INTERACTION OF METAL-IONS WITH HUMIC-LIKE MODELS .12. ADDUCTS OF COBALT(II)-2,6-DIMETHOXYBENZOATES AND NICKEL(II)-2,6-DIMETHOXYBENZOATES WITH HETEROCYCLIC BASES

Adducts formed by polymeric complexes of 2,6-dimethoxybenzoic acid of Ni(II) and Co(II) with base molecules (pyridine, 4-ethylpyridine, pyrazine, 2-methylpyrazine, imidazole and 1-methylimidazole) have been synthesized and characterized by analytical and spectroscopic methods. Bis-adducts have been isolated with all the bases. Only pyrazine affords also mono-adducts where the base adopts a bridging bidentate behaviour. As deduced by spectroscopic and analytical data, the metal coordination geometry is tetragonal in all the hydrated adducts, four oxygens from carboxyls and solvent molecules and two nitrogens from bases being the donor atoms. In the anhydrous Ni adducts the coordination remains tetragonal whereas it is tetrahedral in the analogous Co complexes. The nitrogen basicity is an important factor in determining the strength of the bonds between the metal ion and the base as well as the thermal stability of the adducts. The presence of H2O or EtOH molecules in the metal coordination sphere depends on the metal ion and on the basic strength of the nitrogenous ligand.