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

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

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 double bonds and triple bonds. Stable quadruple bonds are most common among the transition metals in the m ...

. The preparation of chromous acetate once was a standard test of the synthetic skills of students due to its sensitivity to

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

and the dramatic colour changes that accompany its oxidation. It exists as the dihydrate 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 achi ...

forms. Cr₂(OAc)₄(H₂O)₂ is a reddish

diamagnetic Diamagnetic materials 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 ferromagnetic materials are attracted ...

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, macro ...

s can be grown. Consistent with the fact that it is non ionic, Cr₂(OAc)₄(H₂O)₂ exhibits poor solubility in

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

and

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...

Structure

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

chromium 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 metal is valued for its high corrosion resistance and hard ...

, 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 ...

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

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

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. 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. The quadruple bond between the two chromium atoms arises 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) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of s ...

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 orbitals ...

ing components, and the d_''xy'' orbitals give a delta bond. This quadruple bond is also confirmed by the low

magnetic moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electroma ...

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.

History

Eugène-Melchior Péligot first reported a chromium(II) acetate in 1844. His material was apparently the

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...

ic 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 Cu(OAc)2 where AcO− is acetate (). The hydrated derivative, Cu2(OAc)4(H2O)2, which contains one molecule of water for each copper atom, is availab ...

, 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 with the symbol Zn and atomic number 30. Zinc 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 periodi ...

. The resulting blue solution is treated with sodium acetate, which results in the rapid

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

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 institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United State ...

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. The anhydrous form of chromium(II) acetate, and also related chromium(II) carboxylates, can be prepared from chromocene: : 4 RCO₂H + 2 Cr(C₅H₅)₂ → Cr₂(O₂CR)₄ + 4 C₅H₆ This method provides anhydrous derivatives in a straightforward manner. Because it is so easily prepared, Cr₂(OAc)₄(H₂O)₂ is a starting material for other chromium(II) compounds. 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 R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...

s in place of acetate and using different bases in place of the water.

Applications

Chromium(II) acetate has few practical applications. It 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. Because the compound is a good reducing agent, it will reduce the O₂ found in air and can be used as an oxygen scrubber.

References

External links

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

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