The importance of ligand–ligand interactions for molecular geometry and the ligand close-packing model

It is shown that repulsions between vicinal ligands and groups can be of considerable importance in determining molecular geometry, particularly for small central atoms. The importance of such repulsions was first proposed in the 1960s for molecules with a central carbon atom but has much more recently also been shown to be the case for molecules with other small central atoms. Indeed for such molecules the ligands may be considered to be close-packed around the central atom and from the constant ligand–ligand distances in these molecules a ligand radius for the atom bonded to the central atom may be deduced. These radii decrease across the periodic table as the charge on the ligand decreases with increasing electronegativity of the central atom. It is shown that most of the exceptions to the VSEPR model for molecules with non-metal central atoms can be explained if ligand–ligand repulsions, which are not explicitly considered in the VSEPR model, are taken into account. For example, the VSEPR model predicts that the bond angle in PH 3 would be larger than in NH 3 , whereas it is in fact considerably smaller, which is entirely consistent with ligand close packing (LCP) and the small size of the hydrogen ligand. Indeed, the ligand radius enables bond angles to be predicted quantitatively if the bond length is known, whereas the VSEPR model can only make qualitative predictions. It has long been recognized that steric effects between large nearby groups, in particular geminal groups, can be of importance in determining molecular geometry and reaction rates and mechanisms. However, the effect of steric interactions between vicinal atoms or groups has not been so widely recognized. Several authors have maintained that such interactions may be of considerable importance in determining molecular geometry, although this topic has generally only been discussed in terms of the valence bond theory or the VSEPR model. The purpose of this paper is to review previous relevant work and to review and extend our recent work, which provides strong evidence for the importance of the interaction between vicinal ligands (ligand–ligand repulsion) in determining molecular geometry. This evidence led to the development of the LCP model. .

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