COMPLEXATION BEHAVIOR OF HETEROCYCLIC HYDRAZONES .2. EFFECTS OF STERIC FACTORS ON FORMATION-CONSTANTS FOR NICKEL(II) COMPLEXES WITH HETEROCYCLIC HYDRAZONES

The equilibria of nickel(II) ion with thirteen nitrogen-heterocyclic h ydrazones (neutral form: HL) with and without blocking group(s) have b een investigated spectrophotometrically in 28% aqueous dioxane solutio n at 25-degrees-C and an ionic strength of 0.2 (KCl). For a series of 2-pyridinecarbaldehyde 5-substituted-2-pyridylhydrazones, the followin g linear free energy relationships (LFER) were derived between the ove rall formation constants, beta1, beta1', and beta2, of Ni(HL)2+, NiL+, and Ni(HL)22+ complexes, respectively, and the dissociation constants , K(a1) and K(a2), of H3L2+ and H2L+, respectively: log beta1 = 0.95 ( pK(a1) + pK(a2))/2 + 4.60, 1/2 log beta2 = 0.95 x (PK(a1) + PK(a2))/2 + 4.82, and log beta1' = 1.40 (pK(a1) + PK(a2))/2 + 9.73. The steric e ffects of the 6-methyl group in pyridine and the quinolyl ring in plac e of pyridyl group on the complex formation were evaluated on the basi s of LFER. The complex stabilization effect of the pyridyl group subst ituted for formyl hydrogen was also evaluated for Ni(HL)22+, Ni(HL)2+, and NiL+ complexes. These results together with LFER were discussed i n conjunction with the thermodynamic parameters for the formation of N i(HL)2+ complex and the ligand field parameters, D(q), for Ni(HL)2+ an d Ni(HL)22+ complexes.