Trigonal Phosphine Umpolung: Electrophilicity Driven by a Redox‐Active Boron Cluster

group. These reactions are accompanied by a two-electron reduction of the carborane cage and its conversion from a neutral closo- to a dianionic nido- structure, demonstrating that cluster-centered redox activity drives phosphorus-centered electrophilicity. The resulting nido-carboranyl phosphines exhibit enhanced nucleophilicity at the remaining phosphine arm, enabling subsequent trapping with alkyl halides, carbon disulfide, and electron-deficient fluoroarenes. These findings establish redox-active boron clusters as platforms for inducing umpolung and provide a new strategy for accessing ambiphilic reactivity at phosphorus.