}
Chromium(0) Many Cr(0) complexes are known.
Bis(benzene)chromium and
chromium hexacarbonyl are highlights in
organochromium chemistry.
Chromium(II) (Cr3C2) Chromium(II) compounds are uncommon, in part because they readily oxidize to chromium(III) derivatives in air. Water-stable
chromium(II) chloride that can be made by reducing chromium(III) chloride with zinc. The resulting bright blue solution created from dissolving chromium(II) chloride is stable at neutral
pH.
Chromium(III) A large number of chromium(III) compounds are known, such as
chromium(III) nitrate,
chromium(III) acetate, and
chromium(III) oxide. Chromium(III) can be obtained by dissolving elemental chromium in acids like
hydrochloric acid or
sulfuric acid, but it can also be formed through the reduction of chromium(VI) by
cytochrome c7. The ion has a similar radius (63
pm) to (radius 50 pm), and they can replace each other in some compounds, such as in
chrome alum and
alum. Chromium(III) tends to form
octahedral complexes. Commercially available
chromium(III) chloride hydrate is the dark green complex . Closely related compounds are the pale green and violet . If anhydrous violet
chromium(III) chloride is dissolved in water, the violet solution turns green after some time as the chloride in the inner
coordination sphere is replaced by water. This kind of reaction is also observed with solutions of
chrome alum and other water-soluble chromium(III) salts. A
tetrahedral coordination of
chromium(III) has been reported for the Cr-centered
Keggin anion [α-CrW12O40]5–.
Chromium(III) hydroxide (Cr(OH)3) is
amphoteric, dissolving in acidic solutions to form , and in basic solutions to form . It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), a stable oxide with a crystal structure identical to that of
corundum. However, despite several erroneous claims,
chromium hexafluoride (as well as all higher hexahalides) remains unknown, as of 2020.
Sodium chromate is produced industrially by the oxidative roasting of
chromite ore with
sodium carbonate. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of
potassium chromate. At yet lower pH values, further condensation to more complex
oxyanions of chromium is possible. Both the
chromate and dichromate anions are strong oxidizing reagents at low pH: : + 14 + 6 e− → 2 + 21 (ε0 = 1.33 V) They are, however, only moderately oxidizing at high pH: : + 4 + 3 e− → + 5 (ε0 = −0.13 V) (Na2CrO4) Chromium(VI) compounds in solution can be detected by adding an acidic
hydrogen peroxide solution. The unstable dark blue
chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct .
Chromic acid has the hypothetical formula . It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark red
chromium(VI) oxide , 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. == Other oxidation states ==