Molecules with hydride or alkyl ligands and including d(0) transition metal centers: problem cases for the simple VSEPR model

Application of the simple VSEPR model to hydride and alkyl derivatives of M ain Group and transition elements is considered, and successes and failures of the model are outlined. A survey is presented of the difficulties inher ent in the precise location of hydrogen atoms in molecules, and of techniqu es developed to surmount these problems. Consideration is given to a select ion of Main Group and d(0) transition metal molecules which do not comply w ith the expectations of the model. These include halides such as MF2 (M = C a, Sr or Ba), permethyl complexes like WMe6, and mixed-ligand species such as Me2TiCl2 and Me3ReO2. Theoretical attempts to account for this behavior, including extensions to the simple VSEPR model, are described. The phenome non of 'agostic' M ... H interaction is considered in the context of d(0) t ransition metal complexes. Whilst rationalization of the phenomenon is beyo nd the remit of the VSEPR approach, a molecular orbital model is shown to g ive more insight than does an earlier valence-bond approach in explaining t he salient features of the interaction. (C) 2000 Elsevier Science S.A. All rights reserved.