(NH2)]2+ should
increase the reaction rate enormously, it is possible that through
a  bonding intermediate, the stability of the 5-coordinate
complex is increased (Fig. 6.5).
Fig. 6.5: Stabilization of the intermediate 5-coordinate species
through the resonance effects involving NH2 group.
The SN1 CB mechanism does not explain the following
observations. (i) The conjugate base readily dissociates and
releases the ligand L; and (ii) the concentration of the conjugate
base is very low due to the basic nature of the ligands, and
should be present only as a very small fraction of the
concentration of the complex present.
Direct and Indirect Evidences in Favour of Conjugate Mechanism:
Equation 6.1 requires that the reacting complex should have at least
one Photonic hydrogen atom (H+) on a non-leaving ligand so that H+ may
transfer to OH- to form its conjugate acid H2O and conjugate base,
[Co(NH3)4(NH2)Cl]+ of [Co(NH3)5Cl]2+ which acts as an acid. Thus a
complex having no proton should react with OH- much more slowly and the
rate of reaction would be independent of the concentration of OH-. It is
observed that the complexes like [Co(Cn)2Br] and trans [Co-(Py)4Cl2]+
which does not have N-H hydrogen undergo hydrolysis much more slowly
in basic solution at a rate which is independent of [OH-] over a wide range.
Thus in the absence of an acidic portion on the ligands an SN1 CB
mechanism is not possible.
Such complexes undergo rapid base hydrolysis supports the SN 1 CB
mechanism and the acid-base properties of the complexes are more
important to the rate of reaction, than the nucleophilic properties of OH.
Thus both the mechanisms give the same rate laws and the same
hydroxo products in aqueous solution, because water is a good cocoordinating agent. The rate of formation of [Co(en)2(NO2)Y]+ depends only
on the concentration of the base, OH, not on the nature or concentration of
Y-, OH- and piper; dine are used as catalysts while N3-, NO2-, SCN- ion are
used as nucleophiles.
In SN1CB mechanism the reactions of [Co(NH3)5Cl]2+ and OH- in
aqueous solution at 25oC in presence of H2O, when H2O2 is added to the
reaction mixture of [Co(NH3)5Cl]2+ and OH-, the reaction between OH- and
H2O2 occurs as:
OH- + H2O2  H2O + HO2Which increase the rate of base hydrolysis reaction and form peroxo
products.
On the other hand if the reaction occurs by an SN1CB mechanism the
addition of H2O2 to the reaction mixture should reduce the rate of base
hydrolysis reaction compared to OH- because of the reduction in the
concentration of OH- ions. The rate of SN1CB reaction is directly
proportional to the concentration of OH-.
6.3.5 Anation Reaction
The reaction in which an aquo ligand (i.e. H2O molecule) from an
aquo complex is replaced from the co-ordination shell by some axion.
[Co(NH3)5(H2O)]3+ + X- [Co(NH3)5X]2+ + H2O
Thus we find that an anation reaction is the reverse of acid hydrolysis
reaction.
It shows that these are bimolecular reactions with a rate which
depends on the concentration of the complex and X. The same second order
kinetics would be observed for a unimolecular process.
slow
fast
 [Co(NH3)5]3+ 
 [Co(NH3)5X]2++ H2O
[Co(NH3)5(H2O)]3+ 
Let us consider replacement of water in a species containing five nonlabile ligands such as [Co(NH3)5(H2O)]3+ , and let us reverse the
experimental procedure and attempt to infer kinetic behaviour from a
postulated mechanism. This is
k
[L5M(H2O] 
[L5M] + H2O
1
k
[L5M] + Y 
[L5MY]
1
Since Y competes with solvent water for the active intermediate
[L5M], the rate of formation of [L5MY] can be dependent on the
concentration of Y. On the other hand, there should be some high
concentration of Y at which the rate of replacement of water no longer
depends on the concentration of Y. The rate of formation of [L5MY] at this
concentration should be equal to the rate of formation of [L5M] and also
equal to the rate of exchange of water between [L5M(H2O)] and the solvent.
Thus the rate of formation of [L5M] is given by
d
[L5M] = k1[L5M(H2O)] - k-1[L5M][H2O] - k2 [L5M][Y]
dt
According to the steady-state approximation, the concentration of the
very reactive [L5M] remains small and constant during the reactions (i.e.
d
[L5M] = 0 at the steady state). Thus,
dt
[L5M] =
K1[[L 5 M( H 2O ]
K 1[ H 2O ]  K 2 [Y ]
and
d
K K [L M( H 2O][Y ]
[L5MY] = 1 2 5
dt
K 1[ H 2O]  K 2 [Y ]
if k-1 [H2O] > k2[Y]
d
KK
[L5MY] = 1 2 [L5M( H 2O][Y ]
dt
K 1
and a second-order reaction will be observed. On the other hand,
if k2[Y] > k-1[H2O]
d
[L5MY] = k1 [L5 M( H 2O]
dt
giving first-order kinetics with the overall first-order constant equal to
that for the dissociations of the aquo complex.
6.3.6 Reactions without Metal-Ligand Bond-Cleavage
Many a times, replacement of ligand takes place without breaking a
metal-ligand bond. Important examples of this fact are formation of aquocomplex, [Co(NH3)5H2O]3+, from carbon a to complex, [Co(NH3)5CO3] and
nitrito complex [Co(NH3)5ONO]2+ from hydroxo or aquo-complex,
[Co(NH3)5H2O]3+.
The most likely path for the equation of the carbonato complexes
seems to be the electrophilic attack by the proton on the O atom bonded to
the metal, so that no O is found in the complex when the equation is carried
out in presence of H2O (Fig. 6.6).
Similarly the reaction of pentamineaquacobalt (III) with NO2- ion is
explained by the sequence in Fig. 6.7.
Fig. 6.6 Mechanism of substitution of carbonate group by water
through electrophilic attack by H2O+.
Fig. 6.7 Probable mechanism of substitution of [(NH3)5Co(OH)]2+ by
nitrite NO2- through an electrophilic attack.
Check Your Progress-2
Notes : (i) Write your answers in the space given below .
(ii) Compare your answers with those given at the end of the unit.
(a) SN-1 and SN-2 mechanism of substitution reactions differ in (i) In SN-1 rate determining step is.............................process, while that
in SN-2 is .......................process.
(ii) The rate determining step in SN-1 is...................molecular, while
that in SN-2 is ....................... molecular.
(iii) In SN-1 rate = ......................................
while that in SN-2 rate = ......................................
(b)(i) For acid hydrolysis at low pH, the rate Law is ...................................=.................................................
(ii) The rate law of base hydrolysis reactions of an octahedral ammine
complex, by SN-1 CB process is d
= [Co(NH3)5OH] = .................................
dt
= ...................................
(iii) Formation of aquo-complex from a carbonato complex is an
example of substitution...................... bond, and involves..............
attack on....................
6.4
LET US SUM UP

