Phosphorus, Arsenic, Antimony, and Bismuth Multiply Bonded Systems with Low Coordination Number — Their Role as Complex Ligands†

Multiply bonded systems with low coordination number (one to three) involving phosphorus and its homologues, whether isolable or generated in situ at a metal complex, are versatile ligands that can function as two-, four-, six- or eight-electron donors to transition metal fragments and that may be coordinated in η1 (terminal), η2 (side-on), edge- or face-bridging modes. The (p-p)π bonding system of the ligand is thereby rendered capable of addition and intra- or intermolecular cycloaddition reactions. Some of the polycyclic species thus generated represent novel, polydentate ligands.