Vanadium pentafluoride

Properties and structure

The compound is exclusively a monomer in the gas phase. In the gas phase it adopts D_3h symmetric trigonal bipyramidal geometry as indicated by electron diffraction. As a solid, VF₅ forms a polymeric structure with fluoride-bridged octahedral vanadium centers.

The formation enthalpy of VF₅ is -1429.4 ± 0.8 kJ/mol.

It is the only known pentahalide of vanadium.

Synthesis

Vanadium pentafluoride can be prepared by fluorination of vanadium metal:

2 V + 5 F₂ → 2 VF₅

Alternatively, disproportionation of vanadium tetrafluoride yields equal amounts of the solid trifluoride and the volatile pentafluoride:

2 VF₄ → VF₃ + VF₅

This conversion is conducted at 650 °C. It can also be synthesized by using elemental fluorine to fluorinate industrial concentrates and raw materials so as to produce VF₅ on an industrial scale. VF₅ can be synthesized from the reaction of raw materials such as metallic Vanadium, ferrovanadium, vanadium (V) oxide and vanadium tetrafluoride with elemental fluorine.

VF₅ ionises in the liquid state as reflected by the high values of Trouton’s constant and electrical conductivities.

Characteristics and Reactivity

Interest in this highly corrosive compound began in the fifties when there was extensive studies of its physicochemical properties. It is a powerful fluorinating and oxidizing agent. It oxidizes elemental sulfur to sulfur tetrafluoride:.

S + 4 VF₅ → 4 VF₄ + SF₄

Like other electrophilic metal halides, it hydrolyzes, first to the oxyhalide:

VF₅ + H₂O → VOF₃ + 2 HF

Then to the binary oxide:

2 VOF₃ + 3 H₂O → V₂O₅ + 6 HF

Hydrolysis is accelerated in the presence of base. Despite its tendency to hydrolyze, it can be dissolved in alcohols.

It is a Lewis acid, as illustrated by its formation of the hexafluoride:

VF₅ + KF → KVF₆

Vanadium pentafluoride is a weaker acid and mainly undergoes oxidative and fluorinating reactions.

The compound fluorinates unsaturated polyfluoroolefins into polyfluoroalkanes.

The compound dissolves without reaction in liquid Cl₂ and Br₂. VF₅ is moderately soluble in HF.

Other reading

Arnold F. Holleman, Nils Wiberg: Lehrbuch der Anorganischen Chemie, 102. Auflage, de Gruyter, Berlin 2007, S. 1545, ISBN 978-3-11-017770-1.