Kinetics of the acid and mercury(II)-ion induced dissociations of some nickel(II) chelate complexes of substituted pyridine ligands in dimethyl sulphoxide solution

Abstract

Rate data are reported for the H⁺- and Hg^II-ion induced dissociations of mono complexes of nickel(II) ion with the bidentate ligands 2-(aminomethyl)pyridine, (I), 2-(2-aminoethyl)pyridine, (II), 2-(aminomethyl)-6-methyl-pyridine, (III), and 2-[(methylamino)methyl]pyridine, (IV), in dimethyl sulphoxide solution. Limiting ‘neutral’-dissociation rate constants (k_d) at zero [Hg^II] and 292·0 K have been estimated by extrapolation to be 10³K_d= 1·12 ± 0·02, 4·46 ± 0·07, 10·7 ± 0·3, and 2·58 ± 0·01 s^–1 for (I)–(IV) respectively. These results, together with data reported previously for the formation rate constants (K_f), give kinetically determined first stability constants K₁(=K_f/K_d) in dmso a1292 K as log K₁= 6·43, 5·80, 5·54, and 6·74 for (I)–(IV) respectively. In acidic solution the pseudo-first-order rate constants (k) vary with [H⁺]. Limiting rate constants at high [H⁺](K_a) have been estimated and values of K_a/K_d are 54 ± 5, 365 ± 28, 1 565 ± 81, and 102 ± 26 for (I)–(IV) respectively. These results indicate that the formation reactions are ‘normal’[i.e. a chelate-ring-closure (S.C.S.) mechanism does not apply] and a previous suggestion that an S.C.S. mechanism occurs during formation of the mono(2,2′-bipyridine)nickel(II) ion in dmso is reviewed in the light of the marked acid dependence of the reactions studied here. The rate of the Hg^II-ion induced dissociation of the mono(1,10-phenanthroline)nickel(II) ion in dmso (at 321·3 K, K_d= 2·4 × 10^–4 s^–1) indicates that a previous indirect estimate (at 318·1 K, K_d= 1·0 s^–1) is far too large.