Werner was the first to describe the bonding features in coordination compounds. But his theory could not answer basic questions like :
(i) Why only certain elements possess the remarkable property of forming coordination compounds ?
(ii) Why the bonds in coordination compounds have directional properties?
(iii) Why coordination compounds have characteristics magnetic and optical properties ?
Many approaches have been put forth to explain the nature of bonding in coordination compounds viz. Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT).
We shall focus our attention on elementary treatment of the application of VBT and CFT to coordination compounds.
According to this theory, the metal atom or ion under the influence of ligands can use its ( \(n -1) d , ns np\) or ns, np, nd orbitals for hybridisation
to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar and so on.
These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.
\(CoCl _3 \cdot 6 NH _3\) gives 3 moles of AgCl on reacting with \(AgNO _3\).
The correct formula for this compound is :

1. A \(\left[ Co \left( NH _3\right)_6\right] Cl _3\)
2. B \(\left[ CoCl \left( NH _3\right)_5\right] Cl _2 \cdot NH _3\)
3. C \(\left[ CoCl _2\left( NH _3\right)_4\right] Cl \cdot 2 NH _3\)
4. D \(\left[ CoCl _3\left( NH _3\right)_3\right] \cdot 3 NH _3\)