STEREOCHEMICAL CONSEQUENCES OF THE LIGAND-FIELD PHOTOLYSES OF DIHALOTETRAAMINERHODIUM AND AQUOHALOTETRAAMMINERHODIUM(III) COMPLEXES IN AQUEOUS-SOLUTION

Ligand field excitation of the cis-disubstituted tetraamminerhodium(III) complexes cis-Rh(NH3)4XY‘+ (X- = CI- or Br-, Y = X- or H2o) in aqueous solution leads in each case to the corresponding trans-Rh(NH3)4(H2O)X2+ ion as the predominant product. Quantum yields are high, falling in the range 0.24-0.50 mol/einstein, and there is no indication of stable intermediates formed under conditions of continuous photolysis. Hence, at least for the dihalo complexes Rh(NH3)4X2+, photoaquation of a ligand and cis to trans isomerization are concomitant. In contrast, photoaquation of halide from the trans-Rh(NH3)4X2+ ions is stereoretentive. These results are interpreted in terms of the dissociation of halide from a ligand field excited state of Rh(NH3)4X2+ to give a pentacoordinate intermediate Rh(NH3)4X2+, having a square-pyramidal configuration and displaying an energetic preference for the isomer with X- in the apical position. In addition it is argued that the square-pyramidal intermediate undergoing isomerization is in an excited electronic state and that deactivation to the singlet ground electronic state of the same coordination leads to immediate trapping by H2O without further isomerization. The latter point draws some support from the thermochemical studies of the nitrous acid decomposition of the azido complex cis-Rh(NH3)4(N3)Cl+ which proceeds with retention of configuration to give cis-Rh(NH3)4(H2O)Cl2+. Also described are the syntheses of the dithionate salts of the mixed-ligand complexes cis- and trans-Rh(NH3)4(H2O)Cl2+ and cis- and trans-Rh(NH3)4(H2O)Br2+. © 1979, American Chemical Society. All rights reserved.