Chromium(II) acetate

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Chromium(II) acetate
Cr2(OAc)4.svg
Chromium(II)-acetate-dimer-3D-balls.png
Names
IUPAC name
Chromium(II) acetate hydrate
Other names
chromous acetate,
chromium diacetate,
chromium(II) ethanoate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.224.848 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
RTECS number
  • AG3000000
UNII
  • InChI=1S/2C2H4O2.Cr/c2*1-2(3)4;/h2*1H3,(H,3,4);/q;;+2/p-2 Yes check.svgY
    Key: LRCIYVMVWAMTKX-UHFFFAOYSA-L Yes check.svgY
  • InChI=1/2C2H4O2.Cr/c2*1-2(3)4;/h2*1H3,(H,3,4);/q;;+2/p-2
    Key: LRCIYVMVWAMTKX-NUQVWONBAT
  • [Cr+2]1234([OH2])#[Cr+2]([OH2])(O[C-](C)O1)(O[C-](C)O2)(O(C)[C-]O3)O(C)[C-]O4
Properties
C8H16Cr2O10
Molar mass 376.198 g·mol−1
Appearancebrick-red solid
Density 1.79 g/cm3
Melting point dehydrates
soluble in hot water, MeOH
-5104.0·10−6 cm3/mol
Structure
monoclinic
octahedral
counting the Cr–Cr bond
0 D
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
could react exothermically in air
Related compounds
Related compounds
 Rh2(OAc)4(H2O)2
Cu2(OAc)4(H2O)2, molybdenum(II) acetate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Chromium(II) 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.

Contents

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 nonionic, Cr2(OAc)4(H2O)2 exhibits poor solubility in water and methanol.

Chromium(II) acetate (aqueous solution) Chromium(II) acetate.jpg
Chromium(II) acetate (aqueous solution)

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. [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 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. [2]

History

Eugène-Melchior Péligot first reported a chromium(II) acetate in 1844. His material was apparently the dimeric Cr2(OAc)4(H2O)2. [3] [4] The unusual structure, as well as that of copper(II) acetate, was uncovered in 1951. [5]

Preparation

The preparation usually begins with reduction of an aqueous solution of a Cr(III) compound using zinc. [6] 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 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] The anhydrous form of chromium(II) acetate, and also related chromium(II) carboxylates, can be prepared from chromocene:

4 RCO2H + 2 Cr(C5H5)2 → Cr2(O2CR)4 + 4 C5H6

This method provides anhydrous derivatives in a straightforward manner. [8]

Because it is so easily prepared, Cr2(OAc)4(H2O)2 is 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

Chromium(II) acetate has few practical applications. It has been used to dehalogenate organic compounds such as α-bromoketones and chlorohydrins. [9] The reactions appear to proceed via 1e− steps, and rearrangement products are sometimes observed.

Because the compound is a good reducing agent, it will reduce the O2 found in air and can be used as an oxygen scrubber.

See also

Related Research Articles

<span class="mw-page-title-main">Coordination complex</span> Molecule or ion containing ligands datively bonded to a central metallic atom

A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are in turn known as ligands or complexing agents. Many metal-containing compounds, especially those that include transition metals, are coordination complexes.

<span class="mw-page-title-main">Chromium(III) chloride</span> Chemical compound

Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formula CrCl3. It forms several hydrates with the formula CrCl3·nH2O, among which are hydrates where n can be 5 (chromium(III) chloride pentahydrate CrCl3·5H2O) or 6 (chromium(III) chloride hexahydrate CrCl3·6H2O). The anhydrous compound with the formula CrCl3 are violet crystals, while the most common form of the chromium(III) chloride are the dark green crystals of hexahydrate, CrCl3·6H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

<span class="mw-page-title-main">Quintuple bond</span> Chemical bond involving ten bonding electrons

A quintuple bond in chemistry is an unusual type of chemical bond, first reported in 2005 for a dichromium compound. Single bonds, double bonds, and triple bonds are commonplace in chemistry. Quadruple bonds are rarer and are currently known only among the transition metals, especially for Cr, Mo, W, and Re, e.g. [Mo2Cl8]4− and [Re2Cl8]2−. In a quintuple bond, ten electrons participate in bonding between the two metal centers, allocated as σ2π4δ4.

