Understanding the nature of the bonding in transition metal complexes: from Dewar's molecular orbital model to an energy partitioning analysis of the metal–ligand bond

The results of an energy partitioning analysis of three classes of transition metal complexes are discussed. They are (i) neutral and charged isoelectronic hexacarbonyls TM(CO)6q (TMq=Hf2−, Ta−, W, Re+, Os2+, Ir3+); (ii) Group-13 diyl complexes (CO)4FeER (E=B, Al, Ga, In, Tl; R=Cp, Ph), Fe(ECH3)5 and Ni(ECH3)4; (iii) complexes with cyclic π-donor ligands Fe(Cp)2 and Fe(η5-N5)2. The results show that Dewar's molecular orbital model can be recovered and that the orbital interactions can become quantitatively expressed by accurate quantum chemical calculations. However, the energy analysis goes beyond the MO model and gives a much deeper insight into the nature of the metal–ligand bonding. It addresses also the question of ionic versus covalent bonding as well as the relative importance of σ and π bonding contributions.