Valence Bond Theory is unable to explain a number of facts that are summarized below.
- It offers no possibility of predicting magnetic behaviour except the number of unpaired electrons in the complex.
- Complex formation of certain metal ions is totally unsatisfactory, such as Cu2+ forms complex in a d9 species, dsp2 hybridisation is obtained by the promotion of one 3d-electron to a higher level (4d orbital). Hence this should lead to ready oxidation of Cu2+ to Cu3+ a process which occurs rarely.
- The theory does not explain why a particular structure is preferred, such as d8 ion form square planar complexes (dsp2 -hybridisation) after maximum pairing in the excited state. d8 -ions may also form tetrahedral (sp3 -hybridisation) complexes which involves no excitation.
- The theory offers no convincing explanation of causes of maximum pairing.
- In this theory too much stress has been given on the metal ion while the nature of the ligand is not properly stressed.
- This theory cannot explain reaction rate and mechanism of the reactions.
- It does not predict any distortion in symmetrical complexes whereas all the Cu(II) and Ti(III) complexes are distorted.
- It does not explain thermodynamic properties of the complexes.
- It does not attempt to explain the spectra of the complexes.
- It cannot explain the temperature dependent paramagnetism of the complexes.