Inorg. Chem. 1984,the rate constants, kab and kba, for equilibration of the two excited-state pentacoordinate intermediates be larger than those for the competing deactivation pathways, k,, and knb, i.e., have values >lo9 s-*, a reasonable estimated lower limit for the deactivation rates.32 In addition, it should be emphasized that the product stereochemical distribution does not simply represent the equilibrium ratio [sPb*]/ [spa*] but must also include the relative k, values, i.e.Only by assuming that k,, N knb can the product ratio be used to infer the relative energies of sPb* and spa*.The photochemistry of the rhodium(II1) tetraammine complexes has proved very rich both in terms of photostereochemistry and in terms of ligand effects on excited-state reaction dynamics. The stereochemical lability of these complexes upon photosubstitution of a ligand clearly illustrates the difference between the photosubstitution and thermal substitution mechanisms, and consideration of the fine details provides strong circumstantial evidence for a limiting dissociative pathway as the key step of the photosubstitution mechanism. The application of pulse laser excitation techniques has allowed evaluation of the rates of the ligand substitution (dissociation) from the lowest energy (triplet) excited states of these complexes. For analogous complexes the labilization of a ligand X generally follows the order H 2 0 > C1-> Br-> I-, perhaps reflecting the relative abilities of these to K bond to an excited-state metal core with a (d,)5(d,)'(32) For analogous iridium(II1) complexes, cis-and trans-dihalo pairs do not give common product stereochemical distributions, presumably because the higher nonradiative deactivation rates for these heavy-metal complexes are too rapid to allow full equilibration (Talebinasab-Sarvari, M.; Ford, P. C. Inorg. Chem. 1980, 19, 2640).
23, 4538-4545configuration. Similar wbonding considerations may explain the inverse order of these ligands in terms of the labilization rates for NH, loss. The stereochemical positions of these ligands also have major consequences. With regard to NH, substitution, most such labilization in haloammine complexes apparently occurs at a position trans to the halide. In contrast, for dihalo or aquohalo complexes, those having the cis configuration proved to have the greater ES labilities. The opposite is the case for the hydroxohalo complexes. However, both these observations are consistent with the following pattern for the cis and trans pairs Rh(NH3)4XY": The more labile excited-state isomer in each case is also the one that undergoes the greater concomitant isomerization to the final product. A similar apparent coupling of ligand and stereochemical photolability has been reported for the isomers of dihalo trien rhodium(II1) complexes.33 Such observations may suggest some synchronous nature to the ligand dissociation and the isomerization mechanism, a possibility that is not addressed by the mode illustrated in Scheme II.7a38a331 While we remain convinced that this model (or...