Thermodynamic and kinetic study of Ru(II)-Pt(II) complexes with a semi-rigid linker 4’-pyridyl-2,2’:6’,2’’terpyridine (quarterpyridyl (qpy)) with biological nucleophiles was carried out under pseudo first-order conditions as a function of concentration and temperature using UV–visible spectrophotometer. The reactions proceeded via a single step following first-order kinetics with the pseudo first-order rate constant obeying the rate law; \(k_{obs} = k_{2} \left[ {Nu} \right]\) . The study revealed that increase in the overall charges of the complexes is the key reason for the observed increase in the reactivity. Additionally, replacing the cis pyridyl group in Pt1 by Ru(III) polypyridyl to give Pt2 and Pt3 lowers the energy of the LUMO (π*) orbitals and HOMO–LUMO energy gap which influences the reactivity to some extent. The two qpy groups in the trinuclar complex Pt3 only slightly increase the reactivity compared to Pt2. This is because the qpy groups are in orthogonal positions preventing π-electron communication; hence the two Pt(II) centers act independently. The marginal increase in reactivity is due to increased charge and extension of the π-surface. The observed activation parameters support an associative mode of substitution. Their observed lower reactivity towards classical model Platinum-biomolecule interaction than cisplatin revealed their moderate reactivity that makes them less interactive with biomolecules hence potentially less toxic, more effective and selective. Their increased charge makes them better soluble than cisplatin which potentially solves the solubility challenge. The higher binding ability towards 5’-GMP on introduction of the ruthenium polypyridyl moiety indicates their tendency to strongly bind DNA.
