Vanadium carbonyl

Vanadium carbonyl, also known as vanadium hexacarbonyl, is the chemical compound with the formula V(CO)6. This highly reactive species is noteworthy from theoretical and scholarly perspectives. It is a rare isolable homoleptic metal carbonyl that is paramagnetic. Most species with the formula Mx(CO)y follow the 18e rule, whereas V(CO)6 has 17 valence electrons.

Synthesis
Traditionally V(CO)6 is prepared in two-steps via the intermediacy of [V(CO)6]-. In the first step, VCl3 is reduced with metallic sodium under 200 atm CO at 160 °C. The solvent for this reduction is typically diglyme, CH3OCH2CH2OCH2CH2OCH3. This triether solubilizes sodium salts, akin to the behavior of a crown ether:
 * 4 Na +  VCl3  +  6 CO  + 2 diglyme  →  [Na(diglyme)2][V(CO)6]  +  3 NaCl

An improved low pressure synthesis has been developed.

Reactions
The most characteristic reaction for V(CO)6 is a thermally sensitive material. Its primary reaction is reduction to the monoanion [V(CO)6]-, salts of which are well studied. It is also susceptible to substitution by tertiary phosphine ligands, often leading to disproportionation.

V(CO)6 reacts with sources of the cyclopentadienyl anion to give the orange-coloured complex (C5H5)V(CO)4 (m.p. 136 °C). Like many charge-neutral organometallic compounds, this half-sandwich species is volatile. In the original preparation of this species, C5H5HgCl was employed as the source of C5H5.

Structure
V(CO)6 adopts an octahedral coordination geometry. High resolution X-ray crystallography indicates that the molecule is slightly distorted with two (trans) shorter V-C distances of 1.993 (2) vs. four (equatorial) 2.005 (2) Å. Such a distortion could be due to a Jahn-Teller effect.