Transition Metal Complexes with cyclo ‐Triphosphorus (η 3 ‐P 3 ) and tetrahedro ‐Tetraphosphorus (η 1 ‐P 4 ) Ligands

Salts of 3d, 4d, and 5d metals in the presence of the ligands 1,1,1,‐tris(diphenylphosphinomethyl)ethane (triphos) or tris (2‐diphenylphosphinoethyl) amine (np 3 ) react with white phosphorus P 4 (or yellow As 4 ) to produce several mononuclear sandwich and dinuclear triple‐decker sandwich complexes, which contain the unprecedented cyclo ‐triphosphorus (or cyclo ‐triarsenic) unit acting as a trihapto ‐ligand. In these complexes the metal atoms are bonded to the there phosphorus atoms of the phosphane ligand and to the three atoms of the cyclo ‐P 3 or cyclo ‐As 3 unit. The complexes are diamagnetic or have μ eff ‐values corresponding to one or two unpaired electrons. The cyclo ‐P 3 ligand is coordinatively unsaturated as proved by the fact that the mononuclear sandwich compounds may form Lewis‐base adducts with electron‐acceptor fragments. Reaction of the complexes (np) 3 M (M = Ni, Pd) with white P 4 leads to formation of diamagnetic compounds [(np 3 )M(η 1 ‐P 4 )], in which the metal atom is bonded to the three phosphorus atoms of the np 3 ‐ligand and in addition to one P atom of the intact P 4 molecule, which behaves as a monohapto ‐ligand. This article contains a review of the syntheses and structures of these complexes as well as a unified, albeit qualitative, approach to their bonding and properties.