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 CX(3) and CX(4), 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 that each ligand can be approximately represented as a hard sphere we have shown that each ligand can be assigned a radius that decreases in the series BeX(n), BX(n), CX(n) as the ligand charge decreases with increasing electronegativity of the central atom. In this article we present results that further extend this work to other ligands, such as Cl, CH(3) and NH(2) and to other central 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 ligand close packing (LCP) model. (C) 1999 Elsevier Science B.V. All rights reserved.