<span class="mw-page-title-main">Copper(II) acetate</span> Chemical compound

Copper(II) acetate, also referred to as cupric acetate, is the chemical compound with the formula Cu(OAc)2 where AcO− is acetate (CH
3CO−
2). The hydrated derivative, Cu2(OAc)4(H2O)2, which contains one molecule of water for each copper atom, is available commercially. Anhydrous copper(II) acetate is a dark green crystalline solid, whereas Cu2(OAc)4(H2O)2 is more bluish-green. Since ancient times, copper acetates of some form have been used as fungicides and green pigments. Today, copper acetates are used as reagents for the synthesis of various inorganic and organic compounds. Copper acetate, like all copper compounds, emits a blue-green glow in a flame.

<span class="mw-page-title-main">Chromium(III) fluoride</span> Chemical compound

Chromium(III) fluoride is an inorganic compound with the chemical formula CrF3. It forms several hydrates. The compound CrF3 is a green crystalline solid that is insoluble in common solvents, but the hydrates [Cr(H2O)6]F3 (violet) and [Cr(H2O)6]F3·3H2O (green) are soluble in water. The anhydrous form sublimes at 1100–1200 °C.

<span class="mw-page-title-main">Quadruple bond</span> Chemical bond involving eight electrons; has one sigma, two pi, and one delta 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 of covalent bonds: double bonds and triple bonds. Stable quadruple bonds are most common among the transition metals in the middle of the d-block, such as rhenium, tungsten, technetium, molybdenum and chromium. Typically the ligands that support quadruple bonds are π-donors, not π-acceptors. Quadruple bonds are rare as compared to double bonds and triple bonds, but hundreds of compounds with such bonds have been prepared.

<span class="mw-page-title-main">Chromium(II) chloride</span> Chemical compound

Chromium(II) chloride describes inorganic compounds with the formula CrCl2(H2O)n. The anhydrous solid is white when pure, however commercial samples are often grey or green; it is hygroscopic and readily dissolves in water to give bright blue air-sensitive solutions of the tetrahydrate Cr(H2O)4Cl2. Chromium(II) chloride has no commercial uses but is used on a laboratory-scale for the synthesis of other chromium complexes.

<span class="mw-page-title-main">Chromocene</span> Chemical compound

Chromocene is the organochromium compound with the formula [Cr(C5H5)2]. Like structurally related metallocenes, chromocene readily sublimes in a vacuum and is soluble in non-polar organic solvents. It is more formally known as bis(η5-cyclopentadienyl)chromium(II).

<span class="mw-page-title-main">Chromium compounds</span> Chemical compounds containing chromium

Chromium compounds are compounds containing the element chromium (Cr). Chromium is a member of group 6 of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.

<span class="mw-page-title-main">Bis(benzene)chromium</span> Chemical compound

Bis(benzene)chromium is the organometallic compound with the formula Cr(η6-C6H6)2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is the prototypical complex containing two arene ligands.

<span class="mw-page-title-main">Rhodium(II) acetate</span> Chemical compound

Rhodium(II) acetate is the coordination compound with the formula Rh2(AcO)4, where AcO− is the acetate ion (CH
3CO−
2). This dark green powder is slightly soluble in polar solvents, including water. It is used as a catalyst for cyclopropanation of alkenes. It is a widely studied example of a transition metal carboxylate complex.

<span class="mw-page-title-main">Chromium(III) sulfate</span> Chemical compound

Chromium(III) sulfate usually refers to the inorganic compounds with the formula Cr2(SO4)3.x(H2O), where x can range from 0 to 18. Additionally, ill-defined but commercially important "basic chromium sulfates" are known. These salts are usually either violet or green solids that are soluble in water. It is commonly used in tanning leather.

<span class="mw-page-title-main">Potassium octachlorodimolybdate</span> Chemical compound

Potassium octachlorodimolybdate is an inorganic compound with the chemical formula K4[Mo2Cl8]. It is known as a red-coloured, microcrystalline solid. The anion is of historic interest as one of the earliest illustrations of a quadruple bonding. The salt is usually obtained as the pink-coloured dihydrate.

<span class="mw-page-title-main">Molybdenum(II) acetate</span> Chemical compound

Molybdenum(II) acetate is a coordination compound with the formula Mo2(O2CCH3)4. It is a yellow, diamagnetic, air-stable solid that is slightly soluble in organic solvents. Molybdenum(II) acetate is an iconic example of a compound with a metal-metal quadruple bond.

<span class="mw-page-title-main">Chromium acetate hydroxide</span> Chemical compound

Chromium acetate hydroxide is the coordination complex with the formula [Cr2(OH)3(OAc)3]4. A dark violet solid, it crystallizes as the triacontatetrahydrate (34 molecules of water of crystallization). It is water soluble.

