Valance Theory of coordination complexes and its limitations:

According to VBT theory, a coordination entity is formed as a result of coordinate covalent bond formation by electron pairs from ligands (Lewis bases) through overlap of appropriate atomic orbitals (usually hybrid orbitals) of the metal (Lewis acid) and ligand.

A coordination entity is composed of central atom, usually that of metal, to which is attached a surrounding array of other atoms or group of atoms, each of which is called ligand.

Hypothetical sequence of steps for the formation of a coordination entity;

Q. what is the shape you expect for the complex [Be(H₂O)₄]²⁺ ?

Here Be2+ ion can use sp3 hybrid orbitals to receive electron pairs from four water molecules. [Be(H₂O)₄]²⁺ has tetrahedral geometry .

Limitation of the VBT:

1. Fail to explain the colour & characteristics of absorption spectra of complex compounds.
2. Orbital contribution and temperature dependency on magnetic moment of coordination complex are not properly explained by VBT.
3. It is not helpful to predict the mystery of formation of outer or inner orbital coordination complex.
4. VBT fails to predict any distortion in the shapes of the coordination complexes from regular geometry.

Crystal field theory: d-orbital splitting in various ligand fields, spectral properties and magnetism of coordination complexes:

Crystal-field theory:

In the crystal-field theory , a ligand lone pair is modelled as a point negative charge (or as the partial negative charge of an electric dipole) that repels electrons in the d orbitals of the central metal ion. The theory concentrates on the resulting splitting of the d orbitals into groups with different energies, and uses that splitting to rationalize and correlate the optical spectra, thermodynamic stability, and magnetic properties of complexes.