Limitations of valence bond theory.

Though valence   bond theory has many importance it can explain  structure  magnetic properties and 

bonding of some complex compound  but it has some limitations these are

1. It can not explain why      

Co2+( d7 system)    ions form both tetrahedral, octahedral compounds but  Ni2+ (d8 system) hardly form tetrahedral compounds. 

2. It cant explain   why Cu2+ ions form only distorted octahedral   compounds  though the ligands are identical. 

3.  This can not account for the relative rate of reaction for the analogous metal complexes, and mechanism too.

4.  The classification of complexes in the basis of  their magnetic behaviour  into covalent and ionic compounds is not satisfactory and is misleading sometimes.  

 

5.  VBT     fail   to explain  the magnetic properties like magnitude of orbital contribution to magnetic moments.  

6.   It cant interpret the ( color) spectra  of the complexes. 

7.  It cannt exaplain the magnetic properties explain only the unpaired orbitals.