Effect of Substituents on the Relative Stabilities of Fe(III) Complexes of Substituted Salicylic Acids

Chemical Laboratories, University of Allahabad, Allahabad-211 002

The stability constants of Fe(III) complexes of salicylic acid (SA) and ten of its substituted derivatives were determined at constant ionic strength using spectrophoto­metric measurements. Fe(III)-SA system was taken as a reference for comparing the difference in the acid dissociation constants (∆ \(p\)K) and stability constant values (∆ log K). A parallelism is observed, though the magnitude of ∆ log K is less than ∆\(p\)K. While the substitution of an electron donor group increases the stability contaut, it decreases the value of the apparent equilibrium constant. Similarly, the electron withdrawing groups have opposite effect on these two parameters. A hypsochromic shift In the absorption maximum (\(\lambda_{max}\)) of 1 : 1 complex is observed when an electron with­drawing group is substituted in the nucleus of the reference ligand, whereas other groups (or an atom like Hg) give a bathochromic shift. However, exact correlation between \(\lambda_{max}\) and ∆ log K or ∆ \(p\)K. was not observed.