# Bis(benzene)chromium

Bis(benzene)chromium is the organometallic compound with the formula Cr(η$^6$-C$_6$H$_6$)$_2$. 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 containing two arene ligands.

## Historical background

In the late 1910s, Franz Hein started the investigation of "triphenylchromium" by reacting chromium trichloride with a Grignard reagent, phenyl magnesium bromide. Such a reaction gave a mixture of phenyl chromium and Hein suggested that it contained a Cr(VI) species, "(C$_6$H$_5$)$_5$CrBr", generated via valence disproportionation.
    5 C$_6$H$_5$MgBr + 4 CrCl$_3$ → (C$_6$H$_5$)$_5$CrBr + 2 MgBr$_2$ + 3 MgCl$_2$ + 3 CrCl$_2$
This event marked an advance in organochromium chemistry at the time and "(C$_6$H$_5$)$_5$CrBr" was described to have salt-like properties. However, the reported workup procedures for "(C$_6$H$_5$)$_5$CrBr" was challenging and the yield was low. Later scrutinization by Zeiss and Tsutsui revealed that Hein's formulation of the chromium-containing products was flawed.

## Preparation

The actual discovery of bis(benzene)chromium was largely contributed by Ernst Otto Fischer and Walter Hafner in the 1950s. Ernst Otto Fischer postulated that it might be possible to synthesize a neutral chromium(0) complex with two benzene ligands, which has a sandwich structure, similar to that of ferrocene. In 1954, Walter Hafner, a PhD student of Ernst Otto Fischer at the time, put the idea into practice. A reaction of chromium trichloride, aluminium trichloride, aluminium powder in m-xylene resulted in the formation of yellow [Cr(C$_6$H$_6$)$_2$]$^+$, which was then reduced by sodium dithionite in aqueous sodium hydroxide. The resulting solid was determined to be the target, bis(benzene)chromium.
    6 C$_6$H$_6$ + 3 CrCl$_3$ + 2 Al + x AlCl$_3$ → 3 [(C$_6$H$_6$)$_2$Cr][AlCl$_4$]·(x−1)AlCl$_3$
    2 [(C$_6$H$_6$)$_2$Cr]$^+$ + S$_2$O2−4 + 4 OH$^−$ → 2 (C$_6$H$_6$)$_2$Cr + 2 SO2−3 + 2 H$_2$O
It was noted that excess aluminium trichloride is needed to solubilize the product. The substance is air sensitive and its synthesis requires air-free techniques. The reaction, utilizing Al and AlCl$_3$, is so-called the reductive Friedel-Crafts method pioneered by Fischer and his students.
Fischer and Seus soon prepared Hein's [Cr(C$_6$H$_5$−C$_6$H$_5$)$_2$]$^+$ 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.
Using the technique of metal vapor synthesis, bis(benzene)chromium and many analogous compounds can be prepared by co-condensation of Cr vapor and arenes. In this way, the phosphabenzene complex [Cr(C$_5$H$_5$P)$_2$] can be prepared.

## Properties and characterization

Bis(benzene)chromium is thermally stable under an inert gas atmosphere. As predicted, it is diamagnetic with a dipole moment of zero. In 1956, Fischer and Weiss reported the crystal structure of bis(benzene)chromium to be centrosymmetric and has a cubic symmetry. Electrochemical studies of bis(benzene)chromium suggested that the half-wave potential (E$_{1/2}$) of the +1/0 couple is around -1.10 to -1.25 V versus Fc$^+$/Fc at 298.15K, depending on the experimental conditions.

