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)_nL] (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)_nL] (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.