Transition Metal Complexes with cyclo‐Triphosphorus (η3‐P3) and tetrahedro‐Tetraphosphorus (η1‐P4) 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 (np3) react with white phosphorus P4 (or yellow As4) 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‐P3 or cyclo‐As3 unit. The complexes are diamagnetic or have μeff‐values corresponding to one or two unpaired electrons. The cyclo‐P3 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)3M (M = Ni, Pd) with white P4 leads to formation of diamagnetic compounds [(np3)M(η1‐P4)], in which the metal atom is bonded to the three phosphorus atoms of the np3‐ligand and in addition to one P atom of the intact P4 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.