Ligand substitution kinetics in complexes of pyridine and pyrazine derivatives with pentacyanoruthenate(II)

Abstract

The reactivity of the pentacyanoruthenate(II) complexes of derivatives of pyridine and pyrazine (L) are reported. The kinetics of dissociation of the complexes, measured in the presence of an excess of a scavenging ligand, show saturation behaviour with respect to this reactant. The limiting rates, k_–L, are in the range (2–10)× 10^–5 s^–1(25 °C). A dissociative interchange mechanism is postulated, as in related pentacyanoferrate(II) complexes. The rates of formation, starting from [Ru(CN)₅(H₂O)]^3–, are typically first order with respect to the concentration of the ligands, with k=ca. 1 dm³ mol^–1 s^–1(25 °C) for neutral ligands and slightly higher and lower values for positively and negatively charged ligands, respectively. An ion-pair dissociative interchange mechanism is postulated. Activation parameter data are presented for both types of reactions and compared with those from related systems, [Fe(CN)₅L]^n– and [Ru(NH₃)₅L]ⁿ⁺.