Chromium(II) acetate
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| Chromium(II) acetate | |
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| General | |
| Systematic name | Chromium(II) acetate hydrate |
| Other names | chromous acetate, chromium diacetate |
| Molecular formula | C8H16Cr2O10 |
| SMILES | ? |
| Molar mass | 376.2 g/mol |
| Appearance | brick-red solid |
| CAS number | [14976-80-8] |
| Properties | |
| Density and phase | 1.79 g/cm3 |
| Solubility in water | soluble in hot water, MeOH |
| Other solvents | slightly in alcohols |
| Melting point | dehydrates >100C |
| Structure | |
| Molecular shape | quadruple Cr--Cr bond |
| Coordination geometry |
octahedral counting the Cr-Cr bond |
| Crystal structure | monoclinic |
| Dipole moment | 0 D |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | could react exothermically in air |
| NFPA 704 | |
| R/S statement | R: ? S: ? |
| RTECS number | AG3000000 |
| Supplementary data page | |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV,[1] IR, NMR, MS |
| 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 |
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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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[edit] Structure
The Cr2(OAc)4(H2O)2 molecule contains two atoms of chromium, two molecules of water, and four acetate groups. Ignoring the Cr---Cr interaction, each chromium atom has a coordination number of five. The Cr atoms are held together by a quadruple bond, giving the Cr centers overall octahedral geometry. The molecular 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.[2]
The quadruple bond between the two chromium atoms arises from the overlap of the z2 to give a sigma component, the xz and yz orbitals to give the two pi components, and the xy to give the delta bond. This quadruple bond is also confirmed by the low magnetic moment and short intermolecular distance between the two atoms of 2.362(1) Å.The Cr-Cr distances are even shorter, 1.84 Å being the record, when the axial ligand is absent or the carboxylate is replaced with isoelectronic nitrogenous ligands.[3]
[edit] History
Peligot first reported a chromium(II) acetate in 1844. His material was apparently the dimeric Cr2(OAc)4(H2O)2.[4] The unusual structure, as well as that of copper(II) acetate, was uncovered in 1951.[5]
[edit] Preparation
An aqueous solution of a Cr(III) compound is first reduced to the chromous state using zinc as a reductant.[6] 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.[7] 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.
[edit] Applications
Cr2(OAc)4(H2O)2 is used occasionally to dehalogenate organic compounds such as α-bromoketones and chlorohydrins.[8] The reactions appear to proceed via 1e- steps, and rearrangement products are sometimes observed.
Many other applications exist, including those in the polymer industry.[9]
[edit] Links
[edit] References
- ^ 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
- ^ Cotton, F. A.; Walton, R. A. “Multiple Bonds Between Metal Atoms” Oxford (Oxford): 1993. ISBN 0-19-855649-7.
- ^ 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
- ^ Peligot, E. C. R. Acad. Sci. 1844, volume 19, page 609ff. (b) Peligot, E. Ann. Chim. Phys. 1844, volume 12, pages 528ff.
- ^ 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.
- ^ Ocone, L.R.; Block, B.P. (1966). Inorganic Syntheses, 125-129.
- ^ Jolly, W. L. (1970). The Synthesis and Characterization of Inorganic Compounds. Prentice Hall, 442-445.
- ^ 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
- ^ Lee, M. “Graft Copolymerization of Styrene on Rubber Containing Halogen by Chromous Acetate.” Journal of Polymer Science, 1976, volume 14, pages 961-971.
