EARLY STAGES OF THE HYDROLYSIS OF CHROMIUM(III) IN AQUEOUS-SOLUTION .6. KINETICS OF INTRAMOLECULAR INTERCONVERSION BETWEEN SINGLY-BRIDGED AND DOUBLY-BRIDGED HYDROLYTIC DIMERS

The singly-bridged hydrolytic dimer [(H2O)5Cr(mu-OH)Cr(OH2)5]5+ (SBD), undergoes a ring closure reaction which transforms it into the doubly-bridged dimer, [(H2O)4Cr(mu-OH)2Cr(OH2)4]4+ (DBD). The kinetics of this conversion have been studied over the pH range 0-4.93, temperature range 288.15-313.35 K and I = 1.0 M (adjusted with either LiClO4 or NaClO4). The pH dependence of the first-order rate constant, k(obs), was interpreted in terms of three distinct pathways involving ring closure within fully protonated, mono- and doubly-deprotonated SBD and two pathways accounting for acid-catalysed and uncatalysed cleavage of DBD which are important at low pH. Least-squares analysis of the data permitted the determination of rate constants for each of these pathways and the first two acid dissociation constants of the SBD. A reactivity increase of ca four-fold observed on monodeprotonation of SBD was suggested to reflect, at least in part, the greater nucleophilicity of coordinated OH-, cf. coordinated H2O. The more substantial rate enhancement of 25-fold resulting from double deprotonation of SBD was concluded to be due to an increase in the lability of the chromium(III) centre at which substitution occurs. The activation parameters for all pathways are consistent with an associative substitution mechanism, either A or I(a).