The O–O–N–N–O-type pentadentate ligands H31, 3-pd3a and H31, 3-pd2ap (H31, 3-pd3a stands for 1, 3-propanediamine-N, N, N′-triacetic acid; H31, 3-pd2ap stands for 1, 3- propanediamine-N, N′-diacetic-N-3-propionic acid) and the corresponding novel octahedral nickel (II) complexes have been prepared and characterized. H31, 3-pd3a and H31, 3-pd2ap ligands coordinate to nickel (II) ion via five donor atoms (three deprotonated ...