Chromium(VI) oxide peroxide

Chromium(VI) oxide peroxide is the name given to a collection of chromium coordination complexes. They have the formula CrO(O2)2L where L is a ligand. These species are dark blue and often labile. They all feature oxo ligand and two peroxo ligands, with the remaining coordination sites occupied by water, hydroxide, ether, or other Lewis bases.

Preparation and properties
Chromium(VI) oxide peroxide is formed by the addition of acidified hydrogen peroxide solutions to solutions of metal chromates or dichromates, such as sodium chromate or potassium dichromate. The generally yellow chromates or orange dichromates turn to dark blue as "chromium(VI) oxide peroxide" forms:
 * CrO4(2−) + 2 H2O2 + H+ → [CrO(O2)2OH]- + 3 H2O

The structure of the pyridine complex has been determined crystallographically. Adducts with other N-heterocycles have also been characterized similarly.

Aqueous chromium(VI) oxide peroxide decomposes in a few seconds, turning green as chromium(III) compounds are formed.
 * 2 CrO(O2)2 + 7 H2O2 + 6 H+ → 2 Cr(3+) + 10 H2O + 7 O2

Stable adducts of the type CrO(O2)2L include those with L = diethyl ether, 1-butanol, ethyl acetate, or amyl acetate. They form by adding a layer of the organic solvent above the chromate/dichromate solution and shaking during the addition of hydrogen peroxide.

The etherate, bis(pyridine) and pyridine complexes of this compound have been found to be effective oxidants in organic chemistry.