Where are the hydrogen atoms in [(η⁵-C₅H ₅)(PH₃)₂W(H₂SiMe₂)] ⁺? A theoretical study

Quantum-chemical DFT calculations of the tungsten silylene complex [Cp(PH₃)₂W(H₂-SiMe₂)]⁺ have been carried out with the aim to elucidate the structure and bonding situation of the molecule. The W-SiMe₂ interactions have been examined with an energy decomposition analysis. The geometry optimization with the constraint of C_s symmetry gives a classical dihydride structure as the lowest-lying energy minimum form. Four other structures which have up to two bridging hydrogen atoms are only <5 kcal/mol higher in energy than the classical form. The results suggest that the model complex [Cp(PH ₃)₂W(H₂SiMe₂)]⁺ and the real complex [Cp*(dmpe)W(H)₂SiMe₂][B(C ₆F₅)₄], which was synthesized by Tilley, have a very fluxional W(H₂SiMe₂) moiety, which makes it meaningless to classify the structure as classical or nonclassical. The energy decomposition analysis indicates that, in all binding modes, W→SiMe ₂ π-back-donation is very weak. The silylene complex should therefore be considered as a W(d²) compound, where the formal oxidation state of the metal is +4.