Modern inorganic chemistry

THE TRANSITION ELEMENTS 375

by hydrogensulphite. The VO^2 + (aq) cation is probably best repre-

sented as [VO(H 2 O) 5 ]^2 + , with the oxygen occupying one coordina-

tion position in the octahedral complex. However the kVO' entity

is found in many other complexes, both cationic and anionic; an

example of the latter is [VOC1 4 ]~ where the vanadium(V) is 5-co-

ordinate, thus

The V(IV) species are all d^1 complexes, hence their colour. Besides

the *VO' compounds, some halides VX 4 are known, for example

VC1 4 , a liquid with a tetrahedral, covalent molecule and properties

similar to those of TiCl 4 , but coloured (red-brown).

Other oxidation states

In the + 3 oxidation state, vanadium forms an oxide V 2 O 3 , and the

blue [V(H 2 O) 6 ]^3 + cation in acid solution; the latter is obtained by

reduction of V(IV) or V( V):

VO^2 + (aq) + 2H 3 O+ + e~ -» V^3 + (aq) 4- 3H 2 O: E^ = + 0.36V

The hexaquo-cation occurs in the blue-violet alums, for example

NH 4 V(SO 4 ) 2 .12H 2 O

The + 2 oxidation state is achieved by more drastic reduction
(zinc and acid) of the +5, + 4 or + 3 states: thus addition of zinc
and acid to a solution of a yellow vanadate(V) gives, successively,
blue[VO(H 2 O) 5 ]^2 + ,green [VC1 2 (H 2 O) 4 ] +and violet [V(H 2 O) 6 ]2+.
The latter is of course easily oxidised, for example, by air. The oxide
VO is usually non-stoichiometric, but anhydrous halides VX 2 are
known.
The O oxidation state is known in vanadium hexacarbonyl.
V(CO) 6 , a blue-green, sublimable solid. In the molecule V(CO) 6 , if
each CO molecule is assumed to donate two electrons to the van-
adium atom, the latter is still one electron short of the next noble
gas configuration (krypton); the compound is therefore para-
magnetic, and is easily reduced to form [V(CO 6 )]~. giving it the