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Complex ion defined as electrically charged radical whichconsists of a central metal ion surrounded by a group of ions or

neutral moleculesEx. [ Ni(NH3)6]

++, [ Pt (NH3)4Cl2]++

Central ion and ligands The cation to which one or more neutralmolecules or anions are attached is called the central ion while

molecules or ions attached are called ligands

Ex. [ Ni(NH3)6]++, [ Fe (CN)6)]

- - -

Ligands Central metal atom coordinatebonds

Central ion ligand

Should have lone

pair of electrons

Should have vacant

orbitals

attached to

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Donor atom atom of ligand which can donate the electron pair

is called donor or coordination atom

Coordination number Total number of ligands attached to a

central ion is called the coordination number of the ion

Charge of complex ion charge carried by a complex ion is the

algebraic sum of charges carried by central ion and ligandscoordinated to it

[Cu (NH3)6]++ , [Fe (CN)6]

4-

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In coordination compound

Metal exhibits two types of

valency

Primary Secondary

ionizable non-ionizable

Corresponds to

Oxidation state coordination number Fixed for every metal

CO

NH3Cl

NH3

NH3

ClNH3

H3N

H3N

Cl

Example CoCl3

.6NH3

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Effective atomic number (EAN)

In complex formation each ligand donates a electron pair to the

central metal ion. Total number of electrons possessed by the

central metal in complex including those gained by it in bonding iscalled its EAN

Atom At. No. Complex Electron lost

in ion

formation

Electron

gained by

coordination

EAN

Cr

Fe

Co

Cu

Pd

Pt

24

26

27

29

46

78

[Cr (CO)6]

[Fe(CN)6]4-

[CO(NH3)6]+++

[Cu(CN)4]---

[Pd(NH3)6

]4+

[PtCl6]--

? ? ?

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Atom At. No. Complex Electron lost in

ion formation

Electron gained

by coordination

EAN

Cr

Fe

Co

Cu

Pd

Pt

24

26

27

29

46

78

[Cr (CO)6

]

[Fe(CN)6]4-

[CO(NH3)6]+++

[Cu(CN)4]---

[Pd(NH3)6]4+

[PtCl6]--

0

2

3

1

4

4

12

12

12

8

12

12

36

36

36

36

54

86

Valence Bond Theory Developed by Pauling

Coordination compounds contain complex ions in which ligands

from coordinate bonds to the metal.

So, ligand must have lone pair of electrons and metal must have

empty orbital of suitable energy available for bonding.

According to the atomic orbital of the metal used for bonding, shape

and stability of the complex is predicted.

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Crystal field theory

In an isolated metal ion, all the five d orbitals degenerate

On approach of ligand, d orbital electrons will be repelled by

ligands lone pair

Repulsion raises the energy of d orbitals

If field produced by the ligands is spherically symmetrical (allthe ligands are an equal distance from each of the d orbitals),

energy of the d orbital will be raised but they still remain

degenerated hypothetical situation

But, field produced by the ligands is not spherically symmetricalbecause d orbitals differ in their orientations

Energy of the orbitals lying directly in ligands direction will be

raised more than the orbitals lying in between the ligands

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Crystal field theory

So, d orbital splits into two sets of different energies

This crystal field splitting forms the basis of Crystal Field Theory

Relative energies of d orbital depends on the number of ligands

and their arrangement around central metal ion

So crystal field splitting will be different in different structureswith different coordination number

Electrons of the metal ion are repelled by negative field of

electrons, therefore, the metal electrons will occupy those d

orbitals which have their lobes farthest away from the directionof ligand

Complexes with coordination number 6 and 4 are more common

giving octahedral, tetrahedral and square-planar complexes

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+0.6 (O

-0.4 (O

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Crystal

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The d-d transition is the single broad peak with a maximum at20,300 cm-1

1 kJmol-1 = 83.7 cm-1

(o

for [Ti(H2O)6]3+ is 20,300 / 83.7 = 243 kJmol-1

Most convenient way for measuring (o values

Single d electron occupies an energy level 2/5 (obelow the

average energy level

CFSE for this complex = 2/5 x 243 = 97 kJmol-1

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Magnitude of CFS depends on 3 factors

Nature of ligand

Charge on the metal ion Whether the metal is in 1st, 2nd and 3rd row of transition

elements

Magnitude of increases as the charge on the metal ionincreases

M+++ complexes have greater than M++

(o

(o

Oxidation state V Cr Mn Fe Co

(+II) electronic configuration

in cm-1d3

12600

d4

13900

d5

7800

d6

10400

d7

9300

(+III) electronic configuration

in cm-1d2

18900

d3

17830

d4

21000

d5

13700

d6

18600

(o

(o

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Value of CFS increases by about 30% between adjacentmembers down a group of transition elements

Crystal field splitting in one group

values range 7000 30000 cm-1

Nature of Ligand CFS by various ligands

Complex cm-1 kJmol-1

[Co(NH3)6]3+ 24800 296

[Rh(NH3)6]3+ 34000 406

[Ir(NH3)6]

3+

41000 490

Complex cm-1 kJmol-1

[CrCl6]3- 13640 163

[Cr(H2O)6]3+ 17830 213

[Cr(NH3)6]3+ 21680 259

[Cr(CN)6]3-

26280 314

(o

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