Stabilities and Hydrolysis of Some Iron(III) and Manganese(III) Complexes with Chelating Ligands

Abstract

The stability constants of some iron(III) chelate complexes with commercially available phenolic ligands have been determined by means of spectrophotometric measurements. The ligands studied, ethylenediimino-bis(2-hydroxyphenyl)acetic acid (EHPG), ethylene-diiminobis(2-hydroxy-4-methyl-phenyl)-acetic acid (EDDHMA) and N,N'-bis-(2-hydroxybenzyl-trimethylenedinitrilo-N,N'-diacetic acid (BHTDA), are hexadentate with two phenolic, two carboxylic and two amine groups. The stabilities (log K) of the iron(III) complexes with these phenolic ligands are in the range 37.8–39.3. The protonation constants of the ligands, as well as of their iron(III) and manganese(III) complexes have been determined. The hydrolysis of the iron(III) and manganese(III) complexes has also been studied. The tendency to hydrolysis is a function of the stability, i.e. the more stable the complex, the less prone it is to hydrolyze. On the other hand, the protonation constants of the iron(III) and manganese(III) chelates do not follow any regular pattern. The implications of these results for the action of the complexes as plant fertilizers are discussed.

Key Words:

• phenolates
• fertilizers
• alkaline soil
• spectrophotometry