HIGH-VALENT DIPHENYLACETYLENE COMPLEXES OF TUNGSTEN

The W(4f(7/2)) binding energy of [{WCl4(PhC(2)Ph)}(2)] 1 obtained by X-ray photoelectron spectroscopy is similar to that of [{WCl4(NPh)}(2)] and is consistent with a d(0) tungsten (VI) formulation. The reaction of complex 1 and [NEt(4)][WCl5(PhC(2)Ph)] 2 with NaOH-EtOH gave cis-stilbene indicating considerable electron transfer from the metal to the co-ordinated alkyne. Reduction of complex 1 with 2 equivalents of sodium-mercury amalgam in the presence of phosphines gave the complexes [WCl3(PhC(2)Ph)L(2)] (L = PMe(3), PMe(2)Ph or PMePh(2)) with magnetic moments and W(4f(7/2)) binding energies similar to those of the d(1) tungsten(V) organoimido complex [WCl3(NPh)(PMe(3))(2)]. Decomposition of the alkyne complexes with NaOH-EtOH again gave cis-stilbene. The crystal structure of [WCl3(PhC(2)Ph)(PMe(3))(2)] 3 has been determined. The W-Cl bond trans to the alkyne ligand is long [2.479(3) Angstrom], and the W-C bond lengths [2.011(13) and 2.038(12) Angstrom] indicate a four-electron-donor alkyne ligand. The geometry is similar to that of [WCl3(NPh)(PMe(3))(2)]. Reduction of [{WCl4(PhC(2)Ph)}2] using 4 equivalents of sodium-mercury amalgam in the presence of phosphines gave the complexes [WCl2(PhC(2)Ph)L(3)] (L = PMe(3) or PMe(2)Ph) which again produced cis-stilbene on decomposition with NaOH-EtOH. The acetylenic carbon resonance at delta 223.15 in the C-13-{H-1} NMR spectrum of [WCl2(PhC(2)Ph)(PMe(3))(3)] 6 is also indicative of a four-electron-donor alkyne ligand. its W(4f(7/2)) binding energy is similar to [WCl2(NPh)(PMe(3))(3)] and is consistent with tungsten(IV). A crystal structure of complex 6 shows a similar ligand geometry to [WCl2(NPh)(PMe(3))(3)], and the W-C bond lengths [2.019(11) and 2.006(11) Angstrom] indicate a four-electron-donor alkyne ligand. Hartree-Fock and scattered wave X alpha calculations have been performed on the model complexes [WCl5(HC2H)](-) 8, [WCl3(HC2H)(PH3)(2)] 9 and [WCl2(HC2H)(PH3)(3)] 10. Molecular orbital and population analyses indicated that the acetylene-tungsten bond in each involves W(5d(pi)) --> HC2H(pi*) back donation as well as HC2H(pi) --> W(5d(sigma)) and HC2H(pi(perpendicular to)) --> W(5d(pi) forward donation, consistent with a four-electron-donor alkyne formalism. Electron withdrawal from the tungsten to the more electronegative Cl ligand in complexes 8 and 9 is minimised by rotation of the alkyne away from the meridional vectors. In complex 10 the HC2H(pi(perpendicular to) --> W(5d(pi)) donation and phosphine contributions compensate and no rotation is observed. The total d atomic orbitar population of complex 8 is close to that of WCl6, and populations of complexes 9 and 10 step up linearly from this. The computational results support the experimental evidence that [{WCl4(PhC(2)Ph)}(2)] 1, [WCl2(PhC(2)Ph)(PMe(3))(2)] 3 and [WCl3(PhC(2)Ph)(PMe(3))(3)] 6 are complexes of tungsten- (VI), -(V) and -(IV) respectively.