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Abstract

The Mo=PR₂ π* orbital in a Mo phosphenium complex acts as acceptor in a new P^III‐based Lewis superacid. This Lewis acid (LA) participates in electrophilic Si‐H abstraction from E₃SiH to give a Mo‐bound secondary phosphine ligand, Mo‐PR₂H. The resulting Et₃Si⁺ ion remains associated with the Mo complex, stabilized by η¹‐P‐H donation, yet undergoes rapid exchange with an η¹‐Si‐H adduct of free silane in solution. The equilibrium between these two adducts presents an opportunity to assess the role of this new LA in catalytic reactions of silanes: is the LA acting as a catalyst or as an initiator? Preliminary results suggest that a cycle including the Mo‐bound phosphine‐silylium adduct dominates in the catalytic hydrosilylation of acetophenone, relative to a putative cycle involving the silane‐silylium adduct or “free” silylium.