<span class="mw-page-title-main">Chromium(II) sulfate</span> Chemical compound

Chromium(II) sulfate is an inorganic compound with the chemical formula CrSO4. It often comes as hydrates CrSO4·nH2O. Several hydrated salts are known. The pentahydrate CrSO4·5H2O is a blue solid that dissolves readily in water. Solutions of chromium(II) are easily oxidized by air to Cr(III) species. Solutions of Cr(II) are used as specialized reducing agents of value in organic synthesis.

Aluminium triacetate, formally named aluminium acetate, is a chemical compound with composition Al(CH
3CO
2)
3. Under standard conditions it appears as a white, water-soluble solid that decomposes on heating at around 200 °C. The triacetate hydrolyses to a mixture of basic hydroxide / acetate salts, and multiple species co-exist in chemical equilibrium, particularly in aqueous solutions of the acetate ion; the name aluminium acetate is commonly used for this mixed system.

<span class="mw-page-title-main">Metal–metal bond</span>

In inorganic chemistry, metal–metal bonds describe attractive interactions between metal centers. The simplest examples are found in bimetallic complexes. Metal–metal bonds can be "supported", i.e. be accompanied by one or more bridging ligands, or "unsupported". They can also vary according to bond order. The topic of metal–metal bonding is usually discussed within the framework of coordination chemistry, but the topic is related to extended metallic bonding, which describes interactions between metals in extended solids such as bulk metals and metal subhalides.

<span class="mw-page-title-main">Transition metal carboxylate complex</span> Class of chemical compounds

Transition metal carboxylate complexes are coordination complexes with carboxylate (RCO2−) ligands. Reflecting the diversity of carboxylic acids, the inventory of metal carboxylates is large. Many are useful commercially, and many have attracted intense scholarly scrutiny. Carboxylates exhibit a variety of coordination modes, most common are κ1- (O-monodentate), κ2 (O,O-bidentate), and bridging.

Manganese(III) chloride is the hypothetical inorganic compound with the formula MnCl3.

References

  1. ↑ Cotton, F. A.; Walton, R. A. (1993). Multiple Bonds Between Metal Atoms . Oxford: Oxford University Press. ISBN   0-19-855649-7.
  2. ↑ Cotton, F. A.; Hillard, E.A.; Murillo, C. A.; Zhou, H.-C. (2000). "After 155 Years, A Crystalline Chromium Carboxylate with a Supershort Cr–Cr Bond". J. Am. Chem. Soc. 122 (2): 416–417. doi:10.1021/ja993755i.
  3. ↑ Péligot, E.-M. (1844). "Sur un nouvel oxide de chrome" [On a new chromium oxide]. C. R. Acad. Sci. (in French). 19: 609–618.
  4. ↑ Péligot, E.-M. (1844). "Recherches sur le chrome" [Research on chrome]. Ann. Chim. Phys. (in French). 12: 527–548.
  5. ↑ Van Niekerk, J. N.; Schoening, F. R. L. (1953). "X-Ray Evidence for Metal-to-Metal Bonds in Cupric and Chromous Acetate". Nature . 171 (4340): 36–37. Bibcode:1953Natur.171...36V. doi:10.1038/171036a0. S2CID   4292992.
  6. ↑ Ocone, L.R.; Block, B.P. (1966). "Anyhdrous Chromium(II) Acetate, Chromium(II) Acetate 1-Hydrate, and Bis(2,4-Pentanedionato)Chromium (II)". Inorganic Syntheses. Vol. 8. pp. 125–129. doi:10.1002/9780470132395.ch33. ISBN   978-0-470-13239-5.{{cite book}}: |journal= ignored (help)
  7. ↑ Jolly, W. L. (1970). The Synthesis and Characterization of Inorganic Compounds . Prentice Hall. pp.  442–445. ISBN   9780138799328.
  8. ↑ Beneš, L.; Kalousová, J.; Votinský, J. (1985). "Reaction of Chromocene with Carboxylic Acids and Some Derivatives of Acetic Acid". J. Organomet. Chem. 290 (2): 147–151. doi:10.1016/0022-328X(85)87428-3.
  9. ↑ Ray, T. (2004). "Chromium(II) Acetate". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis . New York, NY: J. Wiley & Sons. doi:10.1002/047084289X. hdl: 10261/236866 . ISBN   9780471936237.

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