182They are, however, only moderately oxidizing at high pH:
CrO2−4 + 4 H 2 O + 3 e− → Cr(OH) 3 + 5 OH− (ε 0 = −0.13 V)Chromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide
solution. The unstable dark blue chromium(VI) peroxide (CrO 5 ) is formed, which can be stabilized
as an ether adduct CrO 5 ꞏOR 2.
Chromic acid has the hypothetical formula H 2 CrO 4. It is a vaguely described chemical, despite
many well-defined chromates and dichromates are known. The dark red chromium(VI) oxide CrO 3 ,
the acid anhydride of chromic acid, is sold industrially as "chromic acid". It can be produced by
mixing sulfuric acid with dichromate, and is a strong oxidizing agent.
Chromium(V) and chromium(IV)
The oxidation state +5 is only realized in few compounds but are intermediates in many reactions
involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride
(CrF 5 ). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be synthesized
by treating chromium metal with fluorine at 400 °C and 200 bar pressure. The peroxochromate(V)
is another example of the +5 oxidation state. Potassium peroxochromate (K 3 [Cr(O 2 ) 4 ]) is made by
reacting potassium chromate with hydrogen peroxide at low temperatures. This red brown
compound is stable at room temperature but decomposes spontaneously at 150–170 °C.
Compounds of chromium(IV) (in the +4 oxidation state) are slightly more common than those of
chromium(V). The tetrahalides, CrF 4 , CrCl 4 , and CrBr 4 , can be produced by treating the trihalides
(CrX 3 ) with the corresponding halogen at elevated temperatures. Such compounds are susceptible
to disproportionation reactions and are not stable in water.
Chromium(II)
Many chromium(II) compounds are known, including the water-stable chromium(II) chloride, CrCl 2 ,
which can be made by reduction of chromium(III) chloride with zinc. The resulting bright blue
solution is only stable at neutral pH. Many chromous carboxylates are also known, most famously,
the red chromous acetate (Cr 2 (O 2 CCH 3 ) 4 ), which features a quadruple bond.
Chromium(I)
Most Cr(I) compounds are obtained by oxidation of electron-rich, octahedral Cr(0) complexes.
Other Cr(I) complexes contain cyclopentadienyl ligands. As verified by X-ray diffraction, a Cr-Cr
quintuple bond (length 183.51(4) pm) has also been described. Extremely bulky monodentate
ligands stabilize this compound by shielding the quintuple bond from further reactions.