This is a introductory presentation for the beginners who wish to understand the basics of coordination chemistry.

1. Dr. BHARTENDU K SRIVASTAVA
1
Introduction to Coordination Chemistry
Coordination
Chemistry
Chemistry of
Coordination
compounds
Chemistry of complex
ions
NaCl
H2O
[Ni(H2O)6]2+NiCl2
H2O
+ 2Cl-
NH3
[Ni(NH3)6]2+
+ 2Cl-

2. Alfred Werner (Nobel Prize, 1913) developed a model of coordination complexes which
explains the following observations.
History: Werner's theory of Coordination complexes
CoCl2
NH3
CoCl3.6NH3
Air
CoCl3.5NH3
CoCl3.4NH3
CoCl3.3NH3
+2 +3
CoCl3.6NH3
aq. HCl
Reactivity of ammonia got reduced
CoCl3.6NH3
CoCl3.5NH3
CoCl3.4NH3
CoCl3.3NH3
aq. Ag+
3 AgCl
2 AgCl
1 AgCl
0 AgCl
Conductivity
in aq. solution
4 ions
3 ions
2 ions
0 ions

3. Central metal in the coordination complex exhibits two types of valencies, primary
valency (oxidation state) is ionisable and non directional, secondary valency
(coordination number) is non ionisable and directional (directed towards fixed position
in space), every metal has a fixed number
Central metal tends to satisfy both primary valencies and secondary valencies
[Co(NH3)6]Cl3
[Co(NH3)5Cl]Cl2
[Co(NH3)4Cl2]Cl
[Co(NH3)3Cl3]
[Co(NH3)6]3+ + 3Cl-
[Co(NH3)5Cl]2+ + 2Cl-
[Co(NH3)4Cl2]+ + Cl-
[Co(NH3)3Cl3] + 0Cl- (neutral)
[Co(NH3)6]3+
Coordination
sphere Coordination
number
Central metal/Oxidation state
Ligand
3+

4. Composition of complex entity
Central metal ion Ligands
Must have vacant d-orbitals
(mostly d-block transition metals)
Must have lone pair of electrons
on the donor atom
Monodentate (halides, ammines etc.)
Chelating Ligands
Bidentate (oxalate, ethylenediamine etc.)
Polydentate (dien, EDTA etc.)
Coordination number:
Coordination number decides
the geometry of the of the complexes
C.N. = 2, Linear Geometry
C.N. = 4, Tetrahedral, Square planar geometry
C.N. = 6, Octahedral Geometry
[M(L)n]3+

5. Nomenclature
K4[Fe(CN)6] Potassium hexacyanoferrate(II)
[Co(NH3)6]Cl3 Hexaamminecobalt(III) chloride
[Pt(NH3)2Cl4] Diamminetetrachloroplatinum(IV)
[Pt(NH3)4][PtCl4] Tetraammineplatinum(II)tetrachloroplatinate(II)
[Pt(NH3)2Cl4] (Neutral)
K4[Fe(CN)6] 4K+ + [Fe(CN)6]4-
(Anionic)
[Co(NH3)6]Cl3 [Co(NH3)6]3+ + 3Cl- (Cationic)
[Pt(NH3)4][PtCl4] [Pt(NH3)4]2+ + [PtCl4]2-

6. Isomerism
2. Stereoisomerism
a) Geometrical isomerism
1. Structural Isomerism
a) Coordination isomerism
[Co(NH3)6] [Cr(C2O4)3]
[Cr(NH3)6] [Co(C2O4)3]
b) Linkage isomerism
[Co(ONO)(NH3)5]Cl
[Co(NO2)(NH3)5]Cl
c) Ionisation isomerism
[PtBr(NH3)3]NO2
[Pt(NO2)(NH3)3]Br
d) Hydrate isomerism
[CrCl2(H2O)4]Cl.2H2O bright-green
[CrCl(H2O)5]Cl2.H2O grey-green
Square Planar
Tetrahedral
Octahedral
Ma2b2 / Ma2bc/ M(ab)2 / Mabcd
Ma2b4 / Ma4bc/ Ma3b3 / M(aa)2b2/M(aa)2bc/ Mabcdef
[Pt(NH3)2Cl2]/Square Planar
Cis trans
Mabcd/ 3 isomers

7. Ma2b4 / Ma4bc/ Ma3b3 / M(aa)2b2/M(aa)2bc/ Mabcdef
Ma2b4/[CrCl2(NH3)4]+
Cis/ violet trans/ green
Ma3b3/[RhCl3py3]
Cis form Trans form
[Co(NH3)3(Cl)3]/Octahedral
facial meridional

8. b) Optical isomerism
Square Planar
Tetrahedral
Octahedral
M(aa)3 / M(aa)2b2 /M(aa)2bc/ M(aa)b2c2/ Mabcdef
two optically active isomeric form of the complex [Co(en)3]3+
Coordination complexes which can rotate plane of polarised light are optically
active complexes. The essential thing is to not have a plane of symmetry.

