Kinetic studies on the interconversion of mer-trichloro(o-dimethylaminophenyldimethylarsine-As)(o-dimethylaminophenyldimethylarsine-NAs)rhodium(III) and trans-dichlorobis(o-dimethylaminophenyldimethylarsine-NAs)rhodium(III) chloride

Abstract

The kinetics of the interconversion (i) have been studied in hydroxylic solvents (L and L′=o-dimethylamino-mer-[Rh(L)(L′)Cl₃]⇌trans-[Rh(L)₂Cl₂]⁺+ Cl^–(i) phenyldimethylarsine-NAs and -As respectively). The rate of the forward reaction depends on the concentration of electrolytes in solution (NaClO₄, NEt₄ClO₄, or LiNO₃), according to the rate law, rate =(k_(m→t)1+k_(m→t)2[Salt])[mer-Rh(nas)₂Cl₃], and increases sharply with the polarity of the solvents. A dissociative mechanism involving extensive Rh–Cl bond rupture in the activated complex is proposed. Such a mechanism is supported by the activation parameters. The rate of the reverse reaction shows a linear dependence on LiCl when the total amount of lithium salts is kept constant by addition of LiNO₃, while a square-root dependence on LiCl is observed in the absence of added LiNO₃. An S_N1 (IP) mechanism is proposed for this reaction, involving a precursor ion-pair of the trans-complex with a free chloride ion. A very small solvent effect is observed in this case and is interpreted as the consequence of opposing effects of the polarity of the solvent both on the dissociation of LiCl and on the formation constant of the precursor ion-pair.