Abstract
Pd(bipy)(py)~2~~2~ reacts stepwise with excess thiourea to give Pd(tu)~4~~2~. The kinetics of the second step, which refers to the replacement of bipyridyl in Pd(bipy)(tu)~2~~2~, have been studied in water and in the presence of calf thymus DNA, sodium polyriboadenylate, sodium polyvinylsulfonate or sodium polymetaphosphate at 25 °C and pH = 7 and a fixed sodium chloride concentration. The reaction follows a first order course and a plot of k~obs~ against [thiourea]^2^ affords a straight line with a small intercept. DNA inhibits the process without altering the rate law. The k~obs~ values decrease systematically on increasing the DNA concentration eventually tending to a limiting value. The values are larger at higher ionic strengths and the other polyanions show similar behaviour. The influence of DNA on the kinetics can be related to steric inhibition caused by noncovalent binding with the complex. Upon interaction with DNA, [Pd(bipy)(tu)~2~]^2+^ gives rise to immediate spectroscopic changes in the UV/Vis region as well as induced circular dichroism suggesting that the complex, like similar platinum(II) and palladium(II) species of bipyridyl, intercalates with the double helix. Such a type of interaction hampers the attack of the nucleophile at the metal centre inhibiting the reaction. The decrease in the rate of ligand substitution upon decreasing salt concentration but at a given DNA concentration is due to the influence of ionic strength on the complex−DNA interaction. The reactivity inhibition by single‐stranded poly(A), polyvinylsulfonate or polymetaphosphate can be accounted for in terms of self‐aggregation of the complex induced by the polyanion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)