Lone electron pairs and stereochemistry

Abstract

THE two basic assumptions of the Nyholm–Gillespie Theory^1,2 are, first, that valence shell electron pairs repel one another, the system adopting a minimum repulsion energy configuration (condition (i)), and second, that lone-pair–lone-pair (LL) repulsions are greater than lone-pair–bonded-pair (LB) repulsions which are, in turn, greater than bonded-pair–bonded-pair (BB) repulsions (condition (ii)). To offer a full explanation of the gross stereochemistry of all octahedral and pyramidal complexes two additional axioms are necessary: first, that lone-pair–bonded pair repulsions are less than the arithmetic mean of the lone-pair–lone-pair and the bonded-pair–bonded-pair repulsions (condition (iii)) and second, that electron pair repulsions at 120° to each other are less than half the corresponding repulsions at 90° to each other (condition (iv)). The four rules together enable a rationalisation of the stereochemistry of all known octahedral and bipyramidal complexes.