Chromium(II) acetate hydrate, also known as chromous acetate, is the

coordination compound A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

Cr₂(CH₃CO₂)₄(H₂O)₂. This formula is commonly abbreviated Cr₂(OAc)₄(H₂O)₂. This red-coloured compound features a

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 of covalent bonds: double bonds and triple bonds. Stable quadruple bonds are most common among the transit ...

. It exists as the

dihydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

and the

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

forms. Both are diamagnetic. Cr₂(OAc)₄(H₂O)₂ is a reddish

diamagnetic Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagn ...

powder, although diamond-shaped tabular

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s can be grown. Consistent with the fact that it is non

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

ic, Cr₂(OAc)₄(H₂O)₂ exhibits poor solubility in

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

and

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

Structure

The Cr₂(OAc)₄(H₂O)₂ molecule contains two atoms of

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

, two ligated molecules of water, and four

acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic, or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...

bridging ligand In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually r ...

s. The coordination environment around each chromium atom consists of four

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

atoms (one from each acetate ligand) in a square, one water molecule (in an axial position), and the other chromium atom (opposite the water molecule), giving each chromium centre an

octahedral geometry In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The oc ...

. The chromium atoms are joined by a quadruple bond, and the molecule has D_4h symmetry (ignoring the position of the hydrogen atoms). The same basic structure is adopted by Rh₂(OAc)₄(H₂O)₂ and Cu₂(OAc)₄(H₂O)₂, although these species do not have such short M–M contacts. A quadruple bond between the two chromium atoms is proposed to arise from the overlap of four d-orbitals on each metal with the same orbitals on the other metal: the d_''z''² orbitals overlap to give a

sigma bond In chemistry, sigma bonds (σ bonds) or sigma overlap are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals along the internuclear axis. Sigma bonding is most simply defined for diat ...

ing component, the d_''xz'' and d_''yz'' orbitals overlap to give two

pi bond In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbital ...

ing components, and the d_''xy'' orbitals give a

delta bond In chemistry, a delta bond (δ bond) is a Covalent bond, covalent chemical bond, in which four lobes of an atomic orbital on one atom orbital overlap, overlap four lobes of an atomic orbital on another atom. This overlap leads to the formation o ...

. This quadruple bond is also confirmed by the low

magnetic moment In electromagnetism, the magnetic moment or magnetic dipole moment is the combination of strength and orientation of a magnet or other object or system that exerts a magnetic field. The magnetic dipole moment of an object determines the magnitude ...

and short intermolecular distance between the two atoms of 236.2 ± 0.1 pm. The Cr–Cr distances are even shorter, 184 pm being the record, when the axial ligand is absent or the carboxylate is replaced with isoelectronic nitrogenous ligands. In the anhydrous chromous acetate, the Cr-Cr distance is 2.288 Å, noticeably shorter than Cr-Cr contact in the dihydrate.

History

Eugène-Melchior Péligot Eugène-Melchior Péligot (24 March 1811 – 15 April 1890), also known as Eugène Péligot, was a French chemist who isolated the first sample of uranium metal in 1841. Péligot proved that the black powder of Martin Heinrich Klaproth was not ...

first reported a chromium(II) acetate in 1844. His material was apparently the dimeric Cr₂(OAc)₄(H₂O)₂. The unusual structure, as well as that of

copper(II) acetate Copper(II) acetate, also referred to as cupric acetate, is the chemical compound with the formula where is acetate (). The hydrated derivative, , which contains one molecule of water for each copper atom, is available commercially. Anhydrous co ...

, was uncovered in 1951.

Preparation

The preparation usually begins with reduction of an aqueous solution of a Cr(III) compound using

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

. The resulting blue solution is treated with

sodium acetate Sodium acetate, CH3COONa, also abbreviated Sodium, NaOxygen, OAcetyl, Ac, is the sodium Salt (chemistry), salt of acetic acid. This salt is colorless, deliquescent, and hygroscopy, hygroscopic. Applications Biotechnological Sodium acetate is u ...

, which results in the rapid

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

of chromous acetate as a bright red powder. :2 Cr³⁺ + Zn → 2 Cr²⁺ + Zn²⁺ :2 Cr²⁺ + 4 OAc⁻ + 2 H₂O → Cr₂(OAc)₄(H₂O)₂ The synthesis of Cr₂(OAc)₄(H₂O)₂ has been traditionally used to test the synthetic skills and patience of inorganic laboratory students in

universities A university () is an educational institution, institution of tertiary education and research which awards academic degrees in several Discipline (academia), academic disciplines. ''University'' is derived from the Latin phrase , which roughly ...

because the accidental introduction of a small amount of air into the apparatus is readily indicated by the discoloration of the otherwise bright red product.

Anhydrous chromium(II) acetate

Heating dihydrated chromium(II) acetate at 100 °C gives the brown anhydrous compound, which is particularly sensitive to oxygen. The anhydrous form of chromium(II) acetate, and also related chromium(II) carboxylates, can be prepared from chromocene with elimination of

cyclopentadiene Cyclopentadiene is an organic compound with the chemical formula, formula C5H6. It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp−. This colorless liquid has a strong and unpleasant odor. At room temperature, ...

: : 4 RCO₂H + 2 Cr(C₅H₅)₂ → Cr₂(O₂CR)₄ + 4 C₅H₆

Reactions

Chromium(II) acetate is a starting material for other chromium(II) compounds. For example, it reacts with

acetylacetone Acetylacetone is an organic compound with the chemical formula . It is classified as a 1,3-diketone. It exists in equilibrium with a tautomer . The mixture is a colorless liquid. These tautomers interconvert so rapidly under most conditions that ...

to give chromous acetylacetonate: : Also, many analogues have been prepared using other

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

s in place of acetate and using different bases in place of the water. Chromium(II) acetate has been used to dehalogenate organic compounds such as α-bromoketones and

chlorohydrin In organic chemistry a halohydrin (also a haloalcohol or β-halo alcohol) is a functional group in which a halogen and a hydroxyl are bonded to adjacent carbon atoms, which otherwise bear only hydrogen or hydrocarbyl groups (e.g. 2-chloroethanol ...

s. The reactions appear to proceed via 1e⁻ steps, and rearrangement products are sometimes observed.

References

External links

*http://www.molecules.org/coordcpds.html#Cr2OAc4H2O

(outdated) *http://wwwchem.uwimona.edu.jm/courses/chromium.pdf {{Acetates Chromium(II) compounds Acetato complexes Aqua complexes Reducing agents Chemical compounds containing metal–metal bonds Chromium–oxygen compounds