Stability Order in Metal Chelate Compounds. IV. 2-Pyridylmethanol Complexes

Abstract

Abstract The complexing behavior of 2 - pyridylmethanol with magnesium(II), manganese(II), cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) has been studied in aqueous media at 25.0±0.1°C and an ionic strength of 0.10 m (KNO3). The stability constants for the 1 : 1 (ligand to metal) complexes of manganese(II), cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) with the neutral 2-pyridylmethanol and also for the 2 : 1 complexes of nickel(II) and copper(II) have been evaluated. The resulting stability order with respect to the bivalent metals has been found to follow the sequence: Cd(II)<Mn(II)<Zn(II)<Co(II)<Ni(II)<Cu(II) The higher stability of the present complexes relative to the corresponding pyridine complexes may be ascribed to the formation of a chelate ring which involves the hydroxyl group of the ligand. After the complex formation between metal ions and the neutral ligand has proceeded to a certain extent, the liberation of a hydroxyl proton from the complex was observed to occur in several metal complex systems. This tendency was greatest for the copper complex, followed by the zinc, nickel and cobalt complexes in this order. Acid dissociation constants for the 1:1 and 2:1 copper complexes have also been evaluated. Several pieces of evidence have been obtained in support of the coördination ability of the hydroxyl group of the present ligand in its anionic form. The neutral copper chelate, bis(2-pyridylmethanolato)copper(II), as well as other various copper 2-hyridylmethanolate chelates which involve acetate, chloride, iodide, nitrite and thiocyanate ions as the third component, has been isolated. Acetato(2-pyridylmethanolato)-copper(II) has been found to exist as the tetrameric form in chloroform.