Everything about Chromium Ii Acetate totally explained
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Chromium(II) acetate, better known as chromous acetate, is the compound Cr
2(CH
3CO
2)
4(H
2O)
2. This formula is commonly abbreviated Cr
2(OAc)
4(H
2O)
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.
Cr
2(OAc)
4(H
2O)
2 is a reddish
diamagnetic powder, although diamond-shaped tabular
crystals can be grown. Consistent with the fact that it's non-
ionic,
Cr
2(OAc)
4(H
2O)
2 exhibits poor solubility in
water and
methanol.
Structure
The Cr
2(OAc)
4(H
2O)
2 molecule contains two atoms of
chromium, two
ligated molecules of water, and four mono
anionic
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 don't 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 z
2 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.
History
Eugene Peligot first reported a chromium(II) acetate in
1844. His material was apparently the
dimeric Cr
2(OAc)
4(H
2O)
2. The unusual structure, as well as that of
copper(II) acetate, was uncovered in
1951.
Preparation
An aqueous solution of a Cr(III) compound is first reduced to the chromous state using
zinc as a
reductant. The resulting blue chromous solution is treated with
sodium acetate. Immediately chromous acetate
precipitates as a bright red powder.
» Cr
6+ + 2Zn → Cr
2+ + 2Zn
2+
2 Cr
2+ + 4 OAc
- + 2 H
2O → Cr
2(OAc)
4(H
2O)
2
The synthesis of Cr
2(OAc)
4(H
2O)
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. An alternative route to related chromium(II) carboxylates starts with
chromocene:
» 4 HO
2CR + 2 Cr(C
5H
5)
2 → Cr
2(O
2CR)
4 + 4
C5H6
The advantage to this method is that it provides anhydrous derivatives.
Because it's so easily prepared, Cr
2(OAc)
4(H
2O)
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
Cr
2(OAc)
4(H
2O)
2 is used occasionally to dehalogenate organic compounds such as α-bromoketones and
chlorohydrins. The reactions appear to proceed via 1e
- steps, and rearrangement products are sometimes observed. Because the molecule contains Cr in a +2 oxidation state it's a good reducing agent. For this reason it'll reduce the O
2 found in air, and so can be used as an oxygen scrubber.
Many other applications exist, including those in the
polymer industry.
Further Information
Get more info on 'Chromium Ii Acetate'.
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