Dibenzenechromium

Bailey, M. F. Dahl, L. F. (1965) "Three-dimensional crystal structure of benzenechromium tricarbonyl with further comments on the dibenzenechromium structure, Inorg. Chem. 4, 1314-1319. 

Retention in dibenzenechromium was studied by Baumgartner, Zahn and Seeholzer who found 11.8% retention in samples processed by dissolution and then sublimation, and 19.4% in samples sublimed directly. The difference was interpreted as resulting from a rapid thermal annealing during the sublimation. 

Dibenzenechromium was studied by Baumgartner et al. 15). They found that the yield of Cr(Ph)2 was 11.8%. [One cannot fail to be struck by the similarity in yields of FeCp2, RuCp2, and Cr(PhH)2, although it may well be merely coincidence.] On heating the irradiated samples to 110°C, they found the yield to increase to 19.4%. It was found that dissolution of the radioactive crystals in benzene yielded no further Cr(PhH)2. This stands in contrast to the above-mentioned results of Zahn and Harbottle which, though not strictly comparable, show Cp to be quite reactive toward ruthenium atoms. Dibenzenechromium was also formed in low yield 14) from neutron irradiation of PhHCr(CO)3, as will be discussed in more detail later. 

The transfer of charge from the metal to the ligand caused by back-donation can also be seen from a comparison of the ionization potentials of chromium in different complexes. The ionization potential of complex compounds is higher than that of the uncomplexed coordination center (6.76 eV), e.g. for dibenzenechromium ) 7.07 eV, and for hexacarbonyl-chromium 8.03 eV 43). It may be expected that a decrease in the net positive charge at the coordination center will give rise to an increase in ti-EPD properties. Hence the Fe-C distance will be shorter in [Fe(CO) 4] than in Fe(CO) 5. Likewise iron is more strongly coordinated in ferrocene than in the ferrocinium ion. 

Figure 8.21 An energy level diagram for dibenzenechromium. The positions of ring and chromium orbitals on this diagram are only approximate.

The cation can be reduced to dibenzenechromium with sodium dilhionile, Na-jS-.Oj. The dibenzene complexes of Cr, Mo. and W are all air sensitive and those of Mo and W are especially so. Dibenzenechromium is a black solid that melts at 280 °C. 

Although dibenzenechromium is thermally quite stable, it is less so than ferrocene and melts with decomposition at 285° to give benzene and metallic chromium. Furthermore, it appears to lack the aromatic character of either benzene or ferrocene as judged by the fact that it is destroyed by reagents used for electrophilic substitution reactions. 

Reductive methods are often employed, for example in the preparation of dibenzenechromium(O) Cr(C6H6)2 ... 

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