The valence bond theory has the following limitations:
- The valence bond theory is unable to explain the magnetic moment data of some of the complexes.
- The valence bond theory does not explain the color of transition metal ions, absorption spectra, and heats of formation, etc.
- In the metal ions which involve dsp2 hybridization, the reason for the non-involvement of third p orbital to give dsp3hybrid orbitals cannot be explained on the basis of valence bond theory.
- One of the major drawbacks in valence bond theory is that it does not explain the bonding in Cu2+ complexes which are supposed to utilize dsp2 hybrid orbitals. But Cu+2 is a d9 ion and the complex derived from it e.g., [Cu(NH3)4]SO4 has square planer structure according to valence bond theory. It is clear from the following representation of electronic arrangements that one electron from 3d orbital must be promoted to higher energy orbital (4p) and should result in an unstable complex. The electron in 4p orbital should be easily given out to give Cu2+ ions which is against the facts e.g., Cu3+ ions are not formed and Cu2+ complexes are stable. This is a serious drawback in valence bond theory.
Due to the serious drawbacks in valence bond theory as applied to coordination compounds, other theories were looked for their structural elucidation.
Stability of complexes is depicted by 18 electron rule. According to 18 electron rule, the atomic number of transition metal – 18e + no. of electrons donated by ligands = 18. The complexes which obey this rule are more stable.