Dibenzene-chromium

In dibenzene chromium, the chromium atom is sandwiched between two benzene rings Figure 13.3)... 

Dibenzene chromium(O), physical properties, 6 528t Dibenzo-crown-6, 5 710 molecular formula, 5 713t Dibenzo[18]crown-6, 7 576 24 40, 41 Dibenzo[30]crown-10, 24 43 Dibenzocrown ethers, 24 50 Dibenzofurans, 27 152 Dibenzopyrenquinone dyes, 9 336 Dibenzothiophenes, 27 152 Dibenzoyl peroxide (BPO), 74 283-284 78 427... 

Subsequently, similar calculations have been made on both ferrocene and dibenzene chromium by Shustorovich and Dyatkina (65), on the ring-metal bonding in the tricarbonylarene chromiums (63), and on ring-metal bonding in the general sandwich molecule (CWHW)2M, with n = 4,..., 8 and M a transition metal (66). The same author has also discussed charge-transfer effects in complexes of the type [CwH M(CO)3] and concludes that maximum transfer occurs for n = 6 (67). The general results of these theories are reasonable but detailed comment is reserved for Sec. III. 

In the case of dibenzene chromium and its cation, the most complete investigation of the electronic spectra is due to Yamada and co-workers (76) who report both solution and crystal spectra in detail for dibenzene chromium iodide and ditoluene chromium iodide. Four intense bands were observed in the region 3 to 6 eV, and a detailed discussion of these in terms of molecular-orbital assignments has been given by Berry (78), although no direct comparison with the energy-level scheme of Shustorovich and Dyatkina (75) is reported. The intense band at 5.51 eV (log max = 4.14) for... 

Similarly, dibenzene chromium and its cation give moments which are in agreement with the simple molecular orbital scheme, viz.,... 

The magnetic properties of dibenzene chromium have also been discussed by the Russian workers (94). 

Diarene metals generally decompose under conditions of normal electrophilic substitution, but dibenzene chromium can be metalated with amyl sodium in alkanes, and subsequent reaction with carbon dioxide and dimethyl sulfate results in a complex mixture of mono-, di-, and poly-substituted dibenzene chromium methyl esters. It is interesting that these polysubstituted compounds are homoannular in contrast to the corresponding ferrocene compounds. In view of the scanty evidence and ease of oxidation, it is impossible to conclude whether the diarene metals are less aromatic than the dicyclopentadienyl metals as predicted by simple theory (Sec. III.A). 

The rings of the dibenzene chromium, molybdenum, and lungsien complexes are eclipsed and have a small rotational barrier. Unlike ferrocene, these complexes have labile rings which can be displaced-... 

Many other unsaturated organic compounds can form 7r complexes with transition metals. A substance that is in some ways analogous to ferrocene is the complex of two benzene molecules with chromium metal, called dibenzene-chromium. The bonding involves zerovalent chromium and the tt electrons of... 

Dibenzene chromium, [Cr(C6H6)2], has an eclipsed sandwich structure with D6h symmetry. It has 3 x 25 - 6 = 69 normal vibrations ... 

The prismatic cyclopentadienyl and benzene complexes of transition metals are reminiscent of the polycyclic hydrocarbon pris-manes. Figure 3-28 shows ferrocene, (C5H5)2Fe, for which both the barrier to rotation and the free energy difference between the prismatic (eclipsed) and antiprismatic (staggered) conformations are very small [75], Figure 3-28 also presents a prismatic model with Ay, symmetry for dibenzene chromium, (C6F[6)2Cr. 

Figure 3-28. Prismatic molecular models. Ferrocene prismatic (D5h) and antipris-matir. (D5d) dibenzene chromium prismatic (Ay,)-...

SYNS CHROMIUM, BIS(BENZENE)-(8CI) CHROMIUM, BIS(eta )-BENZENE)-(9CI) CHROMIUM(II), DIPHENYH DIBENZENE-CHROMIUM DIPHENYLCHROMIUM... 

The cation could be isolated by precipitation as the picrate or the reineckate. By reduction with sodium dithionite it could be converted directly into brown-black dibenzene chromium, Cr(C6He)2, w hich can be sublimed 53,54) ... 

Dibenzene chromium is prepared by the methods which have already been mentioned (53, 107, 237, 228). The compound may be sublimed in a... 

Dibenzoic acid chromium is reported as produced when the mixture formed in the preparation of dibenzene chromium by the Grignard reaction is treated with carbon dioxide (227). Substitution reactions for the hydrogen atoms in the rings of the complex, such as are known to occur readily for ferrocene, do not occur in this case (90). 

The preparation of a compound of this type, C6HeCr(CO)3, was first achieved by the interaction of chromium carbonyl and dibenzene chromium in benzene in an autoclave (67). This substance was also obtained more simply by the reaction (68)... 

X-ray work was particularly important in settling the problem of tne structure of dibenzene chromium, and showed that this compound, unlike the dicyclopentadienyl metal compounds studied earlier, possessed a... 

On the basis of chemical properties and physicochemical inves.tigations dibenzene chromium is considered to have the same spatial distribution of bonds as ferrocene. X-ray studies have shown that the molecule is centrosymmetric, and that all the C-C and Cr-C distances are practically equal. One may therefore decide with certainty on the double cone structure (see Fig. 9). As already mentioned, it may be concluded that the metal-carbon bonds have an approximately octahedral arrangement and that the benzene rings have threefold symmetry. 

Ernst Otto Fischer (bom 1918) was a student of Walter Hieber at Technische Hochschule Miinchen where he received his Ph.D. degree in 1948. He elucidated the molecular stmcture of ferrocene shortly after this compound was discovered. Further highlights in his life s work were the synthesis of dibenzene-chromium (C6H5)2Cr in 1955, the discoveries of the first metal carbene (1967), and the first me-tal-carbyne complex (1971). In 1964 he succeeded Walter Hieber to the chair of inorganic chemistry at Technische Hochschule Miinchen from which he retired in 1985. He received the Nobel prize for chemistry jointly with Geoffrey Wilkinson (Imperial College London) in 1973. 

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