Rhodium complexes photochemistry

Like the ammines, rhodium complexes of ligands like bipy and phen have a significant photochemistry. Therefore, on irradiation, solutions of cw-[Rh(L-L)2X(H20)]2+ (X = halogen) gradually convert to c/s-[Rh(L-L)2X(H20)]2+, though much more slowly than with the ammines [101]. 

Ruthenium complexes used to lead research in photochemistry of metal compounds, but rhodium complexes have recently overtaken them as the key target compounds due to their applications in OLEDs. This is a lively and ever-changing field for example, over 90% of luminescent iridum(III) complexes have been reported only in the six years to the beginning of 2009. With their luminescence tuneable through ligand choice, iridium complexes are firm candidates for optical display applications. 

Kinetics of aquation of [Ru(LLLL)X2], with LLLL = cyclam, 2,3,2-tet, en2, or (NH3)4, and X = Cl or Br, have been followed by cyclic voltammetry. Rate constants, and activation parameters (A// and A5 ) have been evaluated, and compared with kinetic parameters for reactions of analogous compounds of ruthenium(III) and cobalt(III). Similar trends obtain for all three sets of complexes. There is retention of stereochemistry, rates decrease as the extent of chelation in LLLL increases, and trans complexes are less labile than cis analogs. Reactivities are determined by solvation of the initial and transition states, by nephelauxetic effects, and by a-trans effects. A limiting dissociative (D) mechanism is proposed for the ruthenium complexes, with square-pyramidal geometry for the transient intermediate [cf. rhodium(III) photochemistry below. Section 5.8.10]. Differences in isomer lability have also been described for... 

The synthesis and photochemistry of related rhodium(lll) bis(alkyl) complexes cA,cA-[Rh(R)2(I)(CO)(dmb)] (R = Me, Tr) have been studied. The complexes showed an IT (tv - r (dmb)) absorption band at ca. 295 nm. There was another band at ca. 370nm for both complexes, which was assigned to an XTCT (p(l)7r (dmb)) transition. Irradiation of both complexes in solution resulted in Rh-R bond homolysis, as evidenced by IR, UV-vis, and spin-trap EPR investigations. The photoreaction was proposed to occur after crossing from the XLCT state to the reactive SBLCT state. 

Reviews have appeared of the photophysics of molybdenum complexes, primary and secondary processes in organometallic chemistry, flash photolysis of Pe(CO)5 and Cr(CO)g, dinuclear manganese carbonyl compounds, the photochemistry of metal complexes isolated in low temperature matrices, cluster complexes, diene complexes, photoproduction of coordinativeiy unsaturated species containing rhodium or iridium, and redox chemiluminescence of organometallic compounds.Synthetic and metal organic photochemistry in industry has also been reviewed. 

Further Studies of the Spectroscopic Properties and Photochemistry of Binuclear Rhodium(I) Isocyanide Complexes... 

The final reference in this photochemistry section concerns trans — cis isomerization of coordinated stilbenecarboxylate, L, in [Ir(NH3)5L] , as discussed in connection with the analogous rhodium(III) complex (Section 5.8.10). ... 

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 ... 

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

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