SYNTHESIS, STRUCTURE AND REACTIONS OF 7-COORDINATE, PHOSPHINE-SUPPORTED, HALIDE-ACTIVATED CARBONYL-COMPLEXES AND ALKYNE-COMPLEXES OF NIOBIUM(I)

The complexes (Hal)Nb(CO)3(PR3)3 (PR3 = PEt3, Hal = I; PR3 = PMe2Ph, H al = Cl, Br, I) and (Hal)Nb(CO)4/2(dppe)1/2 (Hal = Br, I) have been pr epared by oxidative halogenation of carbonylniobate with pyridinium ha lides (Hal = Cl, Br) or iodine (Hal = I). In the tricarbonyls, one CO and one PR3 are labile and can be displaced by a four-electron donatin g alkyne to give all-trans-[(Hal)Nb(CO)2(RCCR')(PR3)2] (PR3 = PMe2Ph; Hal = Cl, Br, I: R, R' = H, Et, Ph; R = H, R' = Ph. PR3 = PEt3, Hal = I: R, R' = Pr; R = H, R' = Bu, Ph; R = Me, R' = Et). In the case of ac etylene, INb(CO)(HCCH)2(PEt3)2 is also formed. PR3 can be displaced by P(OMe)3. In the tetracarbonyls, two CO ligands are replaced by two is onitriles to form INb(CO)2(CNR)2dppe (R = (t)Bu, Cy), or by one alkyne to form (Hal)Nb(CO)2(PhCCPh)dppe (Hal = Br, I). In these complexes, t he remaining CO ligands occupy cis positions. The structure of BrNb(CO )2(dppe)2 . THF, INb(CO)2(dppe)2 . hexane and INb(CO)2(PEt3)2(MeC=CEt) have been determined by a single crystal X-ray diffraction study. The alkyne complexes are best regarded as octahedral with the centre of t he alkyne ligand occupying the positions trans to the halide and the C =C axis aligned with the OC-Nb-CO axis. The complexes (Hal)Nb(CO)2(dpp e)2 adopt a trigonal prismatic structure with the halide capping the t etragonal face spanned by the four phosphorus functions. The crystal s tructure of a by-product, Br2Nb(CO)(H2CPhPCH2CH2PPh2)2 . 1/2THF has al so been determined. The geometry is pentagonal bipyramidal, with one o f the bromine atoms and the CO on the axis. Some Nb-93 NMR data for th e Nb(I) complexes are presented, and preliminary observations on the r eactions between the pi-alkyne complexes and H-2 or H- are reported.