Chromocene

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]

Chromocene

Chromocene 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)	1 0 0
Related compounds
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

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

1. 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.
2. "The Nobel Prize in Chemistry 1973". Nobel Foundation. Retrieved 3 December 2012.
3. 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.
4. 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.
5. Long, N. J. (1998). Metallocenes: Introduction to Sandwich Complexes. London: Wiley-Blackwell. ISBN 978-0632041626.
6. 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.
7. 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
8. Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Wiley-VCH: Weinheim. ISBN 3-527-28165-7.
9. 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.
10. 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.