Aj. Nielson et al., HIGH-VALENT DIPHENYLACETYLENE COMPLEXES OF TUNGSTEN, Journal of the Chemical Society. Dalton transactions, (7), 1995, pp. 1153-1161
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-s
tilbene indicating considerable electron transfer from the metal to th
e co-ordinated alkyne. Reduction of complex 1 with 2 equivalents of so
dium-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) t
ungsten(V) organoimido complex [WCl3(NPh)(PMe(3))(2)]. Decomposition o
f the alkyne complexes with NaOH-EtOH again gave cis-stilbene. The cry
stal structure of [WCl3(PhC(2)Ph)(PMe(3))(2)] 3 has been determined. T
he W-Cl bond trans to the alkyne ligand is long [2.479(3) Angstrom], a
nd 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 equiv
alents of sodium-mercury amalgam in the presence of phosphines gave th
e 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 [WC
l2(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. Molec
ular orbital and population analyses indicated that the acetylene-tung
sten bond in each involves W(5d(pi)) --> HC2H(pi) back donation as we
ll as HC2H(pi) --> W(5d(sigma)) and HC2H(pi(perpendicular to)) --> W(5
d(pi) forward donation, consistent with a four-electron-donor alkyne f
ormalism. Electron withdrawal from the tungsten to the more electroneg
ative Cl ligand in complexes 8 and 9 is minimised by rotation of the a
lkyne away from the meridional vectors. In complex 10 the HC2H(pi(perp
endicular to) --> W(5d(pi)) donation and phosphine contributions compe
nsate and no rotation is observed. The total d atomic orbitar populati
on of complex 8 is close to that of WCl6, and populations of complexes
9 and 10 step up linearly from this. The computational results suppor
t 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 t
ungsten- (VI), -(V) and -(IV) respectively.