ABSTRACT
The elementary idea of a metal-ligand bond was put forward first of all by Werner which was advanced by the various theories from time-to-time. The theories suggesting covalent bonding model are: Lewis Acid-base Theory and Valence Bond Theory. Molecular orbital theory covers all the possible aspects of bonding and has a final say. The concept of hybridisation is supported by the experimental result which states all the metal-ligand bonds are of the same length and strength. There are two types of octahedral complexes according to this theory: the inner orbital complexes and the outer orbital complexes. The valence shell configuration of the central metal ion is controlled by the ligands in complexes. The crystal field theory is based on electrostatic model but it successfully explains shapes, spectra and magnetic properties of complexes. The acceptability of molecular orbital theory to chemists has increased over the years.
