Ligand close packing and the geometries of A(XY) 4 and some related molecules

Abstract

Molecules of the type A(OX) 4, A(NX 2) 4 and A(CY 2X) 4 are not regularly tetrahedral about the central atom A. Two of the angles about this central atom are smaller than tetrahedral and four are larger, or four are smaller and two larger. By considering that the ligand atom (O, N or C) that is bonded to the central atom A has three intramolecular ligand radii and minimising the number of ligand–ligand contact distances, as described in the theory of ligand close packing, we are able to account for the S 4 and D 2d geometries observed for C(OCH 3) 4, C(OPh) 4 and CEE 4 and related molecules. The ligand close packing model also rationalises the C 2 geometries of SO 2(OR) 2 molecules and the differences in O–C–R angles and C–R bond lengths in R 3COX molecules. The lengths of these interligand radii can be determined either by calculating the molecular geometry or by deriving them from experimental geometries. The radii depend on the charge of the ligand, the sizes of the Y and Z groups and the angle A–X–Z. The relative sizes of these radii determine the preference for D 2d or S 4 geometries and the degree of distortion of the bond angles from the ideal tetrahedral angle.