Chromium(II) acetate
Encyclopedia
Chromium acetate hydrate, also known as chromous acetate, is the coordination compound with the formula
Cr2(CH3CO2)4(H2O)2. This formula is commonly abbreviated Cr2(OAc)4(H2O)2. This red-coloured compound features a quadruple bond
. The preparation of chromous acetate once was a standard test of the synthetic skills of students due to its sensitivity to air and the dramatic colour changes that accompany its oxidation. It exists as the dihydrate and the anhydrous
forms.
Cr2(OAc)4(H2O)2 is a reddish diamagnetic powder, although diamond-shaped tabular crystal
s can be grown. Consistent with the fact that it is non-ion
ic, Cr2(OAc)4(H2O)2 exhibits poor solubility in water
and methanol
.
, two ligated
molecules of water, and four monoanionic acetate
ligand
s. 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.
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 bond
ing component, the xz and yz orbitals overlap to give two pi bond
ing 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.
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.
. The resulting blue solution is treated with sodium acetate
, which results in the rapid precipitation of chromous acetate as a bright red powder.
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. An alternative route to related chromium(II) carboxylates starts with chromocene
:
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 acid
s in place of acetate and using different bases in place of the water.
Many other applications exist, including those in the polymer
industry.
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
Cr2(CH3CO2)4(H2O)2. This formula is commonly abbreviated Cr2(OAc)4(H2O)2. This red-coloured compound features a quadruple bond
Quadruple bond
A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the middle members transition metal elements such rhenium, tungsten, molybdenum...
. The preparation of chromous acetate once was a standard test of the synthetic skills of students due to its sensitivity to air and the dramatic colour changes that accompany its oxidation. It exists as the dihydrate and the anhydrous
Anhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
forms.
Cr2(OAc)4(H2O)2 is a reddish diamagnetic powder, although diamond-shaped tabular crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s can be grown. Consistent with the fact that it is non-ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ic, Cr2(OAc)4(H2O)2 exhibits poor solubility in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
.
Structure
The Cr2(OAc)4(H2O)2 molecule contains two atoms of chromiumChromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, two ligated
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
molecules of water, and four monoanionic acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s. The coordination environment around each chromium atom consists of four oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
. The chromium atoms are joined together by a quadruple bond
Quadruple bond
A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the middle members transition metal elements such rhenium, tungsten, molybdenum...
, 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
Rhodium(II) acetate
Rhodium acetate is the chemical compound with the formula Rh24, where AcO− is the acetate ion . This emerald green powder is a catalyst for cyclopropanation of alkenes...
and Cu2(OAc)4(H2O)2
Copper(II) acetate
Copper acetate, also referred to as cupric acetate, is the chemical compound with the formula Cu2 where OAc- is acetate . The hydrated derivative, which contains one molecule of water for each Cu atom, is available commercially. Anhydrous Cu2 is a dark green crystalline solid, whereas Cu22 is...
, although these species do not have such short M---M contacts.
The quadruple bond between the two chromium atoms arises from the overlap of four d-orbitals
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
on each metal with the same orbitals on the other metal: the z2 orbitals overlap to give a sigma bond
Sigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
ing component, the xz and yz orbitals overlap to give two pi bond
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
ing components, and the xy orbitals give a delta bond
Delta bond
In chemistry, delta bonds are chemical bonds of the covalent type, where four lobes of one involved atomic orbital overlap four lobes of the other involved atomic orbital...
. This quadruple bond is also confirmed by the low magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
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 PeligotEugène-Melchior Péligot
Eugène-Melchior Péligot , also known as Eugène Péligot, was a French chemist who isolated the first sample of uranium metal in 1841....
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
Copper(II) acetate
Copper acetate, also referred to as cupric acetate, is the chemical compound with the formula Cu2 where OAc- is acetate . The hydrated derivative, which contains one molecule of water for each Cu atom, is available commercially. Anhydrous Cu2 is a dark green crystalline solid, whereas Cu22 is...
, was uncovered in 1951.
Preparation
An aqueous solution of a Cr(III) compound is first reduced to the chromous state using zincZinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
. The resulting blue solution is treated with sodium acetate
Sodium acetate
Sodium acetate, CH3COONa, also abbreviated NaOAc, also sodium ethanoate, is the sodium salt of acetic acid. This colourless salt has a wide range of uses.-Industrial:...
, 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 and patience of inorganic laboratory students in universities
University
A university is an institution of higher education and research, which grants academic degrees in a variety of subjects. A university is an organisation that provides both undergraduate education and postgraduate education...
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
Chromocene
Chromocene is an organochromium compound with the formula Cr2, often abbreviated Cp2Cr. This sandwich compound is structurally similar to ferrocene but does not follow the 18-electron rule because it only has 16 electrons. It also paramagnetic and highly reducing...
:
- 4 HO2CR + 2 Cr(C5H5)2 → Cr2(O2CR)4 + 4 C5H6CyclopentadieneCyclopentadiene is an organic compound with the formula C5H6. This colorless liquid has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction...
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 acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s 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. 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 is a good reducing agent. For this reason it will reduce the O2 found in air, and so can be used as an oxygen scrubber.Many other applications exist, including those in the polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
industry.
External links
- http://www.molecules.org/coordcpds.html#Cr2OAc4H2O
- http://alpha.chem.umb.edu/chemistry/ch370/documents/CH371chromiumacetate06.pdf (outdated)
- http://wwwchem.uwimona.edu.jm/courses/chromium.pdf