Advanced Inorganic Chemistry:
Coordination Chemistry
Prof. Dr. Claudia Wickleder
1. Introduction
Electronic configuration of the 3d transition elements
Sc Ti V Cr Mn Fe Co Ni Cu Zn
4s
3d
2
1
2
2
2
3
1
5
2
5
2
6
2
7
2 1 2
8 10 10
Electronic configuration of the Lanthanides
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf
6s
5d
4f
2
1
0
2
1
1
2 2
0 0
3 4
2
0
5
2
0
6
2 2
0 1
7 7
2 2 2 2 2 2 2 2
0 0 0 0 0 0 1 2
9 10 11 12 13 14 14 14
2
1. Introduction
3
Frost diagrams of the 3d elements in acid solution
1. Introduction
Definition:
A complex or coordination compound is a
compound in which an atom (called “central
atom”) is bound to more groups (called
“ligands”) than expected with respect to its
charge and position in the periodic table.
The number of ligands around a central atom is
called the “coordination number”.
4
2. Basic Nomenclature
Formulas:
- Complexes are marked with parenthesis (square brackets [])
- 1.) metal ion 2.) charged ligands
3.) neutral ligands (sometimes changed atoms to show bonds, like in [Fe(OH2)6]2+)
Names:
1.) First ligands in alphabetic order, the add of “o” marks ionic ligands
Trivial names of ionic ligands:
FClBrI-
fluoro
chloro
bromo
iodo
OHO22S2HS-
hydroxo
peroxo
thio
mercapto
SCNCH3OCH3SNO2-
thiocyanato (rhodano)
methoxo
methylthio
nitro
O2-
oxo
CN-
cyano
NO3-
nitrato
Names of neutral ligands:
H2O
CO
aquo
carbonyl
NH3
NO
ammin
nitrosyl
5
2. Basic Nomenclature
Number of identical ligands in greek as prefix:
1
2
3
mono
di
tri
4
5
6
tetra
penta
hexa
7
8
9
hepta
octa
nona
10
11
12
deca
undeca
dodeca
Bridging ligands with µ!
If there could be a confusion: bis, tris, tetrakis as prefix
(CH3)2N- = dimethylamino is unambiguous, but: (H2N(CH3))2 = bis(methylamin), not dimethylamin!
2.) After ligands name of the central ion, anionic complexes: + “at”
sometimes usual name of central ions for neutral and cationic complexes, but Latin names for anionic
ones (Fe, Au, Pb, Sn)
3.) For anionic complexes: name of the cation in front of the complex
4.) After the name the oxidation state of the central ion in roman numbers may be given in parenthesis
or/and: Arabian numbers after the complex marks the charge of the complex
5.) First (in front of name): prefix which indicates the structure, like cis/trans
6
2. Basic Nomenclature
Examples: Potassiumtetrafluorooxochromate
Tris(ethylendiamin)cobalt(II)sulfate
Tetrakis(trifluorphosphin)nickel(0)
Tetraammincopper(II)chloride
Examples:
Na[Al(OH)4]
[Ni(CO)4]
K3[CrOF4]
[CoCl(NH3)5]Cl2
[PtCl4(NH3)2]
(NH4)2[PbCl6]
[Al(OH)(H2O)5]Cl2
[Pt(NH3)4][PtCl6]
K[AuCl4]
7
2. Basic Nomenclature
Including chelating large ligands:
tion
(O)
identify the donor atoms
8
2. Basic Nomenclature
General: If all ligands are identical: homoleptic complexes, else: heteroleptic complexes! 9
3. Coordination polyhedra
Geometry of complexes
The main structural characteristics of complexes are their co-ordination numbers and their
co-ordination polyhedra.
L
Z
L
1. Co-ordination number 2
Complexes with co-ordination number 2 are rare. They are only formed by central atoms
of the group Cu+, Ag+ and Au+.
The complexes are linear. Bent geometries as they are found in three-atomic molecules
like H2O have never been seen with complexes.
10
3. Coordination polyhedra
2. Co-ordination number 3
Complexes with co-ordination number 3 are seldom.
Examples are HgI3-, [Pt(P{C6H5}3]3.
