Spectrophotometric Study of the Formation and Protolytic Equilibria for Iron(II) Complexes of Di-2-pyridylmethanone 2-Pyridylhydrazone and Di-2-pyridylmethanone 2-Quinolylhydrazone

The stoichiometries, stability constants and deprotonation constants of the complexes formed between iron(II) and either of two heterocyclic hydrazones, di-2-pyridylmethanone 2-pyridylhydrazone (DPPH) or di-2-pyridylmethanone 2-quinolylhydrazone (DPQH), have been determined spectrophotometrically at 25°C and, μ=0.2. Iron(II) reacts with DPPH and DPQH in acidic media to form a dipositively charged bis complex, Fe(HL)22+, which undergoes successive deprotonation to monocationic (Fe(HL)L+) and neutral bis (FeL2) complexes, along with a decrease in the solution acidity. The acid-strengthening effect of coordinated metal on the imino group of the ligand is briefly discussed. There was no evidence for the formation of mono complexes. The logarithmic stability constants of the neutral complexes, FeL2, were 36.8 and 33.0 for the DPPH and DPQH complexes, respectively.