The nature of the transition metal-carbonyl bond and the question about the valence orbitals of transition metals.: A bond-energy decomposition analysis of TM(CO)6q (TMq = Hf2-_, Ta-, W, Re+, Os2+, Ir3+)

The equilibrium geometries and bond-dissociation energies for loss of one CO and loss of six CO from TM(CO)(6)(q) (TMq = Hf2-, Ta-, W, Re+, Os2+, Ir3+) have been calculated at the BP86 level using Slater type basis sets. The bonding interactions between TM(CO)(5) and one CO and between TMq in the t(2g)(6) valence state and the ligand cage (CO)(6) were analyzed in the framework of Kohn-Sham MO theory with the use of the quantitative ETS energy-partitioning scheme. The BDEs exhibit a U-shaped curve from Hf(CO)(6)(2-) to Ir(CO)(6)(3+), with W(CO)(6) having the lowest BDE for loss of one CO while Re(CO)(6)(+) has the lowest BDE for loss of 6 CO. The stabilizing orbital interaction term, Delta E-orb, and the electrostatic attraction term, Delta E-elstat, have comparable contributions to the (CO)(5)TMq-CO bond strength. The largest orbital contributions relative to the electrostatic attraction are found for the highest charged complexes, Hf(CO)(6)(2-) and Ir(CO)(6)(3+). The contribution of the (CO)(5)TMq<--CO sigma donation continuously increases from Hf(CO)(6)(2-) to Ir(CO)(6)(3+) and eventually becomes the dominant orbital interaction term in the carbonyl cations, while the (CO)(5)TMq-->CO pi-back-donation decreases in the same direction. The breakdown of the contributions of the d, s, and p valence orbitals of the metals to the energy and charge terms of the TMq<--(CO)(6) donation shows for a single AO the order d much greater than s > p, but the contributions of the three p orbitals of TMq are larger than the contribution of the s orbital.