A theoretical study of the effect of ansa-bridges on the energy barrier to hydrogen exchange in a series of biscyclopentadienyl tungsten trihydride cations

Abstract

Density functional calculations have been carried out on the structures and pathway for hydrogen exchange of the tungsten trihydride cations, [W(η-C₅H₅)₂H₃]⁺, I, [W{(η-C₅H₄)₂SiH₂}H₃]⁺, II, and [W{(η-C₅H₄)₂CH₂}H₃]⁺, III, as models for the characterised trihydride cations [W(η-C₅H₅)₂H₃]⁺, 1, [W{(η-C₅H₄)₂SiMe₂}H₃]⁺, 2, and [W{(η-C₅H₄)₂CMe₂}H₃]⁺, 3. The most significant difference found for the ground state structures is the increase in inter-ring angle in the order I<II<III. The barrier to pairwise exchange of the central and a lateral hydride is found to decrease in the order I>II>III. Also the ease of the in-plane bending motion decreases in the same order. For III, a dihydrogen hydride structure is found as an intermediate on the bending pathway. The exchange energy barriers for II and III of 63 and 51 kJ mol^-1 are consistent with the large temperature dependent H1H coupling constants previously reported for cations 2 and 3, which have been attributed to quantum mechanical exchange coupling. The barrier calculated for I, of 96 kJ mol^-1, lies outside the range wherein such unusual coupling is expected and 1 is reported to have a small temperature independent coupling constant. The relative ease of approach calculated for the two hydrogens is also consistent with the view that the quantum mechanical exchange is facilitated by a short tunnelling path. The underlying reasons for the differences found for the series I–III are twofold. As the inter-ring angle increases the rings become more tightly bound and the hydrides less tightly bound. The exchange pathway involves reduction of the metal from a to a configuration, and the d orbital involved becomes more stable on bending the rings.