Rates of formation and dissociation, and the stability of some manganese(II) and zinc(II) complexes with bipyridyl-type ligands in dimethyl sulphoxide solution

Abstract

The rates of formation and dissociation of 1:1 complexes of manganese(II) and zinc(II) ions with bipyridyl-type ligands have been examined in dimethyl sulphoxide (dmso) solution by the stopped-flow method at temperatures just above the freezing point of dmso. In some cases the reactions are too fast to measure {e.g. the reaction between the [Mn(dmso)₆]²⁺ ion and 2,2′-bipyridine (bipy)}. Rate data are reported for the formation and the mercury(II)-ion induced dissociations of [Mn(phen)(dmso)₄]²⁺(phen = 1,10-phenanthroline) and [Zn(bipy)(dmso)₄]²⁺ ions, and the kinetically determined first stability constants in dmso (I= 0.20 mol dm^–3, Na[ClO₄]) are log (K₁/dm³ mol^–1)= 2.63 and 3.36 respectively. The rate constants for dmso solvent exchange (k_ex) are calculated to be log (k_ex/s^–1)= 4.89 (291.7 K) and 5.94(292.2K) for Mn²⁺ and Zn²⁺ respectively. The reaction between a large excess of [Mn(dmso)₆]²⁺ ion and 2,2′:6′,2″-terpyridine (terpy) is complicated; an initial very rapid reaction is followed by a much slower process which has been examined by repetitive-scan spectrophotometry. Rate data are reported for the two steps, and a mechanism is proposed in which the initial rapid reaction involves the formation of a binuclear intermediate and the slow step is associated with final chelate-ring closure.