Modern inorganic chemistry

""HE TRANSITION ELEMENTS 383

CHROMIUM(H) CHLORIDE, CrCl 2

This is prepared by passing dry hydrogen chloride over chromium,

or hydrogen over anhydrous chromium(III) chloride. It is a white

solid. If pure chromium is dissolved in dilute hydrochloric acid in the

absence of air, a blue solution of the hydrated chloride, containing

the hexaaquo-ion [Cr(H 2 O) 6 ]^2 ^. is obtained. The same solution is

also obtained by reduction of the + 6 oxidation state (through the

-f 3) using a solution of a dichromate(VI) and reducing with zinc

and hydrochloric acid:

Cr 2 O^ ~ -> Cr^3 + (aq) -> Cr^2 + (aq)

orange green blue

(cf. the colour change when vanadium(V) is similarly reduced, p. 375)

Other oxidation states

Chromium forms a white solid, hexacarhonyl Cr(CO) 6 , with the
chromium in formal oxidation state 0; the structure is octahedral,
and if each CO molecule donates two electrons, the chromium
attains the noble gas structure. Many complexes are known where
one or more of the carbon monoxide ligands are replaced by other
groups of ions, for example [Cr(CO) 5 I]~.
In dibenzene chromium, the chromium atom is "sandwiched'
between two benzene rings (Figure 13.3}

Cr

Figure 13.3

Here also the rings are uncharged, and the complex contains
chromium (0).

TESTS FOR CHROMIUM

Fusion of any chromium compound with a mixture of potassium
nitrate and carbonate gives a yellow chromate(VI)*.

• Fused potassium nitrate is a powerful oxidising agent (cf. the oxidation of
  manganese compounds, p. 386)