9. Bonding in Coordination Complexes: Valence Bond Theory
[Co(NH3)6]3+ C.N. = 6; O.S. = +3 (Diamagnetic)
27Co = [Ar] 3d7 4s2
Co3+ = [Ar] 3d6 4s0
Co(III) ion
6 empty orbitals
for coordination
number 6
3d 4s 4p
6 d2sp3 (equal energy)
NH3 NH3 NH3 NH3 NH3 NH3
3+
Low Spin
Complex

10. [CoF6]3- C.N. = 6; O.S. = +3(Paramagnetic)
27Co = [Ar] 3d7 4s2
Co3+ = [Ar] 3d6 4s0
Co(III) ion
3d 4s 4p
6 sp3d2 (equal energy)3-
Valence Bond Theory: Shortcomings
F- F- F- F- F- F-
High Spin Complex
4d

11. Bonding in Coordination Complexes: Crystal Field Theory
Bonding between a central metal ion and its ligand arises from purely
electrostatic interactions
𝐵. 𝐸. = −
𝑞1 𝑞2
𝑟

12. 6Dq
4Dq
∆ 𝑜 = 10Dq
eg; E = 6 Dq
t2g; E = -4 Dq
ENERGY
State I
State II
State III
Crystal Field
Splitting
Energy(∆ 𝑜)
6Dq
4Dq
eg; E = 6 Dq
t2g; E = -4 Dq
ENERGY
State I
State II
State III
Crystal Field
Stabilization
Energy = 4Dq∆ 𝑜 = 10Dq
Crystal Field Splitting in octahedral complexes

13. 6Dq
4Dq
eg; E = 6 Dq
t2g; E = -4 Dq
ENERGY
State I
State II
State III
Crystal Field
Stabilization
Energy = 8Dq
6Dq
4Dq
eg; E = 6 Dq
t2g; E = -4 Dq
ENERGY
State I
State II
State III
Crystal Field
Stabilization
Energy = 12Dq
∆ 𝑜 = 10Dq
∆ 𝑜 = 10Dq

14. 6Dq
4Dq
eg; E = 6 Dq
t2g; E = -4 Dq
ENERGY
State I
State II
State III
Crystal Field
Stabilization
Energy = 16Dq
6Dq
4Dq
eg; E = 6 Dq
t2g; E = -4 Dq
ENERGY
State I
State II
State III
Crystal Field
Stabilization
Energy = 6Dq
∆ 𝑜 = 10Dq
∆ 𝑜 = 10Dq
∆ 𝒐 > P OR ∆ 𝒐 < PConcept of pairing energy
∆ 𝒐 > P; Low
spin complex/
Strong field
complex
∆ 𝒐 < P; High
spin complex/
Weak field
complex

15. eg
t2g
ENERGY
State I
State II
State III
eg
t2g
ENERGY
State I
State II
State III
Crystal Field
Splitting
energy∆ 𝑜 (F)
∆ 𝒐 > P; Low
spin complex/
Strong field
complex
∆ 𝒐 < P; High
spin complex/
Weak field
complex
[CoF6]3- C.N. = 6; O.S. = +3(Paramagnetic)
[Co(NH3)6]3+ C.N. = 6; O.S. = +3 (Diamagnetic)
∆ 𝑜 (NH3) > ∆ 𝑜 (F)
∆ 𝑜 (NH3)

16. 4Dq
6Dq
t2; E = 4 Dq
e; E = -6 Dq
ENERGY
State I
State II
State III
∆𝑡 = −
4
9
∆o
4Dq
6Dq
t2; E = 4 Dq
e; E = -6 Dq
ENERGY
State I
State II
State III
Crystal Field
Stabilization
Energy = 6Dq
Crystal Field Splitting in Tetrahedral complexes

17. Factors affecting the magnitude of crystal field splitting:
Nature of Ligands: large negative charge, small size, good sigma
donor and pi acceptors
Oxidation state of the metal: higher for higher oxidation state
Size of d orbitals: 5d > 4d > 3d
Geometry of the complex: Crystal field splitting energy in
octahedral
Complexes will always be more than the tetrahedral complexes

18. Take home message
• What kind of metals are involved in coordination complexes?
• What kind of Ligands are involved in coordination complexes?
• How to do naming of coordination compounds and do they possess isomerism?
Nomenclature
Isomerism
• What kind of bonding is involved? theories and explanation
VBT (Valence Bond Theory)
CFT (Crystal Field Theory)
• Therapeutic importance of coordination compounds

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