[3+2] Fragmentation of a Pentaphosphido Ligand by Cyanide

The activation of white phosphorus (P4) by transition metal complexes has been studied for decades, but the functionalization and release of the resulting (organo)phosphorus ligands has rarely been achieved. Herein we describe the formation of unusal diphosphanides from a P5 ligand by treatment with cyanide. New cobalt diorganopentaphosphido complexes have been synthesized by a stepwise reaction sequence involving a low-valent dimine cobalt complex, white phosphorus and diorganochlorophosphanes. Reactions of such complexes with tetraalkylammonium or potassium cyanide afford the first cyclotriphosphido cobaltate anion 5 and rare 1-cyano-diphosphan-1-ide anions [R2PPCN]− (6-R). The molecular structure of a related product 7 suggests a novel reaction mechanism, where cyanide anion coordination to cobalt induces a ligand rearrangement. This is followed by nucleophilic attack of a second cyanide anion at a P atom, releasing the P2 fragment.