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

The results of an energy partitioning analysis of three classes of transiti on metal complexes are discussed. They are (i) neutral and charged isoelect ronic hexacarbonyls TM(CO)(6)(q) (TMq = Hf2-, Ta-, W, Re+, Os2+, Ir3+); (ii ) Group-13 diyl complexes (CO)(4)Fe-ER (E = B, Al, Ga, In, Tl; R = Cp, Ph), Fe(ECH3)(5) and Ni(ECH3)(4); (iii) complexes with cyclic pi -donor ligands Fe(Cp)(2) and Fe(eta (5) -N-5)(2). The results show that Dewar's molecular orbital model can be recovered and that the orbital interactions can becom e quantitatively expressed by accurate quantum chemical calculations. Howev er, the energy analysis goes beyond the MO model and gives a much deeper in sight into the nature of the metal-ligand bonding. It addresses also the qu estion of ionic versus covalent bonding as well as the relative importance of sigma and pi bonding contributions. (C) 2001 Elsevier Science B.V. All r ights reserved.