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 .