Bent's Rule - Exceptions

Exceptions

According to Bent’s rule, “Atomic s character concentrates in orbitals directed towards electropositive substituents”. However, this general statement is only true for main group elements. For the main group elements, atomic p-orbitals are directed towards more electronegative substituents. This can be rationalized by the fact that the decreased angles of bonds with more p character coincide with the decreased steric demands of more electronegative atoms. Also, electron density can be more easily withdrawn (by electronegative substituents) from higher lying p-orbitals than from s-orbitals.

Transition metal complexes are the exception to Bent’s rule. It has been experimentally determined that the group 4 transition metal compounds of Ti-Hf do not rigorously follow Bent’s rule. With these complexes, the more electronegative substituents have larger bond angles (indicating greater s character), which goes against Bent’s rule. This can be explained by the fact that with transition metals, the energy levels of the d-orbitals are generally below the energy of the s-orbitals, thus the more electronegative substituents will be attracted to the higher lying s-orbitals. Transition metal bonds are essentially sdx hybridized, with very little contribution from the p-orbitals.

A generalized form of Bent’s rule can be stated as follows: “The energetically lower lying valence orbital concentrates in bonds directed toward electropositive substituents.” This satisfies both main group and transition metal complexes.