In order to convert reactions in to products it is necessary that the
groups or the atoms linked in what ever manner in the reactants
molecules should separate and then reunite in the form of the
products.

On the thermodynamic basis, Gibbs free energy for the reaction
should decrease, in order to the reaction takes place, i.e.  G should
be negative.

Since,
 G =  H - T  S, hence the possibility of conversion of reactants in
to products is only when the state of disorder, and the bond energies
in the products, are relatively high;  H should be negative and  S
should be positive.

Complexes are generally classified as labile and inert with reference
to their reactivity. The ability of a complex to engage itself in the
reactions involving the replacement of one or more ligands in the
coordination sphere by other ligand is called lability of the complex.

The complexes that undergo rapid substitution are termed labile;
where as these with law rates of substitution are called inert.

According to VBT, the inner orbital complexes (using d 2sp3
hybridisation for octahedral complication) are inert while the outer
orbital complexes (using sp3d2 hybridisation) are labile.

The inner orbital complexes may be labile only when they have at
least one d-orbital in t2g set is vacant; e.g. [V(NH3)6]3+ ion,

According to CFT, complexes with high values of CFSE are inert,
while those with small values of CFSE are labile.

Substitutions of ligands generally follow one of the two, SN-1 or SN-2
mechanisms.

In SN-1 or dissociation mechanism, the rate determining slow step is a
metal-ligand bond breaking step, since the coordination number of the
complex, MX5Y (=6) is decreased to 5 in the intermediate, MX5,
complexes:
Y
Z
[MX5Y] 
[MX5] 
[MX5Z]
(C.N.=6)
(C.N.=5)
(C.N.=6)
Thus, the rate of SN-1 mechanism is first order with respect to
MX5Y, i.e. the rate determining step is unimolecular.

