The importance of ligand-ligand interactions in determining molecular geometry: the ligand close packing model

In 1960 Bartell proposed that interligand repulsions in some simple organic molecules of the types CX3 and CX4, are much more important in determining molecular geometry than had previously been generally supposed. In recent work we have shown that this concept can be extended to analogous molecules of beryllium and boron where X is F, OH, or O. Assuming as Bartell did tha t each ligand can be approximately represented as a hard sphere we have sho wn that each ligand can be assigned a radius that decreases in the series B eXn, BXn, CXn as the ligand charge decreases with increasing electronegativ ity of the central atom. In this article we present results that further ex tend this work to other ligands, such as Cl, CH3 and NH2 and to other centr al atoms, in particular nitrogen and oxygen which, unlike Be, B and C, have lone pairs in their valence shells. Bartell's original suggestion has now been developed into a useful and widely applicable model that we call the l igand close packing (LCP) model. (C) 1999 Elsevier Science B.V. All rights reserved.