Dicobalt complexes, peroxo-bridged

The reaction of a dicobalt complex bridged by a peroxo ligand with the sulfite ion has been reported. In this reaction, a dicobalt complex containing a bridging sulfato ligand is formed, along with free sulfate (196). 

The oxidation of aqueous solutions of cobalt(II) salts in the presence of ligands may give a variety of products depending on the conditions. Air oxidation of Co(en)2(H20)22+ in the presence of charcoal as catalyst yields Co(en)33 +, and without charcoal catalyst the reaction gives either the peroxo-bridged dicobalt(III) complex (en)2Co(0H)(02)-Co(en)23+ or the tetranuclear complex Co (OH)2Co(en)2 36+, depending on the conditions. 

The Co(III) complex formed in reaction 28.4 can be considered to contain coordinated [02], but the presence of the axial base, L, is crucial to the formation of the monomeric product. In its absence, a dicobalt species with a Co—0—0—Co peroxo-bridge (i.e. analogous to those discussed in Section 21.10) is formed. 

Peroxo-bridged dicobalt(III) complexes are often regarded as models for biological oxygen carriers, "" "" and there has been considerable interest in the formation and breakdown of such complexes in aqueous solution. 

Some qualitative information on the lability of several A -peroxo-dicobalt ammine or amine compounds can be gleaned from a primarily spectroscopic paper dealing with these species. Further mention of kinetic studies of bridged dicobalt complexes will be found later in this chapter, in the sections on metal-ion-catalysed aquation and on formation reactions. 

Redox equilibria between jU-peroxo-/i-hydroxo dicobalt(III) complexes and their oxidized superoxo-bridged form have been discussed in a recent review article by Fallab and Mitchell (119). Corresponding dirhodium(III) complexes have recently been reported (121-124). Reduction of peroxo- (or superoxo)-bridged dicobalt(III) complexes provides a method for inserting a hydroxo bridge, as shown in reaction Eq. (26) (323). 

The kinetics of formation from and decomposition into Co(II) and 02 of jU-hydroxo-jU-peroxo dicobalt(III) complexes have been studied in detail, and the mechanistic aspects have been discussed in a recent article by Fallab and Mitchell (119). Two different mechanisms have been invoked to explain the kinetics of the first bridge cleavage of L4Co(0H)(02)CoL43+ species. One mechanism involves hydroxo bridge cleavage and formation, as shown in Eq. (71). The parameter values... 

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