Rhodium complexes photosubstitution

Figure 2.47 The limiting photosubstitution mechanism for rhodium(III) ammine complexes. (Reprinted from Coord. Chem. Rev., 94, 151, 1989, with kind permission from Elsevier Science S.A., P.O. Box 564, 1001 Lausanne, Switzerland.)...

Although the models have proved to be useful tools for rationalizing some aspects of the photosubstitutional behavior of simple transition metal complexes, they are not without deficiencies. For example, the predictions of reactivity made with the models are only qualitative. Thus a reaction that is predicted for a particular complex may not occur at all. Another important deficiency of the models was recently discussed by Ford (50). In the series of analogous rhodium(III) j omplexes, Rh(NH3)5X"" (X = NH3, H2O, 0H , Cl, Br, and I"), relative quantum yields of ligand substitution are strongly dependent on the rates of physical radiationless decay of the excited complexes to the ground state species. According to the Zink model, however, relative quantum yields within such a series should reflect "reactivities" of the excited state complexes (44,47). 

Thus the activation volume AV for the rate constant kp of an individual ES reaction pathway can be evaluated if the pressure dependencies of the photoreaction quantum yield, of intersystem crossing and of the ES lifetime can be separately determined. However, such parameterization becomes considerably more complex if several different excited states are involved or if a fraction of the photosubstitution products are formed from states that are not vibrationally relaxed with respect to the medium. Currently, parameterization of pressure effects on photosubstitutions has been attempted for a limited number of metal complexes. These include certain rhodium(III) and chromium(III) amine complexes and some Group VI metal carbonyls, which will be summarized here. 

Another rhodium(III) system for which both quantum yield and lifetime pressure effects have been measured under analogous conditions is the bis(bipyridine) complex, ds-Rh(bpy)2Cl2 in aqueous solution [86]. The AF , values have also been reported for the photosubstitution quantum yields of Rh(NH3)e + [87] and for the concomitant photoaquation/photo-isomerization reactions of the tetraammine complexes cis- and trans-Rh(NH3)4X2 (X = Cl or Br) [81]. These data are summarized in Table 3. 

There have been several reviews of mechanisms of photosubstitution in rhodium(III) complexes. Bond indexes for ground and excited states have been discussed in relation to D2h species. " The observation of stereospecificity has been discussed in relation to lifetimes for triplet singlet deactivation and geometric rearrangements. Direct evidence has been presented to support the intermediacy of, and role of rearrangement in, five-coordinate intermediates in ligand field irradiation experiments. Rhodium(III) has been discussed in relation to cobalt(III) and iridium(III), and to ruthenium(II) and ruthenium(III) as well. ... 

The major photochemical pathway for cobalt(in) complexes in solution is substitution. By contrast with the complexes of rhodium(IU) and iridium(III), the quantum yields for photosubstitution for cobalt(III) complexes are low. This feature is shown in the homologous ammine complexes M(NH3)e, where the substitution quantum yields for ammine substitution in rhodium(III) and iridium(UI) complexes... 

Two general reviews of photosubstitution and related reactions devote considerable space to the photochemistry of rhodium(III) complexes. ... 

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