Rhodium aqua ions

Shimizu et al. used simple rhodium-aqua ions (Rh3+) immobilized onto polymer-modified electrodes to perform the electrochemical reduction of NAD+ [114]. Rh3+ was loaded onto polymeric anion doped-polypyrrole membranes coated on the surface. Electrochemical reduction of NAD+ with immobilized Rh3+ was performed at —0.85 V, where Rh3+ was reduced to Rh+. NADH was produced without detectable formation of NAD-dimers. 

Rhodium(III) hydroxide is an ill-defined compound Rh(0H)3.nH20 (n 3) obtained as a yellow precipitate by careful addition of alkali to Na3RhCl6-Addition of imidazole solution to suitable aqua ions leads to the precipitation of active rhodium(III) hydroxides formulated as Rh(0H)3(H20)3, Rh2(/x-0H)2(0H)4(H20)4 and Rh3(/z-0H)4(0H)5(H20)5 [31]. Hydrated iridium(III) hydroxide is obtained as a yellow precipitate from Ir3+ (aq.) at pH 8. 

Complexes of trifluoromethanesulfonate anion with cobalt(III) are labile oward substitution under mild conditions, and they have proved to be useful synthetic precursors to a variety of aminecobalt(III) complexes. The pentaammine-(trifluoromethanesulfonato-O)rhodium(III) ion, which is readily prepared from [Rh(NH3)5Cl]Cl2 in hot CF3SO3H, is also versatile as a synthetic precursor. " Its synthesis and solvolysis to give essentially quantitative yields of the penta-ammineaqua- and hexaamminerhodium(III) ions are described below. The aqua complex has previously been prepared by the base hydrolysis or Ag -induced aquation of [Rh(NH3)5Cl]Cl2 in water, but the present method presents a cleaner and more rapid alternative. The methods for preparation of the [RhCNHj) ] ion have evolved from the procedure of J0rgensen. They involve prolonged reaction of [Rh(NH3)5Cl]Cl2 with ammonia in a pressure vessel at elevated temperature. The solvolysis of [Rh(NH3)5(0S02CF3)](CF3S0j)2 in liquid ammonia is a simple, high-yield, and rapid alternative. 

Bond and coworkers [521] have described Au(III) electroreduction and Au(0) oxidation stripping processes in dilute aqua regia utilizing platinum, rhodium, iridium, gold, and glassy carbon electrodes. Sorption of tetrachloroaureate ions on carbon paste electrode modified with montmorillonite has been performed as a preconcentration step in the determination of gold in pharmaceutical preparation [522]. 

Much remains to be done in the fleld of complexes of mixed ligands. All the ruthenium (III) 12) and rhodium (III) 126) chloro-aqua complexes have been separated by ion-exchange techniques, and the mixed complexes, OsClaBr6 a and IrClaBr6 a S by electrophoresis (7, 88). Labile mixed complexes such as bismuth (III) 78) and uranium (IV) 38) chloro-bromides have been studied by spectrophotometry, but the conclusions are obviously more difficult to obtain. 

Z>) Reactions of [Rh(NH ),X. The aquation and anation of [Rh(NH3X]" in acidic aqueous solution (equation 105 was first studied by Lamb who used conductance changes to monitor the interconversion of acido and aqua rhodium complexes , for the chloro and bromopentaanimine-rhodium(III) cations. He found that both aquation and anation are first-order in [Rh ], and that the hydrolysis is accelerated by the presence of OH . The forward form of equation (105) (aquation) has since been studied for X = Cl-, Br , l-, >.=56 oHj, SO, NO3" and Nj . Representative results for the aquation of various pentaammine salts are given in Table 25. For aquations of halides or nitrate ions, rate expressions which are first-order in [Rh] and zero-order in [H+] are found. Poe gives a detailed discussion of the thermodynamic, kinetic and intrinsic-kinetic trans effects, and compares the aquation and anation of [Rh(NH3)5X]-" (X = Cl, Br and I) with the [RhfenjjXY]" system. When X = S04, a two-term rate expression was observed... 

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