Alkene Hydrogenation by a Cationic, Nine-Coordinate Molybdenum(VI) Pentahydride Catalyst

The cationic nine-coordinate pentahydrido complex [MoH₅(depe)₂]BPh₄ (1·BPh₄, depe  =  1,2-(diethylphosphino)ethane) is, to the best of our knowledge, the first early transition metal polyhydride shown to efficiently catalyze the hydrogenation of a diverse set of unactivated alkenes. Preliminary mechanistic investigation by Density Functional Theory (DFT) calculations indicates that turnover relies on the ability of this 18-electron, d° complex to undergo thermally facile H₂ reductive elimination, which opens coordination sites and supplies the metal center the necessary electrons to engage in the reduction of the substrate. According to the computations, ethylene hydrogenation catalysis by 1·BPh₄ follows a sequence of migratory insertion prior to H₂ oxidative addition. These findings demonstrate that high-coordinate early transition metal polyhydrides are not mere structural curiosities but can act as genuine hydrogenation catalysts. The unique reactivity of 1·BPh₄ provides a foundation for new design concepts in base-metal homogeneous catalysis, beyond the current focus on late 3d metals.