Bonding in transition metal complexes:- Valence bond theory, Crystal field theory, Molecular orbital theory

There are three theories of metal to ligand bonding in complexes.

Valence bond theory

 Coordination compounds contain complex ions, in which ligands form coordinate bonds to the metal. Thus the ligand must have a lone pair of electrons, and the metal must have an empty orbital of suitable energy available for bonding. The theory considers which atomic orbitals on the metal are used for bonding. From this the shape and the stability of the complexes are predicted. The theory has two main limitations. Most transition metal complexes are coloured, but the theory provides no explanation for their electronic spectra. Further, the theory does not explain why the magnetic properties vary with temperature. For these reasons it has largely been superseded by the crystal field theory. However it is of interest for study as it shows the continuity of the development of modern ideas from Werner’s theory.

Crystal field theory

The attraction between the central metal and ligands in the complex is considered to be purely electrostatic. Thus bonding in the complex may be ion-ion attraction (between positive and negative ions such as Co³⁺ and Cl^-).  Alternatively, ion-dipole attractions may give rise to bonding (if the ligand is a neutral molecule such as NH₃ or CO).  This theory has been remarkably successful in explaining the electronic spectra and magnetism of transition metal complexes. Particularly when allowance is made for the possibility of some covalent interaction between the orbitals on the metal and ligand. When some allowance is made for covelencey, the theory is often renamed as the ligand field theory. Three types of interaction are possible. The σ overlap of orbitals, π overlap of orbitals, or dπ – pπ bonding (back bonding) due to π overlap of full d orbitals on the metal with empty p orbitals on the ligands.

Molecular orbital theory

Both covalent and ionic contributions are fully allowed for in this theory. Though this theory is the probably the most important approach to chemical bonding, it has not displaced on the other theories. This is because the quantitative calculations involved are difficult and lengthy, involving the use of extensive computer time. Much of the qualitative description can be obtained by other approaches using symmetry and group theory.

Reference: Inorganic chemistry, J.D Lee