Crystal and Molecular Structure of Tetrakis(trimethyl-silylmethyl) bis-μ-trimethylsilylmethylidynediniobium(V)

Abstract

The crystal and molecular structure of tetrakis(trimethylsilylmethyl) bis-μ-trimethylsilylmethylidynediniobium(V), Nb₂(CH₂SiMe₃)₄(CSiMe₃)₂, were determined from threedimensional X-ray diffraction data about 33 years ago using the facilities available at Imperial College, London, UK. The crystals are triclinic with unit-cell dimensions: a = 19.540(3) Å, b = 11.014(2) Å, c = 12.155(2) Å, α = 115.91(1)º, β = 110.14(2)º, γ = 103.76(2)º, space group is P1 and Z = 2. The structure was refined by least-squares methods to R = 0.052. The complex is dimeric and the two crystallographically independent dimers have the same structure. The dimers have 1 crystallographic symmetry and each niobium atom is joined to another niobium atom by two bridging CSiMe₃ groups. The distorted tetrahedral coordination around each metal atom is completed by two terminal CH₂SiMe₃ groups. The mean Nb-C(bridging) and Nb-CH₂ bond lengths are 1.974 and 2.160 Å, respectively. The compound has a quasi-aromatic ring of which the carbene bridges form a part. Although Nb…Nb distances are ca. 2.90 Å, the multiple bond character of the Nb-C(bridging) bonds and the distortion of the NbCCNb ring so as to reduce and Nb-Nb interaction suggest that the metalmetal bonding probably does not occur in this complex. The analogous tungsten complex is found to be isostructural.