L
The complexes are trigonal planar, sometimes
slightly deformed. There is no possibility for the
formation of isomers in complexes of type [ZL2L’] or
[ZLL’L’’]
Some complexes of CN 3 have the form of a trigonal
pyramid like NH3, OR3+ or SR3+ due to a free electron
pair. They are said to be pseudo-tetrahedral as the free
electron pair and the three ligands occupy the four
corners of a tetrahedron.
L
Z
L
Z
L
L
L
11
3. Coordination polyhedra
3. Co-ordination number 4
For the co-ordination number 4 which is very common there are 4 different
structures possible:
L
L
Z
L
tetrahedral
L
L
L
L
Z
L
L
square planar
Z
L
L
L
bisphenoidal
L
L
Z
L
L
tetragonal pyramid
Examples:
tetrahedral: [Al(OH)4]-, [Cd(CN)4]2-, [BF4]square planar: [PtCl4]2-, [Ni(diacetyldioxim)2], [AuF4]bisphenoidal: main group elements with a free electron pair like As or Sb [AsF4][SbCl4]there is the possibility that the bisphenoid becomes distorted towards a tetragonal
pyramid when the electron pair needs more space
12
3. Coordination polyhedra
Sometimes there is a CN of 4 though the formula suggests CN 3
for instance gaseous AlCl3 is dimeric built from two tetrahedra sharing one edge
so that two chloro ligands are bridging and four are end standing
Cl
Cl
Al
Cl
Cl
Al
Cl
Cl
Or in the case of (AuCl3)2 the central atoms
are square planar co-ordinated by 4 chloro
ligands with 2 of them in bridging positions
Cl
Cl
Cl
Cl
Au
Cl
Au
Cl
13
3. Coordination polyhedra
4. Coordination number 5
this coordination number is formed not very often. There are two different
geometries possible:
L
L
Z
L
L
L
L
trigonal bipyramid
L
L
Z
L
L
tetragonal pyramid
In the trigonal bipyramid we can distinguish between equatorial
and apical positions of the ligands
Examples:
trigonal bipyramid: Fe(CO)5, [SnCl5]tetragonal pyramid: [VO(acetylacetonate)2]
14
3. Coordination polyhedra
Slight deformations of the trigonal bipyramid in the indicated way lead to the
formation of the tetragonal pyramid
A
L
Z
A
L
L
L
L
A
Z
L
L
A
L
Z
A
A
L
This can lead to an exchange of the apical and equatorial positions of the ligands
15
3. Coordination polyhedra
C4 axis
5. Co-ordination number 6
of the possible co-ordination geometries
(octahedron, trigonal prismatic, trigonal
antiprismatic and hexagonal planar) only the
octahedron and the trigonal antiprismatic coordination is observed in coordination compounds.
Very often the octahedra are not ideal as not all
edges are equally long.
This may be caused by an elongation or a
compression along the 4 fold axis
or
by an elongation along the 3 fold axis
L
C3 axis
L
L
Z
L
L
L
leading to the trigonal antiprismatic
polyhedron
16
3. Coordination polyhedra
Possible arrangements of 6 ligands L around a central atom Z
L
L
L
Z
L
L
Z
L
L
L
L
L
L
L
L
L
Z
L
L
L
Z
L
L
L
L
L
L
L
17
3. Coordination polyhedra
Possible arrangements of the ligands in an octahedral complex of
composition [ZL4X2]
X
L
L
Z
X
X
L
L
L
Z
L
L
L
X
cis
trans
18
3. Coordination polyhedra
Possible arrangements of the ligands in a trigonal prismatic complex of
composition [ZL4X2]
L
L
L
X
Z
Z
L
L
X
L
L
X
X
L
L
L
L
Z
X
L
X
19
3. Coordination polyhedra
Possible arrangements of the ligands in a hexagonal planar complex
of composition [ZL4X2]
L
L
L
L
X
Z
L
X
Z
X
L
L
L
L
L
X
X
Z
X
L
L
20
3. Coordination polyhedra
Possible arrangements of the ligands in a trigonal antiprismatic
complex of composition [ZL4X2]
X
L
L
X
L
Z
L
X
L
Z
L
L
L
Z
X
L
L
X
L
X
L
21
3. Coordination polyhedra
6. Co-ordination number 7
3 different co-ordination polyhedra exist for CN 7. The energetic difference between
them is low. Sometimes the co-ordination polyhedron changes when the cation changes
L
L
L
L
L
pentagonal bipyramid
L
L
L
L
L
L
L
Z
L
L
L
L
monocapped trigonal prism
Examples:
pentagonal bipyramid: [UO2F5]3-, [HfF7]3moncapped trigonal prism: [TaF7]3monocapped octahedron: [IF6]-, [NbOF6]3-
L
L
Z
L
L
L
monocapped octahedron
22
3. Coordination polyhedra
7. Co-ordination number 8
4 different co-ordination polyhedra exist for CN 8. The energetic differences
between them are low. They become lower with increasing CN.
