Chromocene

Names

IUPAC name

Bis(η⁵-cyclopentadienyl)chromium(II)

Other names

Dicyclopentadienylchromium(II)

Identifiers

CAS Number

1271-24-5^Y

3D model (JSmol)

Interactive image

ChEBI

CHEBI:30677

ChemSpider

71485

ECHA InfoCard

100.013.670

EC Number

215-036-4

Gmelin Reference

3366

PubChem CID

79154

RTECS number

GB7600000

UNII

FI251W8B1L^Y

UN number

1325

CompTox Dashboard (EPA)

DTXSID7024615

InChI

InChI=1/2C5H5.Cr/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2
Key: TYYBBNOTQFVVKN-UHFFFAOYAS

SMILES

[Cr+2].[cH-]1cccc1.c1[cH-]ccc1

Properties

Chemical formula

C₁₀H₁₀Cr

Molar mass

182.186 g·mol⁻¹

Appearance

dark red crystals

Density

1.43 g/cm³

Melting point

168 to 170 °C (334 to 338 °F; 441 to 443 K)

Boiling point

Sublimes (under vacuum)

Solubility in water

Insoluble

Structure

Coordination geometry

Pseudooctahedral
see Ferrocene

Dipole moment

0 D

Hazards

Occupational safety and health (OHS/OSH):

Main hazards

Pyrophoric

GHS labelling:

Pictograms

Signal word

Danger

Hazard statements

H302, H312, H314, H315, H317, H319, H332, H335

NFPA 704 (fire diamond)

Related compounds

Fe(C₅H₅)₂
Ni(C₅H₅)₂
bis(benzene)chromium
chromium(II) acetate

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Infobox references

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).^[1]

Synthesis

Ernst Otto Fischer, who shared the 1973 Nobel Prize in Chemistry for work on sandwich compounds,^[2] first described the synthesis of chromocene.^[3]^[4] One simple method of preparation involves the reaction of chromium(II) chloride with sodium cyclopentadienide:

CrCl₂ + 2 NaC₅H₅ → Cr(C₅H₅)₂ + 2 NaCl

Such syntheses are typically conducted in tetrahydrofuran. Decamethylchromocene, Cr[C₅(CH₃)₅]₂, can be prepared analogously from LiC₅(CH₃)₅. Chromocene can also be prepared from chromium(III) chloride in a redox process:^[5]

2 CrCl₃ + 6 NaC₅H₅ → 2 Cr(C₅H₅)₂ + C₁₀H₁₀ + 6 NaCl

Structure and bonding

The structure of chromocene has been verified by X-ray crystallography. The average Cr–C bond length is 215.1(13) pm.^[6] Each molecule contains an atom of chromium bound between two planar systems of five carbon atoms known as cyclopentadienyl (Cp) rings in a sandwich arrangement, which is the reason its formula is often abbreviated as Cp₂Cr. Chromocene is structurally similar to ferrocene, the prototype for the metallocene class of compounds. Electron diffraction studies suggest that the Cp rings in chromocene are eclipsed (point group D_5h) rather than staggered (point group D_5d), though the energy barrier to rotation is small.^[7]

With only 16 valence electrons, it does not follow the 18-electron rule.^[8] It is a paramagnetic compound.

Reactions

The main reactivity associated with chromocene follow from it being highly reducing and the lability of the Cp ligands.

The complex exhibits diverse reactions, usually involving displacement of one cyclopentadienyl ring. Carbonylation has been examined in detail, leads ultimately to chromium hexacarbonyl. An intermediate is cyclopentadienylchromium tricarbonyl dimer:^[9]

2 Cr(C₅H₅)₂ + 6 CO → [Cr(C₅H₅)(CO)₃]₂ + "(C₅H₅)₂"

Chromocene provides a convenient route for preparing the anhydrous form of chromium(II) acetate,^[10] a useful precursor to other chromium(II) compounds. The reaction involves the displacement of cyclopentadienyl ligands by the formation of cyclopentadiene:

4 CH₃CO₂H + 2 Cr(C₅H₅)₂ → Cr₂(O₂CCH₃)₄ + 4 C₅H₆

Chromocene decomposes on contact with silica gel to give the Union Carbide catalyst for ethylene polymerization, although other synthetic routes exist for the formation of this important catalyst.

Safety

Chromocene is highly reactive toward air and could ignite upon exposure to the atmosphere.

References

^ Crabtree, R. H. (2009). The Organometallic Chemistry of the Transition Metals (5th ed.). Hoboken, NJ: John Wiley and Sons. p. 2. ISBN 978-0-470-25762-3.
^ "The Nobel Prize in Chemistry 1973". Nobel Foundation. Retrieved 3 December 2012.
^ Fischer, E. O.; Hafner, W. (1953). "Di-cyclopentadienyl-chrom". Z. Naturforsch. B (in German). 8 (8): 444–445. doi:10.1515/znb-1953-0809. S2CID 98255073.
^ Fischer, E. O.; Hafner, W. (1955). "Cyclopentadienyl-Chrom-Tricarbonyl-Wasserstoff". Z. Naturforsch. B (in German). 10 (3): 140–143. doi:10.1515/znb-1955-0303.
^ Long, N. J. (1998). Metallocenes: Introduction to Sandwich Complexes. London: Wiley-Blackwell. ISBN 978-0632041626.
^ Flower, K. R.; Hitchcock, P. B. (1996). "Crystal and Molecular Structure of Chromocene (η⁵-C₅H₅)₂Cr". J. Organomet. Chem. 507 (1–2): 275–277. doi:10.1016/0022-328X(95)05747-D.
^ Davis, R.; Kane-Maguire, L.A.P. (1982), "Chromium Compounds with η2–η8 Carbon Ligands", Comprehensive Organometallic Chemistry, Elsevier, pp. 953–1077, doi:10.1016/b978-008046518-0.00041-6, ISBN 978-0-08-046518-0, retrieved 2023-03-26
^ Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Wiley-VCH: Weinheim. ISBN 3-527-28165-7.
^ Kalousová, Jaroslava; Holeček, Jaroslav; Votinský, Jiři; Beneš, Ludvík (2010). "Das Reaktionsverhalten von Chromocen". Zeitschrift für Chemie. 23 (9): 327–331. doi:10.1002/zfch.19830230903.
^ 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.