Theoretical study of tungsten carbonyl complexes W(CO)_n⁺ (n = 1-6): Structures, binding energies, and implications for gas phase reactivities

The electronic structure and geometry of W(CO)_n⁺ (n = 1-6) have been studied at the B3LYP and ab initio levels. We find that the ground state of W(CO)_n⁺ is linear with a sextet spin state, that a linear sextet and a bent quartet are nearly degenerate for W(CO)₂⁺, and that doublet states are unambiguously the ground states of W(CO)₃⁺ to W(CO)₆⁺. Successive (CO)_n-1W⁺-CO binding energies have been computed to be larger than any of those previously determined for other transition metals. We compare our results with available experimental data. Electron transfers are very important: (i) σ donation from the CO's to the metal is found to be more favorable when involving 5d rather than 6p leading to a preference for the bent rather than linear structures for W(CO)_n⁺ (n = 2-4); (ii) π back-donation plays a crucial role in shaping these molecules. These effects provide the driving force for the spin changes as the number of ligands increases. Spin lowering is associated with an increasing number of doubly rather than singly occupied 5d_π metal orbitals, which enhances the back-donation ability while reducing the repulsion between σ metal electrons and CO lone pairs. On the basis of our results, we propose an interpretation of the observed differences in gas phase reactivity of W(CO)_n⁺ with small hydrocarbons as a function of n. The rationale for this interpretation is that the initially formed (CO)_nW⁺-(hydrocarbon) complex should either have a ground or a low-lying excited state bearing at least two unpaired electrons on the metal to be able to further activate the hydrocarbon efficiently.