Dicobalt III Complexes

The fission and formation of a variety of mono-, di, and tri-bridged di-cobalt(III) complexes will be discussed in this section, in this order. At the end comparison will be made with a cobalt(III)-iron(II) system, very closely related in that again both metal ions are low-spin centers. 

The kinetics of the equilibrium between tri-(x-hydroxo-bis(l,4,7-triazacy-clononane)cobalt(III) and frfl/z5 -diaquo-di-(x-hydroxo-bis(l,4,7-triazacyclonon-ane)cobalt(III), 1,4,7-triazacyclononane = 45, in acid aqueous solution have 

A study of the kinetics of dissociation of the dinuclear complex [(edta)Co(III)-NC-Fe(II)(CN)4(4,4 -bipy)] forms a key part of an investigation of the reduction of [Fe(CN)5(4,4 -bipy)] by cobalt(II)-edta. The kinetic characteristics of this Co(III)-NC-Fe(II) species in fact show that the redox process cannot involve the intermediacy of this x-cyano species, and indeed must go by an outer-sphere mechanism. 

---

The use of a metalated container in stereoselective transformations has only recently begun to be explored (284). This prompted us to study the bromination of the a,/J-unsaturated carboxylate ligands in dicobalt(III) complexes of the type [Co2(Ll9)( -02CCH = CHR)]3+ (256). 

The acrylato-bridged dicobalt(III) complex 97 (241) was selected in an orienting experiment (Scheme 9). Indeed, bromination of the kinetically inert complex proceeded smoothly and produced complex 98 as the sole product, as established by IR... 

Also, Chin s dihydroxy-bridged dicobalt(III) complex 10 provides one of the largest rate enhancements ever observed for phosphodiester hydrolysis [36]. Reaction with the activated phosphodiester methyl(p-nitrophenyl)phosphate (MNPP) yields 11. A crystal structure is available for the analogous dimethylphosphate complex in which the Co ions are 2.9 A apart. At pH 7 and 45 °C 11 releases p-nitrophenolate with k = 0.1 s 1 which corresponds to a nearly 10I2-fold rate enhancement over spontaneous hydrolysis of the substrate. The product of the reaction is a methylphosphate-bridged complex. Based on the pH rate profile and on... 

A. Hydroxo Bridge Cleavage of Amido-Bridged Dicobalt(III) Complexes... 

Some representative spectral data are given in Table VI. The spectroscopic properties of /x-hydroxo-/i-peroxo and -hydroxo/x-superoxo dicobalt(III) complexes have recently been reviewed by Fallab and Mitchell (119) and seem reasonably well rationalized in terms of the molecular orbital (MO) models formulated by Lever and Gray (120). The spectra of -hydroxo -/x-peroxo and /i-hydroxo-/i-superoxo complexes of rhodium(III) show features similar to those of the cobalt(III) species (121-124). 

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. 

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... 

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. 

---