The reaction between the seven-co-ordinated niobium(I) complexes [NbX(CO)4(dppe)] (dppe = Ph2PCH2CH2PPh2, X = Br or I) or [NbX(CO)3(PEt3)3] (X = Cl, Br or I) and alkynes leads to the d4 niobium alkyne complexes [NbX(CO)2(eta-2-RCCR) (dppe)] (R = Ph) and [NbX(CO)2(eta-2-RCCR) (PEt3)2] (R = H, Me, Et or Ph). These complexes have been characterized by H-1, C-13, P-31 and Nb-93 NMR and IR spectroscopies. Low-field acetylenic proton and carbon resonances confirm the description of the alkyne as a four-electron donor. The Nb-93 NMR resonances exhibit the expected dependence of shielding on the nature of the halide ligand, i.e. Cl < Br < 1. The crystal and molecular structures of [NbI(CO)2(MeCCMe)(PEt3)2] have been determined: triclinic, space group P1BAR, a = 9.000(1), b = 16.906(2), c =17.952(3) angstrom, alpha = 80.07(1), beta = 78.59(1), gamma = 75.50(1)degrees. The molecule has a slightly distorted octahedral geometry with the carbonyl and the phosphine ligands in trans positions. Average distances are: Nb-CO 2.11(5), Nb-C(alkyne) 2.09(9) and Nb-I 2.91(2) angstrom.
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