Bis(benzene)chromium is the
organometallic compound
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and s ...
with the
formula Cr(
η6-C
6H
6)
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
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
containing two
arene
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past groupin ...
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
s.
Preparation
The substance is air sensitive and its synthesis requires
air-free technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less ...
s. It was first prepared by Hafner and Fischer by the reaction of
CrCl3, aluminium, and benzene, in the presence of
AlCl3. This so-called reductive Friedel-Crafts method was pioneered by
E.O. Fischer and his students. The product of the reaction was yellow
r(C6H6)2sup>+, which was then reduced to the neutral complex. Idealized equations for the synthesis are:
: CrCl
3 + 2/3Al + 1/3AlCl
3 + 2C
6H
6 →
r(C6H6)2lCl
4
:
r(C6H6)2lCl
4 + 1/2Na
2S
2O
4 →
r(C6H6)2 + NaAlCl
4 + SO
2
Using the technique of
metal vapor synthesis In chemistry, metal vapor synthesis (MVS) is a method for preparing metal complexes by combining freshly produced metal atoms or small particles with ligands. In contrast to the high reactivity of such freshly produced metal atoms, bulk metals typ ...
, bis(benzene)chromium and many analogous compounds can be prepared by co-condensation of Cr vapor and arene. In this way, the
phosphabenzene
Phosphorine (IUPAC name: phosphinine) is a heavier element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. It is a colorless liquid that is m ...
complex
r(C5H5P)2can be prepared.
Structure elucidation
Compounds closely related to
r(C6H6)2sup>+ had been prepared many years before Fischer's work by
Franz Hein by the reaction of
phenylmagnesium bromide and CrCl
3. Hein's reaction affords cationic
sandwich complex
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic, covalent bonds to two arene (ring) ligands. The arenes have the formula , substituted derivatives (for example ) and heterocyclic derivat ...
es containing
bi- and
terphenyl
Terphenyls are a group of closely related aromatic hydrocarbons. Also known as diphenylbenzenes or triphenyls, they consist of a central benzene ring substituted with two phenyl groups. There are three substitution patterns: ''ortho''-terpheny ...
, which baffled chemists until the breakthrough by Fischer and Hafner. (Indeed, although Harold Zeiss and Minoru Tsutsui of Yale University had earlier proposed the "sandwich" structure, they were unable to convince skeptical journal referees to publish their manuscript until Fischer and Hafner's results were made known.) Fischer and Seus soon prepared Hein's
r(C6H5-C6H5)2sup>+ by an unambiguous route, thus confirming that Hein had unknowingly discovered sandwich complexes, a half-century ahead of the work on
ferrocene. Illustrating the rapid pace of this research, the same issue of
''Chem. Ber.'' also describes the Mo(0) complex.
Reactions
The compound reacts with carboxylic acids to give chromium(II) carboxylates, such as
chromium(II) acetate
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 prepara ...
, which have interesting structures. Oxidation affords
r(C6H6)2sup>+. Carbonylation gives
(benzene)chromium tricarbonyl
(Benzene)chromium tricarbonyl is an organometallic compound with the formula . This yellow crystalline solid compound is soluble in common nonpolar organic solvents. The molecule adopts a geometry known as “ piano stool” because of the plana ...
.
The compound finds limited use in
organic synthesis.
[Herndon, J. W. "Dibenzenechromium" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. .]
References
{{DEFAULTSORT:Bis(Benzene)Chromium
Organochromium compounds
Sandwich compounds