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.
(v) it assumes all bonds are localized bonds . As we will see, this is not a valid assumption.