In SN-2 process the rate determining step involves a metal-ligand
bond making step, with the increase of coordination number from 6 to
7:
Z
Y
[LMX5] 
[L-MX5Z] 
[MX5Z]
(C.N.=6)
(C.N.=7)
(C.N.=6)
Thus, the rate determine step in SN-2 process in bimolecular i.e. its
rate of reactions is second order; first order with respect to [MX5L] and first
order with respect to Z.

For SN-1 mechanism, rate = k [MX5L]
For SN-2 mechanism, rate = k [MX5L][Z]

The substitution reaction in which a ligand is replaced by a H2O
molecule or by OH- group is known as hydrolysis reaction.
The reaction is called 'acid hydrolysis' or 'aquation' when an aquo
complex is formed by the replacement of a ligand by H 2O molecule while
the reaction in which a hydroxo complex is formed by the replacement of a
ligand by -OH group is called base-hydrolysis. Acid hydrolysis occur, in
neutral and acid solutions (pH<3), while base hydrolysis occurs in basic
solutions (pH>10).

The rate of acid hydrolysis reaction is of first order [Co (NH3)5X]2+ + H2O  [Co(NH3)5(H2O)]3+ + XAs in oqueous solutions the concentration of water is always constant,
the rate law, K = K1 [Co (NH3)5X]2+ [55.5] does not indicate whether these
reactions proceed by and SN-2 displacement of X by H2O or by SN-1
dissociation followed by the addition of H2O.

For base hydrolysis;
[Co (NH3)5Cl]2+ + -OH  [Co(NH3)5(OH)]2+ + Cl-
SN-2 mechanism gives rate of the reaction = K[Co(NH3)5Cl]2+ [OH-]
and the rate law 
d
[Co(NH3)5Cl] = KB[Co(NH3)5Cl][OH-].
dt
Gerick proposed SN-1CB mechanism for base hydrolysis reaction. In
this, the -OH ions abstract a proton from a ligand in N5 group, giving
the conjugate base of the ligand. This under goes the dissociative
mechanism:
Fast
 [(NH3)4Co(NH2Cl)]+ + H2O
[(NH3)5CoCl]2+ + OH- 

 [(NH3)4Co(NH2)]2+ + Cl[(NH3)4Co(NH2)Cl]+ Slow
Fast
 [(NH3)5Co(OH)]2+
[(NH3)4Co(NH2)]2+ + H2O 
Thus, the rate determining step is the dissociation of the amido group.
The rate law will bed
[Co(NH3)5OH] =
dt
K1K 2 [Co(NH3 )5 Cl][OH- ]
K 1[ H 2O] 2 K 2[ H 2O]
= K [Co(NH3)5Cl][OH-]

The reactions involving removal of coordinated water molecule are
known as 'anation' reactions:
[[Co(NH3)5(H2O)]3+ + X-  [Co(NH3)5X]2+ + H2O
This reaction is reverse of acid hydrolysis reaction. The same second
order kinetics would be observed for a unimolecular process:
 [Co(NH3)5]3+
[Co(NH3)5(H2O)]3+  Slow
H 2O

Fast

 X
[Co(NH3)5X]2+
Many a times replacement of ligand takes place without breaking a
metal-ligand bond, e.g. formation an aquo-complex from a carbonato
complex. These involve the electrophilic attack by the proton on the
O-atom bonded to the metal.
6.5
CHECK YOUR PROGRESS: THE KEY
1(a)(i)  G should be negative.
(ii)  G =  H - T  S
That is .......................should be very high i.e.  H should be
negative and T  S should be positive.
(b)(i) Labile complexes are outer orbital complexes....................inert
complexes are inner orbital complexes.
(ii) One orbital in t2g set:
e.g. [V(NH3)6]3+
(iii) Have high values of CFSE.
2.(a)(i) Is metal-ligand bond breaking process that in SN-2 is metal-ligand
bond making process.
(ii) SN-1 is Unimolecular.
SN-2 is bimolecular.
(iii) SN-1, rate = K[MX5Y]
SN-2, rate = K[MX5Y][Z]
(b)(i) Rate law is -
d
[Co(NH3)5X] =KA[Co(NH3)5X]
dt
K1 K 2 [Co(NH 3 ) 5 Cl][OH - ]
(ii) =
K 1[ H 2 O] 2  K 2 [ H 2 O]
= K [Co(NH 3 )5 Cl][OH - ]
(iii) Without breaking M-L bond.
Involves electrophilic attack of proton on oxygen bonded with
metal.