Evidence for a rate-determining chelate-ring-closure mechanism during the formation of the (2,2′-bipyridine)nickel(II) ion in dimethyl sulphoxide solution

Abstract

In dimethyl sulphoxide (dmso) as solvent, the rate constant at 298·1 K for formation of the mono-complex of Ni^II ion with the unidentate ligand 4-phenylpyridine (k_f= 1·64 × 10³l mol^–1 s^–1) is ca. 24 and 63 times as large as corresponding values for the polydentate ligands 2,2′-bipyridine (bipy) and 2,2′,2″-terpyridine (terpy) respectively, and nearer to the solvent-exchange rate (k_ex= 7·4 × 10³ s^–1). These results are in marked contrast to values reported in aqueous solution where the rates of formation are about the same for py, bipy, and terpy, and a rate-determining chelate-ring-closure mechanism is postulated for the polydentate ligands in dmso. This postulate is supported by kinetic studies of the, Hg^II and H⁺ induced dissociations of the (2,2′-bipyridine)nickel(II) ion in dmso solution which indicate that, for 2,2′-bipyridine at 298·1 K, the rate of chelate-ring closure (rate constant k_rc) is ca. 60 times slower in dmso (k_rcca. 47 s^–1) than in water (k_rcca. 3 × 10³ s^–1).