Valence Bond theory of coordination compounds :

(i) The suitable number of atomic orbitals of central metal ion (s, p, d) hybridise to provide empty hybrid orbitals.

(ii) These hybrid orbitals accept lone pair of electrons from the ligands and are directed towards the ligand positions according to the geometry of the complex.

(iii) When inner d-orbitals i.e. (n-1) d orbitals are used in hybridization, the complex is called – inner orbital or spin or hyperligated complex.

(iv) A substance which do not contain any unpaired electron is not attracted by 2 magnet. It is said to be diamagnetic. On the other hand, a substance which contains one or more unpaired electrons in the electrons in the d-orbitals, is attracted by a magnetic field [exception O₂ and NO]. It is said to be paramagnetic.

Paramagnetism can be calculated by the expression, μ_s = √n(n+2), where μ magnetic moment.

s= spin only value and n= number of unpaired electrons.

Hence, if n = 1 , μ_s = √1(1+2) = 1.73 B.M., If n = 3, μ_s = √3(3+2) = 3.87 B.M. and so on

On the basis of value of magnetic moment, we can predict the number of unpaired electrons present in the complex. If we know the number of unpaired electrons in the metal complex, then it is possible to predict the geometry of the complex species.

(v) There are two types of ligands namely strong field and weak field ligands. A strong field ligand is capable of forcing the electrons of the metal atom/ion to pair up (if required). Pairing is done only to the extent which is required to cause the hybridization possible for that co-ordination number. A weak field ligand is incapable of making the electrons of the metal atom/ ion to pair up.

Strong field ligands: CN^–, CO, en, NH₃, H₂O, NO^–, Py.

Weak field ligands: I^–, Br^–, Cl^–, F^–, NO₃^–, OH^–, C₂O₄^2–, NH₃, H₂O.

Limitations of valence bond theory

The valence bond theory was fairly successful in explaining qualitatively the geometry and magnetic behaviour of the complexes. But, it could not explain the following :

(i) The origin of their absorption spectra could not be explained.

(ii) Why did different complexes of the same metal show different colours.

(iii) Relative stabilities of different complexes could not be explained.

(iv) Why should certain ligands form high spin, while others low spin complexes.