Vanadium hexacarbonyl is the inorganic compound with the formula V(CO)<sub>6</sub>. It is a blue-black volatile solid. This highly reactive species is noteworthy from theoretical perspectives as a rare isolable homoleptic metal carbonyl that is paramagnetic. Most species with the formula M<sub>x</sub>(CO)<sub>y</sub> follow the 18-electron rule, whereas V(CO)<sub>6</sub> has 17 valence electrons.
V(CO)<sub>6</sub> adopts an octahedral coordination geometry and is isostructural with chromium hexacarbonyl, even though they have differing valence electron counts. High resolution X-ray crystallography indicates that the molecule is slightly distorted with two (axial) shorter VâÂÂC distances of 1.993(2) àvs. four (equatorial) 2.005(2) à. Even though V(âÂÂI) is a larger ion than V(0), the VâÂÂC distances in are 0.07 àshorter than in the neutral precursor.
According to the original synthesis by Calderazzo, V(CO)<sub>6</sub> is prepared in two-steps via the intermediacy of . In the first step, VCl<sub>3</sub> is reduced with metallic sodium under 200 atm CO at 160 ðC. The solvent for this reduction is typically diglyme, CH<sub>3</sub>OCH<sub>2</sub>CH<sub>2</sub>OCH<sub>2</sub>CH<sub>2</sub>OCH<sub>3</sub>. This triether solubilizes sodium salts, akin to the behavior of a crown ether:
The resulting anion is oxidized with acid:
Vanadium hexacarbonyl is thermally unstable. Its primary reaction is reduction to the monoanion , salts of which are well studied. It is also susceptible to substitution by tertiary phosphine ligands, often leading to disproportionation.
V(CO)<sub>6</sub> reacts with sources of the cyclopentadienyl anion to give the orange four-legged piano stool complex (C<sub>5</sub>H<sub>5</sub>)V(CO)<sub>4</sub> (m.p. 136 ðC). Like many charge-neutral organometallic compounds, this half-sandwich species is volatile. In the original preparation of this species, C<sub>5</sub>H<sub>5</sub>HgCl was employed as the source of .