Chromium(II) acetate
Chemical compound
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Cr2(OAc)4(H2O)2 is a reddish powder, although diamond-shaped tabular crystals can be grown. Consistent with the fact that it is nonionic, Cr2(OAc)4(H2O)2 exhibits poor solubility in water and methanol.
Structure
The Cr2(OAc)4(H2O)2 molecule contains two atoms of chromium, two ligated molecules of water, and four acetate bridging 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 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.
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 dz2 orbitals overlap to give a sigma bonding component, the dxz and dyz orbitals overlap to give two pi bonding components, and the dxy 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 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.
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History
Eugène-Melchior Péligot 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, was uncovered in 1951.
Preparation
The preparation usually begins with reduction of an aqueous solution of a Cr(III) compound using zinc. The resulting blue solution is treated with sodium acetate, which results in the rapid precipitation 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 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.
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:
- 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 to give chromous acetylacetonate:
- Cr2(O2CCH3)4(H2O)2 + 4 H2C(COCH3)2 → 2 Cr(CH3C(O)CHC(O)CH3)2 + 8 HO2CCH3 + 2 H2O
Also, many analogues have been prepared using other carboxylic acids 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 chlorohydrins. The reactions appear to proceed via 1e− steps, and rearrangement products are sometimes observed.