In the nineteen sixties, Ralph Pearson developed the
Type A and and Type B logic by explaining the
differential complexation behaviour of cations and ligands in terms of
electron pair donating Lewis bases and electron pair accepting Lewis acids:
Lewis acid + Lewis base
Lewis acid/base complex
Pearson classified Lewis acids and Lewis bases as
hard, borderline or soft.
According to Pearson's hard soft [Lewis] acid base (HSAB) principle:
Hard [Lewis] acids prefer to bind to hard [Lewis] bases
and
Soft [Lewis] acids prefer to bind to soft [Lewis] bases
At first sight, HSAB analysis seems
rather similar to the Type A and Type B system.
However, Pearson classified a very wide range of
atoms,
ions,
molecules and
molecular ions
as hard, borderline or soft Lewis acids or Lewis bases,
moving the analysis from traditional metal/ligand inorganic chemistry
into the realm of organic chemistry.
Hard Acids
Hard Bases
Borderline Acids
Borderline Bases
Soft Acids
Soft Bases
Most metals are classified as Hard acids or acceptors.
Exceptions: acceptors metals in red box are always soft .
Green boxes are soft
in low oxidation states,
hard in high..
Solubilities: AgF(S-H) > AgCl > AgBr >AgI (S-S)
Orange boxes are
But: LiBr (H-S) > LiCl > LiI > LiF (H-H) soft in high oxidation
states.
Log K for complex formation
hard
soft
softness
Chatt’s explanation: soft metals ACIDS have d electrons available for p-bonding
Model: Base donates electron density to metal acceptor. Back donation, from
acid to base, may occur from the metal d electrons into vacant orbitals on the
base.
Higher oxidation states of elements to the right of transition metals
have more soft character.
There are electrons outside the d shell which interfere with pi bonding.
In higher oxidation states they are removed.
For transition metals:
high oxidation states and position to the left of periodic table are hard
low oxidation states and position to the right of periodic table are soft
Soft BASE molecules or ions that are readily polarizable and have vacant d or π* orbitals
available for π back-bonding react best with soft metals
Tendency to complex with hard metal ions
N >> P > As > Sb
O >> S > Se > Te
F > Cl > Br > I
Tendency to complex with soft metal ions
N << P > As > Sb
O << S > Se ~ Te
F < Cl < Br < I
The hard-soft distinction is linked to polarizability, the degree to which the
electrons in a molecule or ion may be easily distorted by interaction with other
molecules or ions.
Hard acids or bases are small and non-polarizable
Hard acids are cations with high positive charge (3+ or greater),
or cations with d electrons not available for π-bonding
Soft acids are cations with a moderate positive charge (2+ or lower),
Or cations with d electrons readily availbale for π-bonding
The larger and more massive an ion, the softer (large number of internal electrons
shield the outer ones making the atom or ion more polarizable)
Soft acids and bases are larger and more polarizable
For bases, a large number of electrons
or a larger size are related to soft character
Examples
•Harder nucleophiles like alkoxide ion, R-O–, attack the acyl (carbonyl) carbon.
•Softer nucleophiles like the cyanide ion, NC–, and the thioanion, R-S–, attack
the "beta" alkyl carbon
S-S
H-H
Further Development
Pearson and Parr defined the chemical hardness, h, as the second derivative for
how the energy with respect to the number of electrons.
