Improving our understanding of molecular geometry and the VSEPR model through the ligand close-packing model and the analysis of electron density distributions

Considerable improvements in our understanding of molecular geometry have been made in recent years as a result of the recognition in the ligand close-packing (LCP) model of the importance of ligand-ligand repulsions and the information gained by the study of electron density distributions. These improvements have enabled several aspects of the VSEPR model to be better understood and explanations of exceptions to the model to be proposed. The LCP model shows that particularly for the small atoms of the period 2 elements, ligand-ligand interactions are very important in determining geometry. For example LCP is the reason for the nonVSEPR octahedral geometry of molecules such as SeCl62-. Atomic charges determined from electron density distributions affect the size of a ligand atom which can therefore change from molecule to molecule and allowance for this must be made in applying the LCP model. Electron density distributions also enable us to find the shape of a ligand which is such that in B(OH)(4)(-), for example, the bond angles at boron deviate considerably from the tetrahedral angles. (C) 2000 Elsevier Science S.A. All rights reserved.