Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium(III) chloride (also called chromic chloride) describes any of several chemical compounds with the formula CrCl3 · xH2O, where x can be 0, 5, and 6. The anhydrous compound with the formula CrCl3 is a violet solid. The most common form of the trichloride is the dark green hexahydrate, CrCl3 · 6 H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.
Chromium(III) oxide is an inorganic compound with the formula Cr
2O
3. It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.
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 preparation of chromous acetate once was a standard test of the synthetic skills of students due to its sensitivity to air and the dramatic colour changes that accompany its oxidation. It exists as the dihydrate and the anhydrous forms.
Chromium(III) fluoride is the name for the inorganic compounds with the chemical formula CrF3 as well as several related hydrates. The compound CrF3 is a green crystalline solid that is insoluble in common solvents, but the coloured hydrates [Cr(H2O)6]F3 and [Cr(H2O)6]F3•3H2O are soluble in water. The trihydrate is green, and the hexahydrate is violet. The anhydrous form sublimes at 1100–1200 °C.
Chromium(II) chloride describes inorganic compounds with the formula CrCl2(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 air-sensitive solutions of the tetrahydrate Cr(H2O)4Cl2. Chromium(II) chloride has no commercial uses but is used on a laboratory-scale for the synthesis of other chromium complexes.
Chromocene is the organochromium compound with the formula [Cr(C5H5)2]. Like structurally related metallocenes, chromocene readily sublimes in a vacuum and is soluble in non-polar organic solvents. It is more formally known as bis(η5-cyclopentadienyl)chromium(II).
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 the prototypical complex containing two arene ligands.
Chromium(III) acetylacetonate is the coordination compound with the formula Cr(C5H7O2)3, sometimes designated as Cr(acac)3. This purplish coordination complex is used in NMR spectroscopy as a relaxation agent because of its solubility in nonpolar organic solvents and its paramagnetism.
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 organochromium complexes encompass the entire range of possible oxidation states from –4 (d10) in Na4[Cr–IV(CO)4] to +6 (d0) in oxo-alkyl complexes like Cp*CrVI(=O)2Me.
Metal acetylacetonates are coordination complexes derived from the acetylacetonate anion (CH
3COCHCOCH−
3) and metal ions, usually transition metals. The bidentate ligand acetylacetonate is often abbreviated acac. Typically both oxygen atoms bind to the metal to form a six-membered chelate ring. The simplest complexes have the formula M(acac)3 and M(acac)2. Mixed-ligand complexes, e.g. VO(acac)2, are also numerous. Variations of acetylacetonate have also been developed with myriad substituents in place of methyl (RCOCHCOR′−). Many such complexes are soluble in organic solvents, in contrast to the related metal halides. Because of these properties, acac complexes are sometimes used as catalyst precursors and reagents. Applications include their use as NMR "shift reagents" and as catalysts for organic synthesis, and precursors to industrial hydroformylation catalysts. C
5H
7O−
2 in some cases also binds to metals through the central carbon atom; this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III).
Chromium(II) sulfate refers to inorganic compounds with the chemical formula CrSO4·n H2O. Several closely related hydrated salts are known. The pentahydrate is a blue solid that dissolves readily in water. Solutions of chromium(II) are easily oxidized by air to Cr(III) species. Solutions of Cr(II) are used as specialized reducing agents of value in organic synthesis.
Chromium(II) fluoride is an inorganic compound with the formula CrF2. It exists as a blue-green iridescent solid. Chromium(II) fluoride is sparingly soluble in water, almost insoluble in alcohol, and is soluble in boiling hydrochloric acid, but is not attacked by hot distilled sulfuric acid or nitric acid. Like other chromous compounds, chromium(II) fluoride is oxidized to chromium(III) oxide in air.
Chromium pentafluoride is the inorganic compound with the chemical formula CrF5. It is a red volatile solid that melts at 34 °C. It is the highest known chromium fluoride, since the hypothetical chromium hexafluoride has not yet been synthesized.
Chromium(II) oxalate is an inorganic compound with the chemical formula CrC2O4.
Chromium(I) hydride, systematically named chromium hydride, is an inorganic compound with the chemical formula (CrH)
n. It occurs naturally in some kinds of stars where it has been detected by its spectrum. However, molecular chromium(I) hydride with the formula CrH has been isolated in solid gas matrices. The molecular hydride is very reactive. As such the compound is not well characterised, although many of its properties have been calculated via computational chemistry.
Chromium(II) hydride, systematically named chromium dihydride and poly(dihydridochromium) is pale brown solid inorganic compound with the chemical formula (CrH
2)
n. Although it is thermodynamically unstable toward decomposition at ambient temperatures, it is kinetically metastable.
Chromium(III) iodide, also known as chromium triiodide, is an inorganic compound with the formula CrI3. It is a black solid that is used to prepare other chromium iodides.
Chromium(II) iodide is the inorganic compound with the formula CrI2. It is a black solid. The compound is made by thermal decomposition of chromium(III) iodide. Like many metal diiodides, CrI2 adopts the "cadmium iodide structure" motif, i.e., it features sheets of octahedral Cr(II) centers interconnected by bridging iodiide ligands. Reflecting the effects of its d4 confiiguration, chromium's coordination sphere is highly distorted.
Chromium(II) sulfide is an inorganic compound of chromium and sulfur with the chemical formula CrS. The compound forms black hexagonal crystals, insoluble in water.
