Equilibrium and kinetic study of pyridine binding to phthalocyaninatoiron(II) in dimethyl sulphoxide

he reaction between pyridine (py) and phthalocyaninatoiron(II), [Fe(pc)], in dimethyl sulphoxide (dmso) has been studied from the kinetic and the equilibrium viewpoints at 20 ± 0.5 °C. In the presence of a large excess of pyridine the adduct [Fe(pc)(py)2] is formed via pseudo-first-order kinetics. The observed rate constants have the general form kobs.=k1f[py]+k1r, with k1f?? 1.5 × 104 dm3 mol-1 s-1 and k1f?= 35.5 ± 0.7 dm3 mol-1 s-1 and k1r?= 0.15 ± 0.02 s-1, for the binding and dissociation of the first and second py molecules, respectively. From the values of the equilibrium constant [K1?=(3.9 ± 0.1)× 105 dm3 mol-1] and of the second-order rate constant (k1f?) for the binding of the first py molecule, the dissociation rate constant has been estimated to be k1r?? 4 × 10-2 s-1. The value of the equilibrium constant for the binding of the second py (K1?= 300 ± 28 dm3 mol-1), determined directly, is in good agreement with the corresponding value (240 ± 40 dm3 mol-1) obtained from the ratio k1f?/k1r?). The data obtained in the present study are discussed in the light of related previous studies.