cube
Z
square antiprism
Z
dodecahedron
Examples:
cube: seldom, but [UF8]3square antiprism: more stable than cube [TaF8]3- , [ReF8]3dodecahedron: [Mo(CN)8]4- , [W(CN)8]4hexagonal bipyramid: [UO2(acetylacetonate)3]-
Z
Z
hexagonal bipyramid
23
4. Isomerism
Isomerism in coordination compounds
if two or more molecules or ions have the same molecular formula
but the atoms are arranged differently we call them isomers.
The structures of isomers are not superimposable.
Isomers have different physical and/or chemical properties.
We distiguish between
•structural isomers which contain the same number and kind of
atoms, but the connectivity between the atoms is different and
•Stereoisomers which contain both the same number and kind of
atoms and the same connectivity between the atoms but the spatial
arrangement of the atoms is different
24
4. Isomerism
Structural isomers
• Ionization isomerism
complex salts which show ionization isomerism
are composed in such a way that a ligand and a
counter ion change their places
[CoCl(NH3)5]SO4
[CoSO4(NH3)5]Cl
25
4. Isomerism
Structural isomers
• hydrate isomerism
this a special case of the ionization isomerism. Here
water molecules are present as ligand in one case
and as water of crystallzation in the second case
[Cr(H2O)6]Cl3
[CrCl(H2O)5]Cl2.H2O
[CrCl2(H2O)6]Cl.2H2O
26
4. Isomerism
Structural isomers
• Co-ordination isomerism
if in a complex salt both anion and cation are complexes
there can be an exchange of ligands between cation and
anion
[Co(NH3)6] [Cr(CN)6]
[Cr(NH3)6] [Co(CN)6]
27
4. Isomerism
Structural isomers
• Linkage isomerism
if a ligand containes more than one atom with a free
electron pair, the ligand may be bound to the central
atom via the different atoms.
C
N
S C
N
N
O
O
bonding via C
bonding via N
cyanoisocyano-
bonding via S
bonding via N
thiocyanatoisothiocyanato-
bonding via N
bonding via O
nitronitrito-
28
4. Isomerism
Stereoisomers can be divided in two groups:
• Enatiomers, i.e. stereoisomers that have a non-superimposable
mirror image
• Diastereoisomers, i.e. all stereoisomers that are not
enantiomers
29
4. Isomerism
Diastereoisomers
• cis - trans isomerism
if a square planar or an octahedral complex containes two
ligands of the same type, they can be arranged so that the angle
L - Z - L is 90° (cis) or 180° (trans)
square planar
Cl
Cl
Pt NH3
NH3
cis
Cl
NH3
octahedral
Pt NH3
Cl
trans
Co
cis
Cl
Cl
Cl
Co
Cl
trans 30
4. Isomerism
Diastereoisomers
• fac - mer isomerism
if an octahedral complex containes three ligands of the same
type they can be arranged such that they all are in a cis
position (fac) or that two of them are in a trans position (mer)
Cl
Co
fac(ial)
Cl
Cl
Cl
Co
Cl
Cl
mer(idional)
31
4. Isomerism
Enantiomers
• stereoisomers that have a non-superimposable
mirror image are called enantiomers
mirror plane
Co
Cl
Cl
Cl
Cl
Co
32
4. Isomerism
The corresponding trans complex is not an enantiomer
mirror plane
Cl
Cl
Co
Co
Cl
Cl
33
4. Isomerism
If a molecule or complex is either
asymmetric, i.e. has no symmetry at all, or
dissymmetric, i.e. has no center of inversion or mirror
plane or other Sn*,
it is called chiral.
