NIOBIUM COMPLEXES WITH TERDENTATE DIPHOSPHINO-AMIDO AND DIAMINO-AMIDO LIGANDS - CRYSTAL-STRUCTURE OF [(NBCL3[N(CH(2)CH(2)PME(2))(2)])2]CENTER-DOT-C(6)H(5)ME

The reaction of [NbCl4(thf)(2)] (thf = tetrahydrofuran) with bis(tertiary phosphinoethyl)amides Li-[N(CH(2)CH(2)PR(2))(2)] (R = Me or Pr) resulted in the formation of new niobium dialkylamides. The specific complexes generated depend on the stoichiometry of the reaction and the bulk of the tertiary phosphine substituents. Thus, with the bulky diisopropylphosphino function, monomeric paramagnetic mono- and bis-ligand complexes are obtained, [NbCl3{N(CH2CH2PPr2)(2)}] and [NbCl2{N(CH2CH2P-Pr-2)(2)}(2)]. However with the less sterically encumbered dimethylphosphino function only the diamagnetic dinuclear complex, [{NbCl3[N(CH(2)CH(2)PMe(2))(2)}(2)], can be isolated; its crystal structure shows equivalent bridging terdentate amide ligands with two bridging chlorides for each dimeric unit resulting in eight-co-ordinate niobium atoms. In contrast, the bis(2-diethylaminoethyl)amide Li[N(CH(2)CH(2)NEt(2))(2)], analogue of the smaller phosphine, again only gave rise to paramagnetic monomeric derivatives.