Kinetic and mechanistic studies on the interaction of cytidine with hydroxopentaaquarhodium(III) ion

Kinetics of interaction between cytidine and [Rh(OH2)(5)(OH)](2+) has been studied spectrophotometrically as a function of [Rh(OH2)(5)(OH)(2+)], [cyti dine], pH and temperature. The reaction has been monitored at 307 nm, the l ambda(max) of the substituted complex and where the spectral difference bet ween the reactant and the product is maximum. The reaction rate increases w ith an increase in [cytidine] and reaches a limiting value at higher ligand concentration. From the experimental findings an associative interchange m echanism for the substitution process is suggested. The activation paramete rs (Delta W-not equal = 62.7 +/- 4.2kJ mol(-1), Delta S-not equal = -129 +/ - 12 JK(-1) mol(-1)) support the proposition. The negative Delta G degrees (-11.3kJ mol(-1)) for the fast equilibrium step also supports the spontaneo us formation of the outersphere association complex.