Due to the chirality it has a non-superimposable mirror
image
*S
n
inproper rotation axis, S1 is identical to a mirror plane,
S2 ist identical to a center of inversion
34
4. Isomerism
Optical isomerism
• if the lifetimes of the two enantiomers of a chiral
molecule are long enough to be separable they are called
optical isomers
• pure enantiomers are optically active, they rotate the
plane of polarized light in different directions. This is the
only difference in the physical properties of the two
enantiomers
35
5. Bonding in Complexes
Bonding in coordination compounds
• 18 electron rule based on the octet theory of Lewis this is the first attempt
to account for the bonding in complexes
The formation of a complex was described as an acid -base reaction
according to Lewis
The sum of the electrons on the central atom (Lewis acid) including those
donated from the ligands (Lewis base) should be equal to the
number of elctrons on a noble gas
•
Procedure: add the numbers of valence electrons of the metal ion and the
numbers of electrons of the ligands
e.g.: [Fe(NH3)6]2+: Fe2+ d6, 6 NH3 = 6 x 2 = 12, 6 + 12 = 18
36
5. Bonding in Complexes
Examples:
[Co(NO2)6]3-
[PtCl6]2-
[Ag(NH3)4]+
Co3+
6NO2-
Pt4+ 6 e6Cl- 12 e18 e-
Ag+ 10 e4NH3 8 e18 e-
[Cr(NH3)6]3+
[Ni(NH3)6]2+
[CoCl4]2-
Cr3+
6NH3
Ni2+ 8 e6NH3 12 e20 e-
Co2+
4Cl-
6 e12 e18 e-
but
3 e12 e15 e-
7 e8 e15 e-
⇒ Many complexes which do not obey the 18 electron rule! (Not so
rigid as octett rule)
37
5. Bonding in Complexes
The 18 electron rule is helpful for organometallic compounds and
carbonyl complexes, which obey in most cases this rule:
[Cr(CO)6]
[Fe(CO)5]
[Ni(CO)4]
Cr
6CO
Fe
5CO
Ni
4CO
6 e12 e18 e-
8 e10 e18 e-
10 e8 e18 e-
38
5. Bonding in Complexes
metals with odd numbers of electrons form dimers or are reduced
or oxidized
[Mn(CO)6]+
oxidation
[Mn(CO)5][Co(CO)4]-
reduction
[Mn(CO)5]
dimerization
unknown
reduction
[Co(CO)4]
dimerization
[Mn2(CO)10]
[Co2(CO)8]
unknown
oxidation
[Co(CO)5]+
39
5. Bonding in Complexes
Similarly the formation of olefin complexes and metallocenes may
be explained by the 18 electron rule:
olefines donate 2 electrons /double bond
ethylene
2
butadiene
4
benzene
6
cyclopentadienyl radical 5
[Fe(C5H5)2]
Fe
8
2 C5H5 10
18
[Mn(CO)5C2H4]+
Mn+
6
5 CO
10
C2H4
2
18
[Cr(C6H6)2]
Cr
6
2 C6H6 12
18
40
5. Bonding in Complexes
Crystal field theory
Shape of d orbitals
Position of ligands relative to the d orbitals
41
Splitting of the d orbitals in an octahedral field of ligands
5. Bonding in Complexes
Agreement: The energy of the t2g and eg states are relative to an average value (≡ 0)
E
eg
0.6 ∆O
0
t2g
∆O
0.4 ∆O
For d4 to d7 two different possibilities of orbital occupation:
eg
t2g
Low-spin (LS) complexes
eg
t2g
High-spin (HS) complexes
42
5. Bonding in Complexes
If a complex is LS or HS depends on the amount of the splitting ∆O:
eg
t2g
Large ∆O:
⇒ Large splitting
⇒ LS
eg
small ∆O:
⇒ small splitting
⇒ HS
t2g
43
5. Bonding in Complexes
To understand the general trends for the amount of ∆ some empirical rows are established:
1. Ligands
I- < Br- < S2- < SCN- < Cl- < NO3- < F- < OH- < ox2- < H2O < NCS- < CH3CN < NH3 < en <
bipy < phen < NO2- < PPh3 < CN- < CO (bold letters show the bonding atoms)
small ∆
large ∆
= the spectrochemical row (only σ or additional π bonding)
2. Metal Ions
∆O increases with: - increasing oxidation state
- inside a group from top to bottom
⇒ Mn2+ < V2+ < Fe2+ < Ni2+ < Fe3+ < Co3+ < Mn4+ < Mo3+ < Rh3+ < Ru3+ < Pd4+ < Pt4+
Prediction: left metals + left ligands ⇒ weak field complexes (high spin)
right metals + right ligands ⇒ strong field complexes (low spin)
Halides, H2O + 3d ions ⇒ weak field complexes, NH3 mostly, CN- always strong field
complexes!
