Syntheses and properties of homoleptic carbonyl and trifluorophosphane niobates:: [Nb(CO)6]-, [Nb(PF3)6]- and [Nb(CO)5]3-

Reductive carbonylations of NbCl4(THF)(2), THF = tetrahydrofuran, mediated by sodium naphthalene in 1,2-dimethoxyethane, DME, or sodium anthracene in THF, provide [Nb(CO)(6)](-) as the tetraethylammonium salt in 60% or 70% isolated yields, respectively, the highest known for atmospheric pressure syntheses pf this metal carbonyl. Corresponding reductions involving PF3 give about 40% yields of [Et4N][Nb(PF3)(6)], which in the past was only accessible by a photochemical route. Electrochemical data for [Nb(CO)(6)](-) and [Nb(PF3)(6)](-) are compared and show that the PS complex is almost 1 V more difficult to oxidize than the CO analogue. Protonation of [Nb(PF3)(6)](-) by concentrated sulfuric acid yields a volatile, thermally unstable species, which has been shown by H-1 NMR and mass spectral studies to be the new niobium hydride, Nb(PF3)(6)H. Previously unpublished Nb-93 and C-13 NMR studies corroborate prior claims that the sodium metal reduction of [Nb(CO)(6)](-) in liquid ammonia affords [Nb(CO)(5)](3-), the only known Nb(III-) species. The first details of this synthesis and those of [Nb(CO)(5)H](2-), [Nb(CO)(5)SnPh3](2-), [Nb(CO)(5)NH3](-), and [Nb(CO)(5)(CNtBu)](-) are presented.