Hydride complexes vanadium

Several anionic metal carbonyl hydrides stoichiometrically convert acyl chlorides to aldehydes. The anionic vanadium complex [Cp(CO)3VH] reacts quickly with acyl chlorides, converting them to aldehydes [44]. Although no further reduction of the aldehyde to alcohol was observed, the aldehydes reacted further under the reaction conditions in some cases, so a general procedure for isolation of the aldehydes was not developed. 

The vanadium complexes XLI, with a variety of methyl substituents, add hydride at the unsubstituted positions ... 

Dihydrogen evolves from vanadium(II)-cysteine at pH6.0-9.5. This reduction is first order in V11 and independent of pH in the range 7.5-8.5. If cysteamine or cysteine methyl ester is used, dihydrogen is still evolved. The reaction with serine is 1000 times slower than with cysteine even though the half wave potentials are comparable.156 This reaction may be explained by a hydride pathway similar to that proposed for catechol complexes or alternatively Scheme 8. 

The chemistry of hydrido complexes of group V metals seems to reflect the usual tendency for vanadium to behave differently from the other two elements, although generalizations are probably premature in such a new field. Certainly niobium and tantalum form numerous hydrides of similar composition and properties, appearing to have no vanadium counterparts to date, but there have been no systematic investigations involving all three elements under comparable conditions, as is clear from the following discussions. 

Vanadium(TV).—Accumulating structural data have made it clear that the Ballhausen-Dahl model developed to describe the bonding in the complexes Cp2MH2 (M = Mo or W), in which a sterically active non-bonding orbital is directed between the hydride ligands, cannot be extended to d1 and d2 M,v complexes of the type Cp2ML2. 

The first reported synthesis (9,76) (78%) of Cp2V2(CO)5 (7) involved protonation of the dianion [CpV(CO)3]2- made by sodium amalgam reduction of CpV(CO)4. No vanadium hydride is isolated in this reaction. A slightly better yield (89%) results from the photolysis of CpV(CO)4 in THF using a falling-film photoreactor (77). The product 7 is produced via dimerization of an intermediate solvent complex CpV(CO)3(THF) followed by a further carbonyl loss (35). The corresponding Cp complex 7 is also available by an exactly analogous photolysis of Cp V(CO)4 or by protonation of [Cp V(CO)3]2- (JO). 

Hydride derivatives are of interest in coimection with the catalytic properties of vanadium, for example, in alkene dehydrogenation or polymerization. Complexes with sulfhr-donor and possibly also nitrogen-donor atoms are relevant to the role played by vanadium in the poisoning of the Co Mo catalyst used for the hydrodesulfiirization of crude oU. ... 

An early claim (332, 334) to have prepared V(bipy)g+ has been revised (431) the compound V(bipy)3l is considered to be a mixture of V(bipy)3 and V(bipy) 3I2. The reaction of V(bipy ) or of Cr(bipy) + with aluminum hydride gives the neutral tris-bipyridyl complexes, but with other dipositive metals Al(bipy)3 is the major product (357). Attempts to prepare bipyridyl and plienanthroline derivatives of hexacarbonylvanadium led to the disproportionation of vanadium(O) (376). 

Vanadium hexacarbonyl reacts with various arenes yielding complexes [(arene)V(CO)4]" [V(CO)6] from which hexafluorphosphate or tetra-phenylborate salts can be prepared by metathesis reaction (50, 52). The cation [(CeHe)V(CO)4] reacts with NaBH4 to give the neutral cyclo-hexadienyl compound (CeH7)V(CO)4 (51). Similar results are observed with methyl-substituted derivatives. The addition of the hydride ion was shown to occur preferentially at unsubstituted positions of the aromatic ring. 

The reverse reaction, deprotonation of a vanadium hydride complex to produce an anion, is known. Thus, V(CO)4L2H (L2 = a chelating diphosphine" or diarsine ligand) can be converted to [V(CO)4L2] with OH or NEts. (t -l,3,5-Me3C6H3)V(CO)3H can also be deprotonated with hydroxide ion . 

---