## Bonding and electronic structure

Theoretical chemical bonding of bis(benzene)chromium have been investigated since the discovery of this compound. The ground state configuration is (3e$_{2g}$)$^4$(4a$_{1g}$)$^2$ (3e$_{2u}$)$^0$. Analysis of the frontier orbitals suggested that the chromium-benzene interaction is largely contributed by the 𝝅 and/or 𝞭 interactions between the 3d metal orbitals and ligand 𝝅 orbitals. 3e$_{2g}$ (HOMO-1) and 3e$_{1g}$ (HOMO-2) molecular orbitals are 𝞭-bonding interactions between metal 3d𝞭 and ligand 𝝅 orbitals. The highest occupied molecular orbital (HOMO), 4a$_{1g}$, is the non-bonding metal d$_{z2}$ orbital. The lowest unoccupied molecular orbital (LUMO) is 3e$_{2u}$, which is purely ligand 𝝅 orbital. As for 4e$_{1g}$ (LUMO+1) and 4e$_{2g}$ (LUMO+2), they are composed of anti-bonding interaction between 3d𝝅 and ligand 𝝅 orbitals.
Charge Distribution (%) of Ground State Bis(benzene)chromium :
| | | Cr | Cr | Cr | C | C | C | H |
| Molecular Orbital | No. of Electrons | s | p | d | s | p𝝈 | p𝝅 | s |
| 4e$_{1g}$ | 0 | | | 73 | 2 | 2 | 10 | |
| 3e$_{2u}$ | 0 | | | | | | 73 | |
| 4a$_{1g}$ | 2 | 1 | | 82 | | 1 | 1 | 1 |
| 3e$_{2g}$ | 4 | | | 50 | | 2 | 21 | |
| 3e$_{1g}$ | 4 | | | 16 | | 1 | 52 | |
| 4e$_{1u}$ | 4 | | 4 | | | | 49 | |
| 2e$_{2u}$ | 4 | | | | | 60 | | 17 |
| 2e$_{2g}$ | 4 | | | 1 | | 60 | | 18 |
| 4a$_{2u}$ | 2 | | 4 | | | | 61 | |
| 3a$_{1g}$ | 2 | 8 | | | | | 53 | |
3d orbitals population of chromium(0) in bis(benzene)chromium was investigated, utilizing NBO analysis. While e$_{2g}$ largely results from electron donation from the metal to the ligand, e$_{1g}$ is mainly composed of the electrons donated from the benzene ligands.
3d orbital population of Cr in Bis(benzene)chromium :
| Molecular Orbitals | NBO | X-ray |
| 4a$_{1g}$ | 1.896/36% | 1.62(1)/35% |
| 3e$_{2g}$ | 2.412/45% | 1.953(7)/42% |
| 3e$_{1g}$ | 1.026/19% | 1.112(7)/24% |
In contrast to ferrocene, where 𝝅-interactions dominate the metal-ligand bonds, 𝞭-interactions play a significant role in bis(benzene)chromium.
Energy Decomposition Analysis at BP86/TZP of Ferrocene and Bisbenzenechromium :
| | | Ferrocene | Bis(benzene)chromium |
| Electrostatic Interaction % | Electrostatic Interaction % | 51.1 | 37.9 |
| Covalent Interaction % | Covalent Interaction % | 48.9 | 62.1 |
| Contribution to Covalent Interaction % | 𝝅-interactions (e$_{1g}$) | 64.7 | 14.7 |
| Contribution to Covalent Interaction % | 𝞭-interactions (e$_{2g}$) | 8.3 | 73.4 |

## Reactivity

The compound reacts with carboxylic acids to give chromium(II) carboxylates, such as chromium(II) acetate. Oxidation affords [Cr(C$_6$H$_6$)$_2$]$^+$. Carbonylation gives (benzene)chromium tricarbonyl.
In late 1990s, Samuel and coworkers revealed that bis(benzene)chromium is an efficient organometallic radical scavenger. In contrast to cobaltocene, which traps radicals (R．) to form 19-valence electron species (η$^5$-C$_5$H$_5$)(η$^4$-C$_5$H$_5$R)Co, bis(benzene)chromium reacts with radicals to form 17-valence electron species (η$^6$-C$_6$H$_6$)(η$^5$-C$_6$H$_6$R)Cr (R = H, D, isobutyronitrile).
Subsequently, Bis(benzene)chromium was reported to catalyze hydrosilation of alcohols and aldehydes. Unlike late transition metal catalyzed processes involving oxidative addition, the mechanism of this reaction might involve radicals and hydrogen atom abstraction.
The compound finds limited use in organic synthesis.