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Chromium(II) acetate



Chromium(II) acetate
IUPAC name Chromium(II) acetate hydrate
Other names chromous acetate,
chromium diacetate
Identifiers
CAS number 14976-80-8
RTECS number AG3000000
Properties
Molecular formula C8H16Cr2O10
Molar mass 376.2 g/mol
Appearance brick-red solid
Density 1.79 g/cm3
Melting point

dehydrates >100C

Solubility in water soluble in hot water, MeOH
Structure
Crystal structure monoclinic
Coordination
geometry
octahedral
counting the Cr-Cr bond
Molecular shape quadruple Cr--Cr bond
Dipole moment 0 D
Hazards
Main hazards could react exothermically in air
Related Compounds
Related compounds Rh2(OAc)4(H2O)2
Cu2(OAc)4(H2O)2
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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.

Additional recommended knowledge

Contents

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 D4h 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
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chromium(II)_acetate". A list of authors is available in Wikipedia.
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