Photochemistry of Cobalt III Complexes

The photochemistries of the mixed ligand cf complexes Rh(NH3)5X (X = Cl, Br, I), Rh(NH3)5L (L=CH3CN, py) and Ru(NH3)5L (L=N2, py, H2O) have been widely studied. In contrast to the photochemistry of cobalt(III) complexes where both substitution and redox reactions are observed, the photochemistry of the analogous rhodium(III) complexes results in substitution ... 

It will become evident from this discussion of cobalt(III) and chromium(III) photochemistry that a fortunate combination of many factors makes these systems especially amenable to mechanistic investigation this is not the case for most other inorganic systems. For this reason, the progress made toward understanding the photochemical reactions of cobalt(III) and chromium(III) complexes may not be easily duplicated in other areas and it is useful to inquire whether conclusions reached here can be generalized. It is my belief that they can, and this and the next section are intended to place the subsequent discussion in the perspective of inorganic photochemistry. 

Much less is known about photoinitiated ligand exchange reactions of coordination complexes of cobalt(III) and other de ions than is the case for chromium(III). With the exception of the cobalt(III) ammines, however, available data suggest that photochemical ligand exchange reactions of cobalt(III) and d6 systems involve the thermally equilibrated 17 g and/or 37, states of the complex as intermediates. The reasoning is completely analogous to that described earlier with respect to chromium(III) photochemistry. 

The photochemistry of the water-insoluble [Co(sep)](B(C6H5)4)3 complex was studied in aprotic solvents (THF, AN, and DMF) [309]. When THF solution was irradiated with visible light, the cobalt(III) complex has been reduced by the tetraphenylborate anion to give a white [Co(sep)](B(C6H5)4)2 precipitate. From the residual solution biphenyl and phenol were isolated as the main products. This photoreaction proceeded much slower in DMF solution. 

Weit, S. K. Ferraudi, G. Grutsch, P. A. Kutal, C. Charge-transfer spectroscopy and photochemistry of alkylamine cobalt(III) complexes. Coord. Chem. Rev. 1993, 128, 225-243. 

Photochemistry of cis-[Co(en)2(N02)2] in neat acetonitrile differs from that in water. In aqueous solution excitation of the complex ion in the ligand-to-metal charge transfer band gives the cobalt(II) ion and the nitrito linkage isomer. In acetonitrile, an intermediate cobalt(II) complex coordinated to NO2 is produced. As previous studies have shown, the solvent employed has a marked effect on the reactivity pattern of excited states. The viscosity dependence of the photoaquation of [Co(CN)j " and a variety of Cr(III) complexes has been studied using glycerol-water mixtures. " Cage effects are not important in the photosubstitution of CN in the systems studied. 

Whereas for thermal substitution the mechanistic picture is clearer for cobalt(m) complexes than for chromium(m) complexes, for photochemical reactions the reverse is the case. Currently there is much interest, and some success, in bringing the level of understanding of the photochemistry of cobalt(m) and other octahedral complexes up to that of chromium(in) complexes. A recent review of photochemical reactions of transition-metal complexes concentrates on chromium(iii) complexes, but also contains some information on cobalt(ni) and platinum(n) complexes. 

It is several years since sufficient pattern had emerged in photoreactions of chromium(iii) complexes for Adamson to expound his empirical rules to codify this area of photochemistry. A general pattern for photoreactions of cobalt(m) complexes has been slower to emerge, principally owing to the difficulties attendant on characterizing primary photoproducts of redox processes, since cobalt(ii) is such a labile centre. However, a model based on known antibonding properties of excited states has recently been shown both to rationalize Adamson s empirical rules for chromium(ra) complexes and to... 

The synthesis, structure, and catalytic properties of the square-planar cobalt(III) complex (3) have recently been reported. The photochemistry has... 

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