Kinetic studies on the substitution reactions of (Pyridine-2-carbaldehyde 2'-pyridylhydrazone)nickel(II). A study of stacking interactions in outer-sphere complexes

Abstract

Kinetics of the formation reactions of the mono and bis complexes of pyridine-2-carbaldehyde 2'-pyridylhydrazone (paphy) with nickel((II), the mono complex of 6-methylpyridine-2-carbaldehyde2'-pyridylhydrazone (mpaphy) with nickel(II), and of the ternary complexes of Ni(paphy)²⁺ with terpy, bpy, phen and pada and Ni(paphy)²⁺ and Ni(phen)²⁺ with mpaphy have been studied inaqueous solution at 25°C and ionic strength 0.1 mol dm^-3 (NaNO₃). The rates of the formationreactions of the mono complexes are slower than expected on the basis of the usual water-loss mechanism, and ring closure probably contributes to the observed rates. The formation of the bisand ternary complexes is usually more rapid than predicted by the water-loss mechanism. The enhanced rates are attributed to large outer-sphere association constants due to 'stacking' interactions between the incoming and bound ligands.