Free Metallophosphines: Extremely Electron‐Rich Phosphorus Superbases That Are Electronically and Sterically Tunable**

Abstract We report herein a facile and highly modular access to an intriguing class of free Au‐substituted phosphines (AuPhos), namely (LAu) n PR 3− n (L=singlet carbene ligand; R=H, aryl, alkyl, silyl) ( n =1–3). The Tolman electronic parameter (TEP) values coupled with theoretical investigations showcase that Au‐substitution can boost the electron‐releasing ability of AuPhos, thus leading to an electronically and sterically tunable, extremely electron‐rich phosphorus center. The high basicity of AuPhos is attributed to the d‐p lone pair π‐repulsion arising from interaction between Au substituents and the lone pair at P. A series of multi‐nuclear transition metal complexes (i.e. Rh, Ir, Pd, Au, W, Mn) ligated by AuPhos are readily prepared via a straightforward process. Preliminary catalytic results reveal the facilitation of Pd‐catalyzed C−N coupling reactions and Ir‐catalyzed decarbonylation reactions via AuPhos. This work provides insights for future development of electron‐rich ligands.