Chemical bonding in mononuclear transition metal complexes with Group 13 diyl ligands ER (E = B-Tl) Part X: Theoretical studies of inorganic compounds

Recent advances in the synthesis of stable transition metal complexes with terminal BR, AlR, GaR and InR ligands where R is mostly but not always a pi -donor ligand, gave rise to speculation about metal-ligand binding interact ions. In response to this, theoretical studies have been carried out, which focus on the nature of the chemical bond in transition metal complexes wit h terminal Group 13 diyl ligands ER (E = B-TI). Some of these investigation s were made in cooperation with experimental work which reported new stable complexes. The theoretical work made predictions about the geometries and bond energies of the complexes. The bonding situation of the TM-ER bonds, w hich has been controversial in the literature, has also been examined. The review summarizes the results and the progress in the understanding of the nature of the TM-ER bonds which has been gained in recent theoretical work. Previous discussions focused on the importance of RE-->TM a donation and p articularly the RE<--TM pi-backdonation. The analysis of the bonding situat ions revealed that the pi-backdonation may indeed become as important as RE -->TM a donation, but the total covalent TIW-ER bond order is always less t han 1. The most important conclusion which arises from these theoretical st udies is that the TM-ER bonding interactions are largely ionic, and that th e covalent donor-acceptor interactions play a less important role. This mak es the discussion whether the TM-ER bond should be considered as a single o r triple bond irrelevant. (C) 2000 Elsevier Science S.A. All rights reserve d.