Stable Paramagnetic Half-Sandwich Mo(V) and W(V) Polyhydride Complexes. Structural, Spectroscopic, Electrochemical, Theoretical, and Decomposition Mechanism Studies of [Cp*MH₃(dppe)]⁺ (M = Mo, W)

Compounds Cp*MH₃(dppe) (M = Mo, 1; W, 2) are oxidized chemically and electrochemically to the corresponding 17-electron cations 1⁺ and 2⁺. Analogous oxidations of 1-d₃ and 2-d₃ provide 1⁺-d₃ and 2⁺-d₃, respectively. Complex 2⁺ is stable in CH₂Cl₂, THF, and MeCN at room temperature. A single-crystal X-ray analysis of the PF₆^- salt of 2⁺ shows a geometry for the cation which is intermediate between octahedral and trigonal prismatic, which is reproduced by geometry optimization of the [CpWH₃(PH₂CH₂CH₂PH₂)]⁺ model at the B3LYP/LANL2DZ level. Identical calculations on the neutral analogue also reproduce the previously reported trigonal prismatic structure for 1. A blue shift in the M−H stretching vibrations upon oxidation for both Mo and W compounds indicates that a M−H bond strengthening accompanies the oxidation process. The DFT calculations (M−H bond lengths, BDE, and stretching frequencies) are in good agreement with the experimental results. Complex 1⁺ decomposes in solution at room temperature by one or more of three different mechanisms depending on conditions:  H₂ reductive elimination, solvent-assisted disproportionation, or deprotonation. In THF or CH₂Cl₂, a reductive elimination of H₂ affords the stable paramagnetic monohydride Cp*MoH(dppe)PF₆ (3), which adds a molecule of solvent in CH₂Cl₂, THF, and MeCN. EPR studies show that the CH₂Cl₂ molecule coordinates in a bidentate mode to afford a 19-electron configuration. A solvent dependence of the decomposition rate [k(CH₂Cl₂) ≈ 7.8k(THF) at 0 °C] and an inverse isotope effect [k_H/k_D = 0.50(3) in CH₂Cl₂ at 0 °C] indicate the nature of 1⁺ as a classical trihydride and suggest a decomposition mechanism which involves equilibrium conversion to a nonclassical intermediate followed by a rate-determining associative exchange of H₂ with a solvent molecule. In MeCN at 20 °C, a solvent-assisted disproportionation (rate = k_disp[1⁺]², k_disp = 3.98(9) × 10³ s^-1 M^-1) and a deprotonation by residual unoxidized 1 (rate = k_deprot[1⁺][1], k_deprot = 2.8(2) × 10² s^-1 M^-1) take place competitively, as shown by detailed cyclic voltammetric and thin-layer cyclic voltammetric studies. The stoichiometric chemical oxidation of 1 in MeCN leads to a mixture of [Cp*MoH₂(dppe)(MeCN)]⁺ and [Cp*MoH(dppe)(MeCN)₂]²⁺ by the disproportionation mechanism.