Substitution reaction of aqua(ethylenediaminetetraacetato) ruthenium(III) with 2-mercaptobenzoic acid

It was observed that in an aqueous solution, aqua( ethylenediaminetetraacetato) ruthenium(III), [Ru(Hedta)(H2O)], reacts with 2-mercaptobenzoic acid (H2L) in a molar ratio of 1:1 to give a bluish complex (epsilon(595) = 4000 M-1 cm(-1)). The bluish complex was isolated and characterized by pH titration, elementary analyses, IR, and magnetic susceptibility measurements. The experimental results revealed that the +3 state of Ru (low spin d(5)) is retained throughout the reaction and a plausible formula for the reaction product was suggested as H[Ru(H(2)edta) (L)]. 2.5H(2)O. This complex formation reaction is first order with respect to both [Ru(Hedta) (H2O)] and 2-mercaptobenzoic acid. From the pH dependence of an anation constant, k(AN), it was found that the overall reaction rate could be expressed approximately as follows: 2<pH less than or equal to 5 rate=k(c)[Ru(edta)(H2O) (-)][H2L] +k(d)[Ru(edta)(H2O)(-)] [HL-]; 5<pH<9 rate=k(d)[Ru(edta)(H2O) (-)]-[HL-] +k(e)[Ru(edta)(H2O)(-)] [L2- ]; k(c), k(d) and k(e) were evaluated as 33.4+/-1.5 M-1 s(-1), 134 +/- 8 M-1 s(-1) and 153 +/-7 M-1 s(-1) respectively at I= 0.1 M and 15 degrees C. From the activation parameters, it can be said that the association mechanism is supported. The labilization effect of the quinquedentate edta complex was also observed. (C) 1998 Elsevier Science S.A. All rights reserved.