The photochemistry of some bisethylenediamine and cyclam complexes of rhodium(III)

Abstract

JAMES SELLAN and ROBERT RUMFELDT. Can. J. Chem. 54, 519 (1976). The visible and uv photolysis of rt.ntls-[Rh(A4)X2]+ and cis-[Rh(A4)Y2]+ have been studied where A4 is the macromolecule 1,4,8,11-tetraazacyclotetradecane (cyclam) or two ethylenediamine (en) molecules, X is Cl, Br, or I, and Y is Cl. With the exception of cis-[Rh(en)2C12]', all photoreactions proceeded via halide aquation with complete isomeric retention for both ligand field and charge transfer excitation. In the case of cis-[Rh(en)zCI?]+, amine aquation is the principal reaction with some isomerization occurring through a secondary reaction. The quantum yield of halide increased in the order CI < Br < I for rrut1s-[Rh(A4)X2]+, but was found to be allnost equally sensitive to the degree of chelation of the a~nine system so that the photoactivity of rrnt1.~-[Rh(NH~)4CI.l' and /t.nt~s-[RI~(cyclam)Iz]+ were nearly equal ($(CI-) = 0.13 and $(I-) = 0.14). These systenls were also found to be more sensitive to charge transfer excitation than ligand field excitation with cis-[Rh(cyclarn)Clz]+ being the most sensitive complex (&I-) = 0.47 at 254 nm).The photochemistry of the systems studied has been interpreted in terms of a dominant influence of electronic factors in determining the type of photoreaction with the quantitative aspects being significantly influenced by mechanistic factors. On a interprCtC la photochimie des syst'emes CtudiCs en termes d'une inlluence prkdonlinante de facteurs Clectroniques dans la determination du type de photorkaction avec des aspects quantitatifs qui sont influencks d'une faqon importante par des facteurs mkcanistiques.[Traduit par le journal]The photochernistry of transition metal cornplexes has in the main been concentrated on complexes of Cr(II1) and Co(II1); however, a number of interesting studies of Rh(II1) have been reported recently (I, 2), which indicate significant variations in photochemical behaviour from both Cr(111) and Co(II1). Thus, the adherence of mixed ligand systems of Cr(II1) to "Adamson's Etuperical Rules" (3) is not entirely paralleled by Rh(II1) complexes. For example. whereas ammonia release would be predicted from halopentaammine complexes and this was found t o be the case for [Rh(NH,),I]+, with [Rh(NH3),C1]+ chloride aquation is apparently the exclusive process. Whilst this pattern was operative for irradiation of both (/-(I ailti charge transfer to metal (cttm) bands, the curious effect was observed that photoaquation was more efficient for d-(1 than cttm excitation (1).At least a partial explanation for these photochenzical observatioils of the rhodiuln(II1)-haloammines is provided by the photochenlical nod el developed by Zink (4, 5). The model, which is based on fundamental crystal field and molecular orbital theories, a~lalyscs the antibonding properties of the excited states. Whereas Can. J. Chem. Downloaded from www.nrcresearchpress.com by 18.236.120.13 on 05/12/18For personal use only.