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)₆(q) (TM(q) = Hf^2-, 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) (TM(q) = Hf^2-, 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 TM(q) 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)₆³⁺, 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, ΔE(orb), and the electrostatic attraction term, ΔE(elstat), have comparable contributions to the (CO)₅TM(q)-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)₆³⁺. The contribution of the (CO)₅TM(q)←CO σ donation continuously increases from Hf(CO)₆^2- to Ir(CO)₆³⁺ and eventually becomes the dominant orbital interaction term in the carbonyl cations, while the (CO)₅TM(q)→CO π-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 TM(q)←(CO)₆ donation shows for a single AO the order d >> s > p, but the contributions of the three p orbitals of TM(q) are larger than the contribution of the s orbital.