Polarization and Hybridization

Abstract

Two types of polarization that can occur by bond formation are: (i) the result of concentration of charge due to the overlap of atomic orbitals; and (ii) the actual polarization of the basis orbitals. Mulliken¹ has described this second type of polarization and its effect as a function of the overlap integral for certain diatomic molecules. In all cases it was observed that the addition of a given amount of a 2p Slater type orbital (STO) into a hybrid orbital produced the largest change in the overlap if the hybrid orbital was a 2s STO. This observation led to the well-known phrase “A little hybridization goes a long way”. The lowering of the energy of the system due to the increase in the overlap integral must be larger than the increase in energy incurred by the inclusion of the higher energy 2p STO if the polarization is to be useful. We are inclined to reserve the word ‘polarization’ for this type of hybridization for the following reasons: (a) the process described displaces the centroid of the electronic charge distribution from the nucleus; (b) this concept can be readily extended to polyatomic molecules and is not dependent on the ‘localized bond’ approximation (this will prove particularly useful for molecules containing lone-pair electrons); (c) it distinguishes this type of hybridization from the hybrid orbitals obtained from molecular orbitals by a unitary transformation.