ETA-2-NITRILES AS 4-ELECTRON-DONOR LIGANDS FOR TUNGSTEN(II) AND THEIRCOMPARISON TO ETA-2-ALKYNES

A series of octahedral tungsten coordination compounds have been prepa red containing chloride, phosphine, and pyridyl ligands. The electron- rich, 16-electron complex WCl2(PMe3)2(bpy) undergoes chloride substitu tion with acetonitrile, benzonitrile, and pivalonitrile to form comple xes of the type [WCl(PMe3)2(bpy)(L)]+, in which the nitrile (L) is eta 2-bound to the metal. These unusual side-bound nitrile complexes exhib it electrochemical, spectroscopic, magnetic, and structural features s imilar to those of the ubiquitous ''four-electron-donor'' alkyne compl exes of Mo(II) and W(II) previously reported and theoretically evaluat ed. Thus, the model used to describe the bonding interactions of alkyn e complexes is extended to include eta2-nitrile complexes. The nitrile is proposed to stabilize the W(II) center through donation of electro n density from both its pi(parallel-to) and pi(perpendicular-to) orbit als while its pi(parallel-to) orbital simultaneously renders it a pi acid. The W(II) alkyne complexes WCl2(PMe3)3(eta2-2-pentyne) and [W(bp y)(PMe3)2(eta2-2-pentyne)]2+ have also been prepared in order to make a direct comparison with their nitrile analogs. Crystal structures of the complexes [W(bpy)(PMe3)2Cl(eta2-CH3CN)]+ and WCl2(PMe3)3(eta2-CH3C N) show that the nitrogen and carbon of the nitrile group are nearly e quidistant from the metal (ca. 2.00 angstrom) and that the N-C bond is lengthened by as much as 0.12 angstrom compared to that of the free l igand. In addition, crystal structures are reported for WCl2(PMe3)2(bp y) and WCl3(PMe3)2(PY). WCl3(PMe3)2(PY): a (angstrom) = 10.025(3), b ( angstrom) = 22.228(6), c (angstrom) = 8.232(1), orthorhombic, P2(1)2(1 )2(1) (No. 19), Z = 4. WCl2(PMe3)2(bpy): a (angstrom) = 13.975(5), b ( angstrom) = 9.408(3), c (angstrom) = 18.456(6), beta = 103.38(3), (mon oclinic, P2(1)/c (No. 14), Z = 4. WCl2(PMe3)3(eta2-CH3CN): a (angstrom ) = 16.792(5), b (angstrom) = 12.611(3), c (angstrom) = 18.971(5), bet a (deg) = 97.78(3), monoclinic, P2(1)/c (No. 14), Z = 8.