Niobium carbonylate anions

Dinuclear carbonyl anions [M2(CO)8X3] were obtained through a two-electron transfer to protons by treatment of [M(CO)6]- with HX (X = Cl, OAc, OMe) (equation 82).526 The anion has Civ symmetry. The niobium atoms are heptacoordinated with four terminal carbonyl groups each and three bridging chlorides located at the vertices of an approximately equatorial... 

Compounds containing niobium or tantalum in negative formal oxidation states -I and -III are mainly metal carbonyl anions. Although these are organometallic derivatives, the report of efficient procedures for the synthesis of [M(CO)6] since the review of Labinger8 merits mention, as it can be anticipated that these highly reduced and reactive species will be important precursors of a large variety of new coordination compounds and metal clusters. 

The crystal structures of both (PPN)[M(CO)6] derivatives have been determined.717 The coordination polyhedron is octahedral (Nb—C 2.098(5) A CNbC 89.2(2)°). The PPN moiety is constrained to be centrosymmetric, and thus linear. These compounds correspond to the lowest oxidation state of niobium and tantalum for which structural data are available. A single v(CO) is found in the IR (1854 and 1852 cm-1 for Nb and Ta respectively). Comparable spectra are observed for Na[M(CO)6] in pyridine, but in solvents of lower dielectric constants such as tetrahydrofuran, additional bands attributed to distortion of the anion by the countercation are observed. [Nb(CO)6] appears to be the most labile carbonyl of the group VA analogs. 

A recent review gives a chronological survey of the syntheses and characterizations of homoleptic mononuclear metal-carbonyl (see Carbonyl Complexes of the Transition Metals) anions. [M(CO)6] is a key precursor to niobium and tantalum carbonyls. More reliable synthesis of [Ta(CO)6] ... 

The first group 5 cyclopentadienyl complexes with a pendant phosphane ligand were reported very recently by Fryzuk. Ligands 35 and 36 were treated with NbCl3(DME) to give niobium bischelates 207 and 208 in 80% yield (Scheme 35). Carbonylation gave 209 and 210 in 80% yield both were characterized by X-ray structure analyses. Oxidation with PbCL resulted in formation of trichlorides 211 and 212, which can also be obtained from the anionic ligands by treatment with Nb(0)Cl3(THF)2 (Scheme 36). 211 was structurally characterized. ... 

Niobium and tantalum halides yield salts of the 18 electron anions Nb(CO) and Ta(CO), but no neutral binary carbonyls have been isolated. 

Information on the carbonyl chemistry of niobium and tantalum is, to date, very meager. The main difficulty appears to be the reduction of the usual pentavalent derivatives of these metals to the very low formal oxidation states of metal carbonyl derivatives. Nevertheless, the yellow anions [M(C0)6] (M = Nb, Ta) have been obtained by a method analogous to, but more difficult than, one of the preparations of the [VCCO) ]" anion. The method involves reduction of the pentachlorides with sodium metal in diglyme in the presence of high pressures of carbon monoxide (63). The niobium and tantalum derivatives are much more air-sensitive than the analogous vanadium derivative. The niobium derivative has not yet been obtained analytically pure (63). No chemistry of the [Nb(CO)J and the [Ta(CO)6] ions has been reported, even conversion to the neutral carbonyl derivatives [M(CO) (M = Nb or Ta = 1 or 2) or to the carbonyl hydride derivatives HM(CO)6 (M = Nb, Ta) still presenting unsolved problems. 

---