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 π-donor ligand, gave rise to speculation about metal-ligand binding interactions. In response to this, theoretical studies have been carried out, which focus on the nature of the chemical bond in transition metal complexes with terminal Group 13 diyl ligands ER (E = B-Tl). Some of these investigations 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, which 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 σ donation and particularly the RE ← TM π-backdonation. The analysis of the bonding situations revealed that the π-backdonation may indeed become as important as RE → TM σ donation, but the total covalent TM-ER bond order is always less than 1. The most important conclusion which arises from these theoretical studies is that the TM-ER bonding interactions are largely ionic, and that the covalent donor-acceptor interactions play a less important role. This makes the discussion whether the TM-ER bond should be considered as a single or triple bond irrelevant.