The coordination abilities of some heterocyclic N-bases and N-oxides towards bis(1-phenyl-3-methyl-4-benzoylpyrazolonato-5)cobalt(II)

Abstract

The adducts formed between bis(1-phenyl-3-methyl-4-benzolpyrazolonato-5)cobalt(II), Co(PMBP) 2 and a number of heterocyclic N-bases or N-oxides have been identified and their formation constants β 2, n( org.) = [ Co(PMBP) 2 B n] org. [ Co(PMBP) 2] org. [ B] org. n determined by the the solvent extraction method. The results indicate that for the adducts formed with the N-bases β 2, n(org) increases in the following order: 2,6-dimethyl pyridine < 2-methyl pyridine < 2,4-dimethyl pyridine < pyridine < 3-methyl pyridine < 4-methyl pyridine. This trend follows the order of basicities except where steric hindrance to adduct formation is involved. For the adducts formed with the heterocyclic N-oxides β 2,1(org.) increases in the order: 2,6-dimethyl pyridine oxide < 2-methyl pyridine oxide < 3-methyl pyridine oxide < 4-methyl pyridine oxide < pyridine oxide. This trend does not follow the order of basicities of the N-oxides and the results are discussed assuming metal-to-ligand back-bonding. The formation constant of an adduct with an N-oxide was found to be greater than that formed with the corresponding N-base.