Cisplatin is a square planar coordination complex (cis- [Pt (NH₃)₂Cl₂]), in which two similar ligands are in adjacent positions.

The stability of coordination complex is measured in terms of its stability constant (β). Higher the value of stability constant for a complex ion, greater is the stability of the complex ion.

(i) In metal carbonyls, the bond between metal atom and the carbonyl ligand consists of two components.
(ii) The first component is an electron pair donation from the carbon atom of carbonyl ligand into a vacant d-orbital of central metal atom.
(iii) This electron pair donation forms M \(\overset { \sigma \quad bond }{ \longleftarrow } \) CO sigma bond. This sigma bond formation increases the electron density in metal d orbitals and makes the metal electron rich.
(iv) In order to compensate for this increased electron density, a filled metal d-orbital interacts with the empty \(\pi \) * orbital on the carbonyl ligand and transfers the added electron density back to the ligand.

Even though VBT explains many of the observed properties of complexes, it still has following limitations.
(i) It does not explain the colour of the complex.
(ii) It considers only the spin only magnetic moments and does not consider the other components of magnetic moments.
(iii) It does not provide a quantitative explanation as to why certain complexes are inner orbital complexes and the: others are outer orbital complexes for the same metal. For example, [Fe(CN)₆]^4- is diamagnetic (low spin) whereas [FeF₆]^4-is paramagnetic (high spin).