Chromium is a member of

group 6 Group 6 may refer to: *Group 6 element, chemical element classification *Group 6 (racing) Group 6 was the official designation applied by the FIA to two motor racing classifications, the Prototype-Sports Car category from 1966 to 1971 and the Two ...

, of the

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

s. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.

Common oxidation states

Chromium(0)

Many Cr(0) complexes are known.

Bis(benzene)chromium Bis(benzene)chromium is the organometallic compound with the formula Cr( η6-C6H6)2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is th ...

and

chromium hexacarbonyl Chromium carbonyl, also known as chromium hexacarbonyl, is the chemical compound with the formula Cr( CO)6. At room temperature the solid is stable to air, although it does have a high vapor pressure and sublimes readily. Cr(CO)6 is zerovalent ...

are highlights in

organochromium chemistry Organochromium chemistry is a branch of organometallic chemistry that deals with organic compounds containing a chromium to carbon bond and their reactions. The field is of some relevance to organic synthesis. The relevant oxidation states for organ ...

Chromium(II)

Chromium(II) compounds are uncommon, in part because they readily oxidize to chromium(III) derivatives in air. Water-stable

chromium(II) chloride Chromium(II) chloride describes inorganic compounds with the formula Cr Cl2(H2O)n. The anhydrous solid is white when pure, however commercial samples are often grey or green; it is hygroscopic and readily dissolves in water to give bright blue ...

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. Some other notable chromium(II) compounds include

chromium(II) oxide Chromium(II) oxide (CrO) is an inorganic compound composed of chromium and oxygen. It is a black powder that crystallises in the rock salt structure.Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemistry'', Elsevier Hypophosphites ma ...

, and

chromium(II) sulfate Chromium(II) sulfate refers to inorganic compounds with the chemical formula CrSO4·n H2O. Several closely related water of crystallization, hydrated salts are known. The pentahydrate is a blue solid that dissolves readily in water. Solutions of ...

. Many chromium(II) carboxylates are known. The red

chromium(II) acetate Chromium(II) acetate hydrate, also known as chromous acetate, is the coordination compound with the formula Cr2(CH3CO2)4(H2O)2. This formula is commonly abbreviated Cr2(OAc)4(H2O)2. This red-coloured compound features a quadruple bond. The prepara ...

(Cr₂(O₂CCH₃)₄) is somewhat famous. It features a Cr-Cr

quadruple bond A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the transition metals in the m ...

Chromium(III)

A large number of chromium(III) compounds are known, such as chromium(III) nitrate,

chromium(III) acetate Chromium(III) acetate, commonly known as basic chromium acetate, describes a family of salts where the cation has the formula r3O(O2CCH3)6(OH2)3sup>+. The trichromium cation is encountered with a variety of anions, such as chloride and nitrate. ...

, and

chromium(III) oxide Chromium(III) oxide (or chromia) is an inorganic compound with the formula . It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite. Structure and properties has the corundum ...

. Chromium(III) can be obtained by dissolving elemental chromium in acids like

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

, but it can also be formed through the reduction of chromium(VI) by

cytochrome Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of bin ...

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 Chrome alum or Chromium(III) potassium sulfate is the potassium double sulfate of chromium. Its chemical formula is KCr(SO4)2 and it is commonly found in its dodecahydrate form as KCr(SO4)2·12(H2O). It is used in leather tanning. Production a ...

and

alum An alum () is a type of chemical compound, usually a hydrated double salt, double sulfate salt (chemistry), salt of aluminium with the general chemical formula, formula , where is a valence (chemistry), monovalent cation such as potassium or a ...

. Chromium(III) tends to form

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

complexes. Commercially available

chromium(III) chloride Chromium(III) chloride (also called chromic chloride) describes any of several chemical compounds with the formula CrCl3, where can be 0, 5, and 6. The anhydrous compound with the formula CrCl3 is a violet solid. The most common form of the tri ...

hydrate is the dark green complex rCl₂(H₂O)₄l. Closely related compounds are the pale green rCl(H₂O)₅l₂ and violet r(H₂O)₆l₃. If anhydrous violet

is dissolved in water, the violet solution turns green after some time as the chloride in the inner

coordination sphere In coordination chemistry, the first coordination sphere refers to the array of molecules and ions (the ligands) directly attached to the central metal atom. The second coordination sphere consists of molecules and ions that attached in various ...

is replaced by water. This kind of reaction is also observed with solutions of

and other water-soluble chromium(III) salts. A

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

coordination of chromium(III) has been reported for the Cr-centered Keggin anion �-CrW₁₂O₄₀sup>5–.