Expanding with a three point approximation
Related to Mulliken
electronegativity
softness

1
h
IA

2
Energy levels for halogens and relations between
, h and HOMO-LUMO energies
Chemical Hardness, , in electron volt
Acids
Bases
+
Hydrogen
H
Aluminum
Al
Lithium
Li
Scandium
Sc
Sodium
Na
Lanthanum
La
Zinc
Zn
-
infinite Fluoride
F
45.8
Ammonia
NH3
35.1
hydride
H
24.6
carbon monoxide CO
21.1
hydroxyl
OH
15.4
cyanide
CN
5.3
10.8
phosphane
PH3
5.0
Carbon dioxide CO2 10.8
nitrite
NO2
Sulfur dioxide
SO2 5.6
Hydrosulfide
SH
Iodine
I2
Methane
CH3
3+
+
3+
+
3+
2+
3.4
7
6.8
-
6.8
6.0
-
5.6
-
-
-
4.5
4.1
-
4.0
Coordination Chemistry
• General aspects (Ch. 9)
• Bonding (Ch. 10)
• Electronic spectra (Ch. 11)
• Reaction mechanisms (Ch. 12)
Acids and bases (the Lewis concept)
A base is an electron-pair donor
An acid is an electron-pair acceptor
acid
adduct
base
Lewis acid-base adducts involving metal ions
are called coordination compounds (or complexes)
Coordination complexes
Coordinated ligands
Central metal atom
Solv
+n
L
L
L
L
M
L
[A-]n
Solv
L
L
L
Inner coordination sphere counteranion
L
Solv
Solv
M
L
Solv
Inner coordination sphere
The metal cation is the Lewis acid, the ligands are the Lewis bases
Naming coordination complexes
General nomenclature rules in coordination chemistry
•
•
•
•
•
•
•
•
•
Cation first, then anion (as for simple salts) (K3[Fe(CN)6], potassium hexacyanoferrate)
Inner coordination sphere in square brackets in formula. Ligands named before the metal
Hexaaminecobalt(III) chloride: [Co(NH3)6]Cl3
Number of ligand indicated by prefix (di,tri,tetra or bis, tris, tetrakis if ligand in parenthesis)
tris(bipyridine)iron(II) chloride: [Fe(bipy)3]Cl2
Ligands named in alphabetical order ignoring prefix
Anionic ligands are given the suffix -o (chloro-, sulfato-, nitrato-)
while neutral ligands retain name (except aqua for H2O and ammine for NH3)
Metal named after ligands with oxidation state in roman numerals or give overall charge of
coordination sphere
Ex. Fe(III), tetrachloroplatinate(-2)
Cis (adjacent)-trans (opposite) or fac (C3v) –mer (C2v) isomers are indicated with prefix
Bridging ligands are indicated with m (greek mu)
m-oxo for M-O-M
If complex is anionic, use ending “-ate”
-cobaltate, ruthenate, but note ferrate for Fe, argentate for Ag, plumbate for Pb,
stannate for Sn and aurate for Au
Isomerism
• Stereoisomers (enantiomers, diastereomers,
cis/trans, mer/fac, conformational) have
same metal ligand bonds but different 3D
arrangement.
• Hydrate (solvate) isomers, ionization,
linkage, coodination isomers have different
metal-ligand bonds.
Examples of Four Coordinate
Stereoisomers
planar
NH3
NH3
Cl
Pt
Cl
Cl
NH3
Cl
NH3
trans
Pt
cis
Tetrahedral, chirality now possible.
Four different monodentate ligands.
stereoisomers
Chirality in tetrahedral complexes
Very common
L4
L1
L1
M
M
L2
L3
L2
L3
(2 enantiomers if all ligands different)
L4
Examples of Six coordinate
Stereoisomers
How many stereoisomers are there of formula Mabcdef?
For the six sites in the octahedron there are 6! = 6 * 5 * 4 * 3 * 2 * 1
ways of positioning the ligands.
However some of these ways are the same structure; simply rotated.
An octahedron has many rotations which simply interchange ligands: 8
C3, 6 C2, 6 C4 and 3 C2. Thus there are 23 rotated structures to be
generated from an original structure. 6!/(23+1) = 30 stereoisomers.
For some complexes with multidentate ligands there are geometry
constraints which reduce the number of isomers.
Examples of Six coordinate
Stereoisomers
How many stereoisomers are there of formula Maabbcc?
For the six sites in the octahedron there are 6! = 6 * 5 * 4 * 3 * 2 * 1
ways of positioning the ligands.
However some of these ways are the same structure; simply rotated.
An octahedron has many rotations which simply interchange ligands: 8
C3, 6 C2, 6 C4 and 3 C2. Thus there are 23 rotated structures to be
generated from an original structure. 6!/(23+1) = 30 stereoisomers.
For some complexes with multidentate ligands there are geometry
constraints which reduce the number of isomers.
