Reversible Silylium Transfer between P‐H and Si‐H Donors

The Mo=PR 2 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 E 3 SiH to give a Mo‐bound secondary phosphine ligand, Mo‐PR 2 H. The resulting Et 3 Si + ion remains associated with the Mo complex, stabilized by η 1 ‐P‐H donation, yet undergoes rapid exchange with an η 1 ‐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.