Salt effects on the reactivity for ligand substitution reactions of [Ru(CN)5OH2]3− anion with two naphthalene substituted ligands (nitroso-R-salt and α-nitroso-β-naphthol) in presence of Tetrapropylammonium bromide (Pr4NBr) and Sodium chloride (NaCl)

The kinetics of the ligand exchange reaction between aquapentacyanoruthenate(II) [Ru(CN)₅OH₂]³⁻ ion and naphthalene substituted ligands [α-nitroso-β-naphthol (αNβN), and nitroso-R-salt (NRS)] has been studied in aqueous salt solutions of sodium chloride (NaCl) or tetrapropylammonium bromide (Pr₄NBr) salt. The kinetics was monitored spectrophotometrically at 525 nm corresponding to the λ_max of reddish-brown-colored substituted products, [Ru(CN)₅(αNβN)]³⁻ or [Ru(CN)₅(NRS)]³⁻. Increasing the ionic strength of the reaction mixture using NaCl, exerted a negative salt effect on the rate of formation of naphthalene-substituted products. At the same time, an increment in the concentration of Pr₄NBr imparted a positive salt effect on the reaction. The observed rate constant (k_obs) exhibits linear increment with respect to the concentration of NRS or αNβN while remaining invariant with variation in [Ru(CN)₅OH₂]³⁻. The computed activation parameters for NRS (∆H^# = 24.55 kJ mol⁻¹, E_a = 27.03 kJ mol⁻¹, ∆G^# = 87.83 kJ mol⁻¹, and ∆S^# = – 212.5 J K⁻¹ mol⁻¹) and αNβN ((∆H^# = 17.33 kJ mol⁻¹, E_a = 19.81 kJ mol⁻¹, ∆G^# = 87.87 kJ mol⁻¹, and ∆S^# = – 236.7 J K⁻¹ mol⁻¹) also support the proposed mechanism.