HIGHEST CO(III)-CO(II) REDOX POTENTIAL IN CO(II)N6 (S=3/2) COMPLEXES OF TRIDENTATE LIGANDS - PREDOMINANCE OF STERIC OVER ELECTRONIC EFFECT

Using the tridentate ligand 2,6-bis(pyrazol-1-ylmethyl)pyridine and its methyl-substituted derivatives a new group of high-spin grossly octahedral cobalt(II) complexes has been synthesized and characterized. The field strength of these ligands with high-spin cobalt(II) progressively decreases with an increase in the number of methyl substituents near the donor site, as revealed from their crystal-field spectral analysis. The trends in the anodic peak potentials (Co(III)-Co(II) couple with E1/2/E(pa) values of 0.78-1.06 V vs S.C.E.) are rationalized on the basis of predominance of the steric effect over the electronic effect of the methyl groups present near the donor site.