Modern inorganic chemistry

374 THE TRANSITION ELEMENTS

which readily melts and vaporises, to give a monomeric vapour

with a pentagonal bipyramid structure (cf. PF 5 , p. 40). It reacts

directly with potassium fluoride at room temperature to give the

hexafluorovanadate(V), KVF 6 , (containing the octahedral complex

ion VFg). Oxide halides VOX 3 (X = F, Cl, Br) are known (cf.

phosphorus).

Vanadium pentoxide, vanadium(V) oxide, V 2 O 5 , is the most important
compound in this oxidation state. It is a coloured solid (colour
due to charge transfer, p. 60), the colour varying somewhat
(red -» brown) with the state of subdivision; it is formed when
vanadium (or some of its compounds) is completely oxidised, and
also by heating ammonium vanadate(V):
2NH 4 VO 3 -> V 2 O 5 + 2NH 3 + H 2 O
It is extensively used industrially as a catalyst, notably in the
oxidation of sulphur dioxide to the trioxide in sulphuric acid
manufacture. It is an essentially acidic oxide, dissolving in alkalis to
give vanadates; however, addition of acid converts the anionic
vanadate species to cationic species, by processes which are very
complex, but which overall amount to the following:

Approximate
pH range: 14-12 10-7 6-2 below 2
VOl" ^ (VOi)n ^ polyvanadates-^ VO 2 +(aq)
orthovanadate, polymetavanadate yellow dioxovanadium(V)
(tetraoxovanadatefV)) (polytrioxovanadate(V)) red
colourless, colourless,
tetrahedral tetrahedral
coordination
around vanadium

Oxidation state + 4

This is the important state of vanadium in aqueous solution; it is
neither strongly oxidising or strongly reducing and acidic solutions
are stable to atmospheric oxidation:

HSO;

[V(OH 4 )+ + 2H 3 O+ + e~ =± VO^2 + (aq) + 5H 2 O

VlV) oxovanadmm(IV)MnO^4

colourless or vanadyl, blue

As the scheme indicates, the blue 'vanadyl' oxovanadium cation can
be (quantitatively) oxidised to vanadium(V) and the latter is reduced