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Crystal Field Theory

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Crystal Field Theory - Lesson Summary

The crystal field theory was proposed by Hans Bethe and VanVleck. This theory gives satisfactory explanation for the bonding and the properties of complexes than the valence bond theory.

Assumptions of Crystal field theory:

The interaction between the metal ion and the ligand is purely electrostatic.

Negative ligands are treated as point charges and neutral ligands are treated as dipoles.

The five d-orbitals in an isolated gaseous metal atom (or) ion are degenerate.

The repulsions between the electrons in the metal ion and the ligands increases the energy of the d-orbitals and lifts the degeneracy in d-orbitals. This results in the d- orbitals getting split.

Crystal field splitting in octahedral complexes:

In octahedral complexes, the metal ion is at the centre of the octahedron, and the six ligands lie at the six corners of the octahedron along the three axes X, Y and Z.

The dx2- y2 and d z square orbitals are together known as the eg set of orbitals.

The dxy, dxz and dyz orbitals are collectively known as the t2g set of orbitals.

Under the influence of the ligands, the degeneracy of the five d orbitals of the metal ion is lost and they are split into two groups of orbitals of different energies. This effect is known as crystal field splitting.

The pattern of splitting of the d orbitals depends on the number of ligands and their arrangement around the central metal atom or ion.

The magnitude of crystal field splitting depends upon the field strength of the ligand and the charge on the metal ion.

Ligands that cause only a small degree of crystal filed splitting are termed weak field ligands, while ligands that cause a large splitting are called strong field ligands.

Complexes formed with weak field ligands would be high-spin complexes.

Complexes formed with strong field ligands would be low-spin complexes.

Common ligands can be arranged in ascending order of field strength is,

I- < Br - < SCN - < Cl - < S2- < F- < OH - < C2O42- < H2O < NCS - edta4 - NH3 < en < CN- < CO

The order remains practically constant for different metals and this series is called the spectro-chemical series. It is an experimentally determined series based on the absorption of light by complexes with different ligands.

Crystal field splitting in tetrahedral complexes:

In a tetrahedral field, the crystal field splitting for the same metal, the same ligands and metal ligands is

Δt = 4/9 Δ0

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