Cobalt hydroxo-bridged complexes formation

The data in Table XXXV show that common features for these ammonia and amine complexes are very fast isomerization between the cis and trans isomers of the diaqua species and the fact that the trans diaqua isomers are generally more stable than the cis isomers. In the ammine system the activation parameters for k2 and k 2 are consistent with an isomerization process at cobalt(III), but it is at present not clear how this occurs. It need not be a simple cis-trans isomerization occurring at one of the Co(III) centers, but might involve the participation of both metal centers. The isomerization reaction may proceed via intramolecular proton transfer between a water ligand and one of the two hydroxo bridges with simultaneous bridge cleavage and formation... 

Fig. 13. Formation of a (x-peroxo-bridged complex from a nitrogen ligand complex of cobalt(II) and O2. The p,-peroxo-bridged complex is stabilized in the presence of a second bridging ligand, L, such as amino or hydroxo.

Oxidation of mandelic acid by H2O2, catalyzed by Fe ", is only 50% inhibited by the presence of OH radical traps and formation of an active Fe(IV) species may be a possibility. However it is thought more likely that the uninhibited pathway involves cage reaction of newly formed OH radicals. Peroxytungstic acids are formed in the H2O2 oxidation of dmso catalyzed by WO4 . An extensive study of the oxidation of cobalt(II) by m-Cl-perbenzoic acid in 90% acetic acid media has appeared. The mechanism is complex involving a second-order dependence on [Co(II)] and various dimeric and trimeric 0x0- and hydroxo-bridged species, are formed in the course of the reaction. 

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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