Chirality in octahedral complexes
Maabbcc
a
c
b
a
a
a
b
b
c
c
b
b
c
c
b
c
a
c
a
b
a
a
non-chiral
c
b
Have two trans ligands the same.
a
a
a
a
b
c
c
b
b
b
b
a
c
c
a
a
c
c
b
b
c
Do not have two trans ligands the same.
b
a
c
chiral
Multidentate ligands and isomer
count.
Let AA be a multidentate ligand which must bond cis.
For octahederal complex MAAbcde how many stereoisomers?
Permutation count is not 6! but
6 * 4 *4!
# stereoisomers = 6 * 4 *4!/(24*2)
Only four spots
for the second
A to enter.
M
A
For a complex MAABCde
Due to
rotations
Since both
ends of the
AA are the
same.
For a complex MAABCde with multidentate ligands A – A and B - C
Rotation factor
Due to A-A
symmetric ligand
Number of stereoisomers = 6 * 4 * (2 *2 * 2! + 2*3 *2!)/(24 * 2) = 10 stereoisomers
B
Assign first A and
second A in cis position
A
M
A
A
M
A
B
Chirality in octahedral complexes with chelating ligands
Cl
N
N
non-chiral
Co
N
N
Cl
N
N
N
Cl
Co
Co
N
Cl
N
N
Cl
Cl
N
N
chiral
Several chelate rings and chirality
N
N
N
N
N
M
N
N
M
N
N
N
N
N
D isomer
L isomer
Right hand screw
Left hand screw
Conformational Isomers
The chelate rings can have alternative
conformations.
Constitutional Isomers
• Hydrate Isomers: in crystal structure is water
part of the first ligand shell or a hydrate
–
–
–
–
[Cr(H2O)6]Cl3, violet
[CrCl(H2O)5]Cl2.H2O, blue-green
[CrCl2(H2O)4]Cl.2H2O, dark green
[CrCl3(H2O)3].3H2O, yellow green
3+
OH2
H2 O
Cr
2+
Cl
OH2
H2 O
OH2
H2 O
violet
3Cl-
H2 O
Cr
H2 O
OH2
OH2
H2 O
green
+
Cl
-
2Cl
H2 O
Cr
H2 O
OH2
OH2
Cl
green
Cl-
Constitutional Isomers
• Ionization isomerization: different ions produced
in solution
– [Co(NH3)5SO4]NO3 & [Co(NH3)5NO3] SO4
• Coordination Isomers: More than one ratio
of ligand can exist but maintaining overall
ratio
– [Pt(NH3)2Cl2]
– [Pt(NH3)3Cl] [Pt(NH3)Cl3]
• Linkage (ambidentate) isomerism
– Thiocyanate, SCN-, can bind through either
the N (to hard acids) or through S (to soft
acids).
– Nitrite, NO2-, can bond through either the N or
the O
Typical coordination numbers and structures
of coordination complexes
and isomerism
Coordination number 1
Very rare, bulky ligands, linear structures, no possible isomers
Coordination number 2
Also rare, typical of d10, linear structures, no possible isomers
Coordination number 3
Also typical of d10, trigonal planar structures (rarely T-shaped), no possible isomers
Coordination number 4
Very common
L1
L2
M
L1
L1
L2
cis
M
L4
L2
L1
L3
L2
M
L2
Tetrahedral
L1
trans
(2 enantiomers if all ligands different)
Square planar
(2 geometrical isomer
for two types of ligands)
typical of d8
Tetrahedral
Square planar
Coordination number 5
La
La
Le
Le
M
Le
Lb
Lb
M
Lb
Lb
La
Trigonal bipyramidal (tbp)
Square-based pyramidal sbp)
Very similar energies, they may easily interconvert in solution (fluxionality)
Coordination number 6
M
M
Octahedral
most common
Trigonal prism
less common
Some possible isomers in octahedral complexes
A
A
B
A
B
B
M
M
B
B
B
B
A
B
cis-MA2B4
trans-MA2B4
A
A
A
B
M
M
B
B
B
A
B
A
B
A
fac-MA3B3
mer-MA3B3
Some examples of trigonal prismatic structures
Coordination number 7
M
M
M
Pentagonal bipyramidal
Capped octahedral
Capped
trigonal prismatic
Examples of coordination number 7