Chromium dimer

Organometallic chromium dimers have been prepared from CrG2, CrCl3(THF)3, and CrBr2(THF)2 in different reactions. Allyl Grignard reagents form Cr2(allyl)4 (254), while methyllithium in ether produces... 

A comparison of metal-metal bond-length variations for a given metal in differing environments among structures of homonuclear rhenium, molybdenum, and chromium dimers reveals that the flexibility of strong multiple chromium-chromium bonds is distinctly different from the behavior of the two heavier metals. Both rhenium and molybdenum display relatively constant quadruple bond lengths in most structures examined to date, while chromium exhibits a wide range of metal-metal distances. 

The appreciation of the theoretical work notwithstanding, the greatest achievement, however, was undoubtedly the isolation and precise characterization of the novel dinuclear compound 20. Not too long ago, probably nobody would have assumed that simple molecules of the general composition A2B2 with a fivefold bonding between the two atoms A could exist as stable entities under normal conditions. It is a relatively safe bet to predict that Phil Power s chromium dimer was just a start and, as he proposed [152], could be the forerunner of other quintuple bonds . 

These reactions require vigorous conditions . The iron dimer is prepared by refluxing in ethylcyclohexane for 40 h giving 55% yield . The chromium dimer is prepared in 37% yield by a tube furnace reaction for 24 h at 260°C. The molybdenum and tungsten analogues could be prepared in lower yield d. Reaction of Mn(CO)f with PMe2Cl forms a nonacarbonyl (35%) and an octacarbonyl (42%) dimer ... 

A chromium dimer is formed from CrCU and Na cyclooctatetranide in benzene. The Cr2(C8Hg)3 compound exhibits a Cr-Cr distance of 221 pm , consistent with strong metal-metal forces. 

The spirit is to show some of the results, but also to guide users of the approach by pointing to the problems and limitations of the method. The review covers some of the newer applications in the spectroscopy of organic molecules acetone, methylenecyclopropene, biphenyl, bithiophene, the protein chromophores indole and imidazole, and a series of radical cations of conjugated polyenes and polyaromatic hydrocarbons. The applications in transition metal chemistry include carbonyl, nitrosyl, and cyanide complexes, some dihalogens, and the chromium dimer. 

In the introduction it was stated that transition metal-containing systems have often been considered as unusually difficult to treat by theoretical methods. Even though respect for these systems has been exaggerated in many ways, there are definitely cases for which it is still very difficult to obtain results of even qualitative accuracy, and this does not refer to obvious cases of very large molecules. The classically difficult case is the chromium dimer, which has challenged method-oriented theoreticians for at least a decade. However, as solution to the chromium dimer problem is now in sight, and some of the more recent development are described briefly below. 

It is important to emphasize from the outset that metal-metal bonds present a substantirJ challenge to electronic structure theory, particularly where diatomic overlap is weak and the electrons are highly correlated. The chromium dimer, Crj, for example, is a notoriously difficult case and has been the subject of debate for decades [13], Some progress toward a quantitative understanding of these correlation effects has been made through Complete Active Space Self Consistent Field (CASSCF) and related wavefunction-based techniques, but much of our qualitative understanding... 

The rate law for I04(H4l06) oxidation of chromium(III) to chromium(VI) has a second-order dependence on [Cr(III)] concentration and the reaction involves formation of a periodate-bridged chromium dimer [Cr(0H)I04HCr0H] which decays at a rate in excess of 1.29 x 10 s at 25°C and 0.25 M ionic strength with two simultaneous one-electron transfers from the metal centers to the bridge. 

There are, however, situations where the weight of the HF wave function is less than 0,9 (or even much less than 0.9) in the FCI wave function. This is typically the case for molecules where one or several chemical bonds are weak. An extreme example is the chromium dimer where the weight has been estimated to be less than 0.5 at the equilibrium bond distance. For excited states of most molecules this situation is not uncommon. In the late 1970s serious attempts to deal with these cases were made. Instead of forming the HF wave... 

---