ROLE OF THE NH3-AMINOHYDROXAMATE, ALUMINUM(III)-AMINOHYDROXAMATE AND GALLIUM(III)-AMINOHYDROXAMATE INTERACTIONS( MOIETY IN IRON(III))

Stability constants have been determined and the bonding modes and eff ects caused by the side-chain NH3+ moiety in aminohydroxamic acids eva luated for complexes formed in aqueous solution in between iron(III), aluminium(III) and gallium(lll) with alpha-alaninehydroxamic acid (alp ha-Alaha), beta-alaninehydroxamic acid (beta-Alaha), aspartic acid-bet a-hydroxamic acid (Asp-beta-ha) and glutamic acid-gamma-hydroxamic aci d (Glu-gamma-ha). The iron(III)-, aluminium(III)- and gallium(III)-ace tohydroxamic acid (aha) systems were studied as models. Co-ordination of hydroxamate oxygens occurs in the cases of aha, alpha- and beta-Ala ha, while Asp-beta-ha and Glu-gamma-ha are co-ordinated via their hydr oxamate and carboxylate oxygens. The OH- ion was found to be an effect ive ligand in these systems (especially for Ga-III) causing the format ion of both binary and ternary hydroxo complexes. The presence of NH3 in the hydroxamic acids favours the hydrolysis to an extent which dep ends on the distance between the hydroxamate moiety and NH3+. These fi ndings can be explained by the electron-withdrawing effect of NH3+ and electrostatic repulsion between it and the co-ordinating M(3+) ion.