Mechanistic Aspects of Ligand Substitution on cis -diaqua-chloro-tris(Dimethyl Sulfoxide) Ruthenium(II) Complex by Adenosine and Cytidine in Aqueous Medium

The interaction of the cis-diaqua-chloro-tris(dimethyl sulfoxide)ruthenium(II) complex with selected nucleosides such as adenosine (L¹H) and cytidine (L²H) has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand] and temperature. The reaction has been monitored at 295 nm where the difference in spectra between the reactant and product is a maximum. At pH 5.0, the interaction with the two nucleosides shows two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of the nucleosides: both steps are [ligand] dependent. The rate constants for the processes are: k₁ ∼ 10^–3 s^–1 and k₂ ∼ 10^–5 s^–1. The activation parameters were calculated from Eyring plots. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. From the temperature dependence of the outer sphere association equilibrium constant, the thermodynamic parameters were also calculated, which gives a negative ΔG° value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The product of the reaction has been characterised by IR and ESI-mass spectroscopic analysis.