Chromium(II) acetate

Chromium(II) acetate, better known as chromous acetate, is the compound Cr₂(CH₃CO₂)₄(H₂O)₂. This formula is commonly abbreviated Cr₂(OAc)₄(H₂O)₂. This compound and some of its simple derivatives illustrate one of the most remarkable properties of some metals - the ability to engage in quadruple bonds. The preparation of chromous acetate once was a standard test the synthetic skills of students due to its considerable sensitivity to air. It exists as the dihydrate and the anhydrous forms.

Cr₂(OAc)₄(H₂O)₂ is a reddish diamagnetic powder, although diamond-shaped tabular crystals can be grown. Consistent with the fact that it is non-ionic, Cr₂(OAc)₄(H₂O)₂ exhibits poor solubility in water and methanol.

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Structure

The Cr₂(OAc)₄(H₂O)₂ molecule contains two atoms of chromium, two ligated molecules of water, and four monoanionic acetate ligands. The coordination environment around each chromium atom consists of four oxygen 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 together 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.^[1]

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 z² orbitals overlap to give a sigma bonding component, the xz and yz orbitals overlap to give two pi bonding components, and the xy orbitals give a delta bond. This quadruple bond is also confirmed by the low magnetic moment and short intermolecular distance between the two atoms of 236.2±0.1 picometers.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.^[2]

History

Eugene Peligot first reported a chromium(II) acetate in 1844. His material was apparently the dimeric Cr₂(OAc)₄(H₂O)₂.^[3] The unusual structure, as well as that of copper(II) acetate, was uncovered in 1951.^[4]

Preparation

An aqueous solution of a Cr(III) compound is first reduced to the chromous state using zinc as a reductant.^[5] The resulting blue chromous solution is treated with sodium acetate. Immediately chromous acetate precipitates as a bright red powder.

Cr⁶⁺ + 2Zn → Cr²⁺ + 2Zn²⁺

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 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.^[6] An alternative route to related chromium(II) carboxylates starts with chromocene:

4 HO₂CR + 2 Cr(C₅H₅)₂ → Cr₂(O₂CR)₄ + 4 C₅H₆

The advantage to this method is that it provides anhydrous derivatives.

Because it is so easily prepared, Cr₂(OAc)₄(H₂O)₂ is often used as a starting material for other, chromium(II) compounds. Also many analogues have been prepared using other carboxylic acids in place of acetate and using different bases in place of the water.

Applications

Cr₂(OAc)₄(H₂O)₂ is used occasionally to dehalogenate organic compounds such as α-bromoketones and chlorohydrins.^[7] The reactions appear to proceed via 1e^- steps, and rearrangement products are sometimes observed.

Many other applications exist, including those in the polymer industry.^[8]

References

1. ^ Cotton, F. A.; Walton, R. A. “Multiple Bonds Between Metal Atoms” Oxford (Oxford): 1993. ISBN 0-19-855649-7.
2. ^ Cotton, F. A.; Hillard, E.A.; Murillo, C. A.; Zhou, H.-C. "After 155 Years, A Crystalline Chromium Carboxylate with a Supershort Cr-Cr Bond" Journal of the American Chemical Society, 2000 volume 122 , pages 416-417. doi:10.1021/ja993755i
3. ^ Peligot, E. C. R. Acad. Sci. 1844, volume 19, page 609ff. (b) Peligot, E. Ann. Chim. Phys. 1844, volume 12, pages 528ff.
4. ^ van Niekerk, J. N. Schoening, F. R. L. “X-Ray Evidence for Metal-to-Metal Bonds in Cupric and Chromous Acetate” Nature 1953, volume 171, pages 36-37. doi:10.1038/171036a0.
5. ^ Ocone, L.R.; Block, B.P. (1966). Inorganic Syntheses, 125-129. 
6. ^ Jolly, W. L. (1970). The Synthesis and Characterization of Inorganic Compounds. Prentice Hall, 442-445. 
7. ^ Ray, T. "Chromium(II) Acetate" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289
8. ^ Lee, M. “Graft Copolymerization of Styrene on Rubber Containing Halogen by Chromous Acetate.” Journal of Polymer Science, 1976, volume 14, pages 961-971.

Further reading

• Rice, S. F. “Electronic Absorption Spectrum of Chromous Acetate Dihydrate and Related Binuclear Chromous Carboxylates.” Inorg. Chem. 19 (1980):3425-3431.doi:10.1021/ic50213a042