σ-Bonded early transition metal–carbon derivatives. Part II. Methylniobium(V) and methyltantalum(V) oxohalide complexes

Abstract

A range of methylniobium(V) and methyltantalum(V) oxohalide complexes of formula MeMOX₂,2L have been synthesised, most of them in good yields, (i) by the action of MeMgl on NbOCl₃ in toluene–ether [L = OSMe₂, OP-(NMe₂)₃, OP(OMe)₃, and PPh₃], (ii) by reaction of MeMX₄ with an excess of the ligand in the case of oxo-type ligands (M = Nb, X = Cl, L = OSMe₂, ONMe₃, ONC₅H₅, OPMe₃, OPPh₃, OP(NMe₂)₃, O[OP(NMe₂)₂]₂, and OAsPh₃; M = Nb, X = Br, L = OPPh₃; M = Ta, X = Cl, L = OPPh₃). The reaction of MeNbCl₄ with acetone and the reaction of MeTaCl₄ with OP(NMe₂)₃ gave only the MeMCI₄,L adducts. The new compounds were thermally fairly stable. The acceptor character is weaker for MeNbOCl₂ than for NbOCl₃. The fundamental role of ether in the direct alkylation of NbOCl₃ is shown. Evidence for the existence of an NbOCl₃,2OEt₂ adduct is given and the formation of an analogous MeNbOCl₂,2OEt₂ intermediate is suggested. The fact that the ‘labile’ metal–carbon bond remains unaffected throughout the oxygen abstraction–halogen exchange reaction is stressed.