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UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 1

UNIT IV

THEORIES OF BONDING IN COMPLEXES AND CRYSTAL FIELD THEORY

VALENCE BOND (VB) THEORY:

Salient features of the VBT theory: The central metal atom (or) ion has the required number

of vacant orbitals for accommodating the electrons donated by the ligands. The number of

vacant orbitals is equal to the coordination number of the metal ion for a particular complex.

Vacant orbital s, p, d, f orbital. This vacant orbital goes hybridization to form same no. of

hybrid orbitals. Each ligand has at least one orbital containing lone pair of electrons. Vacant

hybrid orbital filled with ligand to form coordination bond. Coordinate bond is stronger if the

overlapping between the orbitals is greater.

The vacant orbitals of the metal atom (or) ion undergo suitable hybridisation to yield a set of

equivalent hybrid orbitals of definite geometry. Geometrical shape depends upon the

hybridization of the metal orbital. If (n-1) d orbitals are used for hybridization, the complexes

is called inner complexes and ‘nd’ called outer complexes.

Structure of complex compounds based on VBT 1. Structure of nickel tetracarbonyl

[Ni(CO)4] 2. Formation of [NiCl4]2-

3. Structure of [Ni(CN)4]2-

4. Structure of [CoF6]3-

5.

Structure of [Co(NH3)6]3+

GEOMETRY OF COMPLEX COMPOUND:

UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 2

OCTAHETRAL COMPLEX ION

SQUARE PLANAR COMPLEX ION

UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 3

TETRAHEDRAL COMPLEX ION ION

Drawbacks of the valence bond theory: a) Does not explain the colour of coordination

compound. b) cannot explain magnetic behavior based on geometry. c) does not explain why

some called inner and outer complex same metal ion in the same oxidation state. d) Fails to

predict the exact geometry of the complexes with the coordination number four. e) Doesnot

distinguish weak field and strong field of ligand. f) It cannot predict exactly the tetrahedral

and square planar structure of 4-cordinate.

CRYSTAL FIELD THEORY

Silent features of CFT: Central metal atom or ion is surrounded by various ligands which are

either negative charge or neutral molecule. Electrostatic interaction between ML. Eg. Fe- and

Co3+.

Central metal atom have 5 degenerated orbital dxy, dyz, dxz, d(x2-y2), dz2. The ligands

approach the metal ion, due to repulsion forces, the degeneracy of d- orbitals is destroyed and

they split into two groups of different energy level t2g and eg orbital. This effect is called

crystal field splitting(∆0 or10Dq). Due to repulsion, the orbitals along the axes of ligands

acquire higher energy while those lying in between the ligands acquire less energy. • It doent

show the overlapping. • From the Crystal field stability energy the stability of the complexes

can be known.

UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 4

Definition: Crystal field splitting- Splitting of 5 degenerated d-orbital. Crystal filed

stabilization energy (CFSE) - Change in energy achieved by filling up electron in orbital in

complex metal atom. High spin complex-(spin free) Greater no. of unpaired electrons and

hence higher value of resultant spin and magnetic moment is called. • Low spin complex •

Pairing energy- The energy required to pair 2 electron against the electron electron repulsion

in the same orbital of a metal atom.

Application of CFT to tetrahedral complexes • In the tetrahedral complex, [MX4]n, the metal

atom or ion is placed at centre of the regular tetrahedron and the 4 ligands, are placed at four

corners of the tetrahedron. • Ligand approach the central Metal atom in between 3 cordinate

x, y, z.

In case of strong field ligands, the electrons prefer to pair up in eg orbital giving low spin

complexes while in case of weak field ligands, the electrons prefer to enter higher energy t2g

orbitals giving more unpaird electrons and hence form high spin complexes.

Application of Crystal field theory to octahedral complexes [MX6]n the metal atom or ion is

placed at the centre of regular octahedron while 6 ligands Occupy the positions at 6 vertices

of the octahedron. • Two orbital dx2-y2 and dz2 are Axial greater repulsion and dxy, dyz, dxz less

repulsion.

UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 5

Five d orbital lose degenracy and split into two point group. The group t2g lower

energy while eg group have higher energy. Experimental calculation show that the energy of

t2g orbital is lowered by 0.4∆0 or 4Dq and energy of eg orbital is increased by 0.6∆0 or 6Dq.

Thus energy difference between t2g and eg orbitals is ∆0 or 10Dq which is crystal field

splitting energy. CFSE increases with the increasing strength of ligands and oxidation state of

central metal ion.

Limitation of crystal field theory: Does not explain the s and p orbital. Does not explain bi-

bonding. Cannot explain partly covalent nature of the metal ligand bond.

Spectrochemical series: The decreasing order of field strength of some of the ligands is, CO >

CN > NH3 > EDTA > H2O > OH- > F- > S2- > Cl- This series depends on the power of

splitting the d orbitals and is called spectrochemical series.