Higher coordinate complexes. 23. Reductive coupling of carbon monoxide ligands to form coordinated bis(trimethylsiloxy)ethyne in seven-coordinate niobium(I) and tantalum(I) [M(CO)2(dmpe)2Cl] complexes

A general procedure for the reductive coupling of carbonyl ligands in [M(CO)2(dmpe)2Cl] complexes, M = Nb or Ta and dmpe = l,2-bis(dimethylphosphino)ethane, is described. Reaction of [M(CO)2(dmpe)2Cl] with 40% sodium amalgam in THF or glyme followed by filtration, addition of Me3SiY, Y = Cl or CF3S03(triflate), and recrystallization from pentane produce [M(Me3SiOC≡COSiMe3)(dmpe)2Y] in 40-70% isolated yields. In an alternative route, the coupled ligand product could be obtained in 25% yield by using activated Mg dust and a group IV (4) metallocene dihalide, such as (C5Me5)2ZrCl2, in place of sodium amalgam. The molecular structures of four [M(Me3SiOC≡COSiMe3)(dmpe)2Y] molecules (M = Ta or Nb, Y = Cl; M = Ta, Y = CF3S03or Cl0.8I0.2) were determined by X-ray crystallography. The metrical parameters thus obtained, average C-C bond length, 1.32 (1) A, M-C-0 angle, 155.6 (7)°, and C-C-0 angle, 132.8 (5)°, parallel those displayed by other complexes containing four-electron donating alkynes. The infrared and NMR spectral properties also support the formal assignment of the coupled ligand as bis(trimethylsiloxy)ethyne. Preliminary mechanistic studies of the reductive coupling reaction suggest that it proceeds through initial formation of the cis-[M(CO)2(dmpe)2]-anion, which can be silylated to form [M-(Me3SiOCCOSiMe3)(dmpe)2Y].