Vanadium hexacarbonyl

Vanadium hexacarbonyl is the inorganic compound with the formula V(CO)₆. 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_x(CO)_y follow the 18-electron rule, whereas V(CO)₆ has 17 valence electrons.[1]

Vanadium hexacarbonyl

Names
IUPAC name hexacarbonylvanadium(0)
Identifiers
CAS Number	20644-87-5^Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:37857^Y
ECHA InfoCard	100.039.928
PubChem CID	519800
UNII	WA8Q7FLK2X^Y
SMILES O=C=[V](=C=O)(=C=O)(=C=O)(=C=O)=C=O
Properties
Chemical formula	C6O6V
Molar mass	219.00 g/mol
Appearance	blue-green crystals yellow solutions
Density	1.7 g/cm^3
Melting point	decomposes
Boiling point	sublimes at 50 °C (122 °F; 323 K) (15 mmHg)
Solubility in water	insoluble
Solubility in other solvents	5 g/L hexane; more soluble in dichloromethane
Structure
Crystal structure	orthorhombic
Coordination geometry	octahedral
Dipole moment	0 D
Hazards
Main hazards	CO source
Related compounds
Related compounds	Cr(CO)6, VCl3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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

The resulting anion is oxidized with acid:[2]

2 V(CO)⁻
₆ + 2 H₃PO₄ → 2 V(CO)₆ + H₂ + 2 H
₂PO⁻
₄

Reactions

Vanadium hexacarbonyl is thermally unstable. 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 four-legged piano stool 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 C
₅H⁻
₅.

Structure

V(CO)₆ 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 V–C distances of 1.993(2) Å vs. four (equatorial) 2.005(2) Å. Even though V(−I) is a larger ion than V(0), the V–C distances in V(CO)⁻
₆ are 0.07 Å shorter than in the neutral precursor.[3]

References

1. Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Weinheim: Wiley-VCH. ISBN 3-527-28165-7.
2. Liu, X.; Ellis, J. E. (2004). "Hexacarbonylvanadate(1−) and Hexacarbonylvanadium(0)". Inorg. Synth. 34: 96–103. doi:10.1002/0471653683.ch3. ISBN 0-471-64750-0.
3. Bellard, S.; Rubinson, K. A.; Sheldrick, G. M. (1979). "Crystal and Molecular Structure of Vanadium Hexacarbonyl". Acta Crystallographica. B35: 271–274. doi:10.1107/S0567740879003332.

Further reading

• Original synthesis: Ercoli, R.; Calderazzo, F.; Alberola, A. (1960). "Synthesis of Vanadium Hexacarbonyl". J. Am. Chem. Soc. 81: 2966–2967. doi:10.1021/ja01496a073.