Chromium(III) hydroxide Chromium(III) hydroxide is a gelatinous green inorganic compound with the chemical formula . It is a polymer with an undefined structure and low solubility. It is amphoteric, dissolving in both strong alkalis and strong acids. : In alkali: Cr ...

(Cr(OH)₃) is

amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphiproti ...

, dissolving in acidic solutions to form r(H₂O)₆sup>3+, 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 Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pres ...

Chromium(VI)

Chromium(VI) compounds are oxidants at low or neutral pH. Chromate anions () and dichromate (Cr₂O₇²⁻) anions are the principal ions at this oxidation state. They exist at an equilibrium, determined by pH: :2 rO₄sup>2− + 2 H⁺ r₂O₇sup>2− + H₂O Chromium(VI) oxyhalides are known also and include chromyl fluoride (CrO₂F₂) and

chromyl chloride Chromyl chloride is the inorganic compound with the formula CrO2Cl2. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal complexes. Preparation Chromyl chloride can be prepared by th ...

(). However, despite several erroneous claims,

chromium hexafluoride Chromium hexafluoride or chromium(VI) fluoride (CrF6) is a hypothetical chemical compound between chromium and fluorine with the chemical formula CrF6. It was previously thought to be an unstable yellow solid decomposing at −100 °C, but th ...

(as well as all higher hexahalides) remains unknown, as of 2020. Chrom(VI)-oxid

Sodium chromate Sodium chromate is the inorganic compound with the formula Na2CrO4. It exists as a yellow hygroscopic solid, which can form tetra-, hexa-, and decahydrates. It is an intermediate in the extraction of chromium from its ores. Production and react ...

is produced industrially by the oxidative roasting of

chromite Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can s ...

ore with

sodium carbonate Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...

. 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 Potassium chromate is the inorganic compound with the formula K2 CrO4. This yellow solid is the potassium salt of the chromate anion. It is a common laboratory chemical, whereas sodium chromate is important industrially. Structure Two crystal ...

. At yet lower pH values, further condensation to more complex

oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...

s of chromium is possible. Both the

chromate and dichromate Chromate salts contain the chromate anion, . Dichromate salts contain the dichromate anion, . They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate i ...

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) Chroman sodný

Chromium(VI) compounds in solution can be detected by adding an acidic

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

solution. The unstable dark blue

chromium(VI) peroxide Chromium(VI) peroxide or chromium oxide peroxide is an unstable compound with the formula CrO5. This compound contains one oxo ligand and two peroxo ligands, making a total of five oxygen atoms per chromium atom. Preparation and properties Chromi ...

(CrO₅) is formed, which can be stabilized as an ether adduct .

Chromic acid The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixt ...

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 An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...

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

Compounds of chromium(V) are rather rare; 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 (CrF₅). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be prepared 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 (K₃ r(O₂)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) are slightly more common than those of chromium(V). The tetrahalides, CrF₄, CrCl₄, and CrBr₄, can be produced by treating the trihalides () with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water. Organic compounds containing Cr(IV) state such as chromium tetra ''t''-butoxide are also known. Most chromium(I) compounds are obtained solely by oxidation of electron-rich,

chromium(0) complexes. Other chromium(I) complexes contain

cyclopentadienyl Cyclopentadienyl can refer to *Cyclopentadienyl anion, or cyclopentadienide, **Cyclopentadienyl ligand *Cyclopentadienyl radical, • *Cyclopentadienyl cation, See also *Pentadienyl In organic chemistry, pentadienyl refers to the organic radic ...

ligands. As verified by

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

, a Cr-Cr

quintuple bond A quintuple bond in chemistry is an unusual type of chemical bond, first reported in 2005 for a dichromium compound. Single bonds, double bonds, and triple bonds are commonplace in chemistry. Quadruple bonds are rarer but are currently known only ...

(length 183.51(4) pm) has also been described. Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions. 5-fold chromium

Notes

References

{{Chromium compounds Chromium Chromium compounds Chemical compounds by element