Substitution reactions between penta-ammineaquaruthenium(II) and thiocyanato, protonated acetato, cyano, and oxalato ligands in aqueous acidic solutions

Abstract

The substitution reactions between penta-ammineaquaruthenium(II) and thiocyanato (SCN^–), protonated acetato (CH₃COOH), cyano (HCN), and oxalato (H₂C₂O₄-HC₂O₄^–) ligands (L) have been investigated in aqueous acidic solutions and at an ionic strength 0.20 mol dm^–3(LiCl). All the reactions obey the simple second-order rate law (i) and are independent of acid concentration in -d[Ru(NH₃)₅(H₂0)²⁺]/dt = k_obs.[Ru(NH₃)₅(H₂O)²⁺][L](i) the range [H⁺]= 1.0 × 10^–3 to 1.5 × 10^–1 mol dm^–3. The observed second-order rate constants (10²k_obs./dm³ mol^–1 s^–1) measured at 25 °C [20.8 ± 1.1(SCN^–), 11.3± 0.7(HCN), 2.2 ± 0.2 (H₂C₂O₄-HC₂O₄^–), and 0.36 ± 0.03 (CH₃COOH)] fall within the characteristic range of substitution rate constants for the substituson of many other ligands previously reported and are therefore insensitive to the nature of the ligand. The activation parameters measured for the substitution reactions between penta-ammineaquaruthenium(II) and SCN^– and HCN respectively are as follows: δH^‡= 88.00 ± 8.4 kJ mol^–1, δS^‡= 35.7 ± 3.1 J K^–1mol^–1; δH^‡= 50.4 + 5.4 kJ mol^–1, δS^‡=–94.0 ± 9.3 J K^–1 mol^–1. A dissociative mechanism is proposed for all-the reactions.