THE TRANSITION ELEMENTS 381
2+ 34-OH 2The compounds also illustrate the very great tendency of triposi-
tive chromium to form complexes, which are usually of the octa-
hedral form [CrX 6 ], for example [Cr(NH 3 ) 6 ]^3 +, [Cr(NH 3 ) 5 NO 2 ]^2 +
[Cr(CN) 6 ]^3 -.
CHROMIUM(III) OXIDE, Cr 2 O 3
Chromium(III) oxide is prepared:
(1) By heating chromium(III) hydroxide (see below).
(2) By heating ammonium dichromate:
(NH 4 ) 2 CrVI 2 O 7 N 2 T 4- 4H 2 OIt is a green powder, insoluble in water and in acids (cf. aluminium
oxide, A1 2 O 3 ). It is not reduced by hydrogen.
It catalyses the decomposition of potassium chlorate(V). Mixed
with zinc oxide, it is used as a catalyst in the manufacture of methanol.
It is used as a pigment, being very resistant to weathering.
CHROMIUM(III) HYDROXIDE, Cr(OH) 3 (HYDRATED)
Chromium(III) hydroxide is obtained as a light green gelatinous
precipitate when an alkali or ammonia is added to a chromium(III)
salt:
Cr^3 + Cr(OH) 3 i (or Cr 2 O 3 .xH 2 O)
This reaction is better represented as a removal of hydrogen ions
from the hydrated Cr3+ ion [equation (13.1)] ; the hydroxyl groups
left are believed to act as bridges, so building up aggregates of ions
[equation (13.2)] these forming first colloidal particles and then
larger aggregates [equation (13.3)*.
- Ions with hydroxyl bridges are probably formed from other hydrated metal
ions, e.g. (A1(H 2 O) 6 )^3 +. (Fe(H 2 O) 6 )^3 +.