Chromium hydroxo-bridged complexes formation

Catalysis by chromium(n) involves the formation of a Cr —OH—Cr hydroxo-bridged species. Some preliminary data are also reported for the analogous nicotinamide system. A similar study involves the decomposition of a uranium(v)-chromium(ra) dinuclem complex in the presence of excess chromium(n). Approximate rate data for the uncatalysed reactions are shown in Table 12. 

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

The easy formation of hydroxo- or oxo-bridged Cr111 polymers in basic aqueous solution, the comparative lability of the Cr—N bond, and the precautions needed to obtain chromium(II) complexes compared with cobalt(II) complexes have meant that the preparative chemistry of chromium(III) is more difficult than that of cobalt(III). A greater variety of non-aqueous solvents is now in use, and there is greater knowledge of chromium(II) chemistry to be exploited in the preparation of chromium(III) complexes generally, but few new methods of preparation of amine complexes have been devised since the early work. 

Di-/Li-hydroxo-bis[bis(ethylenediamine)cobalt(III)] dithionate may be obtained analogously to the chromium(III) salt from cis- [aquabis(ethylenediamine)hydrox-ocobalt(III)] dithionate, either by heating at 110° or by refluxing in acetic anhydride. The formation of the bridged cation of cobalt(III) is much slower than that of chromium(III). In contrast to the chromium(III) complex there is no evidence that the bridged cobalt(III) complex can be formed by aqueous hydrolysis. 

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