44
5. Bonding in Complexes
a)
UV/VIS absorption spectra of
three chromium(III) complexes:
a) [Cr(en)3]3+
b)
b) [Cr(ox)3]3c) [CrF6]3look for the shift of the two
c)
absorption peaks ν1 and ν2 to
lower frequencies.
45
5. Bonding in Complexes
for a t2gx egy configuration:
LFSE = (x 0.4 - y 0.6) ∆O
Table : Ligand field stabilisation energies (LFSE)
dn
example
octahedron
tetrahedron
strong field
weak field
N
LFSE
N
LFSE
N
LFSE
d0
Ca2+, Sc3+
0
0
0
0
0
0
d1
Ti3+
1
0.4
1
0.4
1
0.6
d2
V3+
2
0.8
2
0.8
2
1.2
d3
Cr3+, V2+
3
1.2
3
1.2
3
0.8
d4
Cr2+, Mn3+ 2
1.6
4
0.6
4
0.4
d5
Mn2+, Fe3+ 1
2.0
5
0
5
0
d6
Fe2+, Co3+
0
2.4
4
0.4
4
0.6
d7
Co2+
1
1.8*
3
0.8*
3
1.2
d8
Ni2+
2
1.2*
2
1.2*
2
0.8
d9
Cu2+
1
0.6*
1
0.6*
1
0.4
d10
Cu+, Zn2+
0
0
0
0
0
0
N: number of unpaired electrons
LSFE in units of ∆O or ∆T; ∆T ≈ 0.45 ∆O
* undistorted
46
5. Bonding in Complexes
octahedron
tetrahedron
distorted tetrahedron
47
5. Bonding in Complexes
tetrahedron
cube
48
5. Bonding in Complexes
tetragonal pyramid
trigonal bipyramid
49
5. Bonding in Complexes
octahedron square bipyramid
square
50
5. Bonding in Complexes
Jahn-Teller splitting
Non linear molecules which are in a degenerate electronic
state tend to lower their symmetry to remove the degenerate
state and thereby lower their energy
examples: Cr2+ d4 high spin t2g3 eg1
Co2+ d7 low spin t2g6 eg1
Cu2+ d9
t2g6 eg3
distortion of the octahedron will cause a splitting of the eg as
well as the t2g orbitals thus leading to a lower total energy
51
5. Bonding in Complexes
Jahn-Teller splitting
compressed
elongated
octahedron (along the z-axis)
52
6. Ligand substitution reactions
53
6. Ligand substitution reactions
Classification of metal ions according to their exchange rate of ligands
54
6. Ligand substitution reactions
55
6. Ligand substitution reactions
56
6. Ligand substitution reactions
Series of trans-directing ligands
57
7. Metal Organic Compounds
Hapticity η of a ligand: numbers of atoms which are bonded to a metal ion!
Table: numbers of electrons and hapticity of some metal organic ligands
58
7. Metal Organic Compounds
Metal Carbonyls
Homoleptic carbonyls are known for most of the d-elements, except for Sc, Y, Cu, Ag, Au
Homoleptic carbonyls of Pd and Pt are not very stable and exist only at low temperatures!
Carbonlys are used as educts for other metal organic compounds and for catalysis!
MO diagram of CO
Bonds of CO HOMO and LUMO and metal ion d orbitals
C
O
CO is a weak donor to metal ion and builts a σ bond
Metal electrons are delocalized due to interactions with LUMO , CO acts as π acid
⇒ “π-back donation”, CO is a very strong ligand!
59