Limitations of CFT 1. It does not consider the formation of 7t bonding in complexes.
2. It is also unable to account satisfactorily for the relative strengths of ligands e. g., it does not explain why H2O is stronger ligand than OH-. 3. It gives no account of the partly covalent nature of metal-metal bonds.
Ligand Field or Molecular Orbital Theory
This theory was put forward by Hund and Mulliken. According to this theory, all the atomic orbitals of the atom participating in molecule formation get mixed to give rise an equivalent number of new orbitals, called the molecular orbitals. The electrons are now under the influence of all the nuclei.
Stability of Coordination Compounds
The stability of complex in solution refers to the degree of association between the two species involved in the state of equilibrium. It is expressed as stability constant( K).
The factors on which stability of the complex depends:
( i) Charge on the central metal atom As the magnitude of charge on metal atom increases, stability of the complex increases.
( ii) Nature of metal ion The stability order is 3d < 4d < 5d series.
( iii) Basic nature of ligands Strong field ligands form stable complex.
The instability constant or the dissociation constant of compounds is defined as the reciprocal of the formation or stability Constant.