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

ABSTRACT The cationic nine‐coordinate pentahydrido complex [MoH 5 (depe) 2 ]BPh 4 ( 1 ·BPh 4 , 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 H 2 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 ·BPh 4 follows a sequence of migratory insertion prior to H 2 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 ·BPh 4 provides a foundation for new design concepts in base‐metal homogeneous catalysis, beyond the current focus on late 3d metals.