Kinetics and mechanism of the demetallation of macrocyclic nickel(II) complexes by cyanide

The kinetics and the mechanism of the cyanide-induced demetallation of a series of Ni2+ complexes with macrocyclic ligands of different ring size (12- to 14-membered; see 1-4) and steric constraints was studied. Although the rates differ by almost five orders of magnitude when compared to each other under fixed experimental conditions (pH 10.5, [CN-] = 10(-2) m), all reactions proceed through the relatively rapid formation of cyano adducts [Ni(CN)(n)L] (n = 1, 2), which then react with additional CN- or HCN to give the final products. Of paramount importance for the reaction rate is the geometry and configuration of the cyano adducts [Ni(CN)(n)L] (n = 1,2). cis-Dicyano derivatives with a folded macrocycle react faster than trans-compounds. In the case of (1,4,8,11-tetraazacyclotetradecane)nickel(2+) ([Ni(4)](2+)), which gives a trans- dicyano adduct, the base-catalyzed N-inversion necessary to obtain the cis-dicyano derivative becomes rate determining at high CN- concentrations.