Coordination modes of hydroxamic acids in copper(II), nickel(II) and zinc(II) mixed-ligand complexes in aqueous solution

The stability constants and coordination modes of the mixed-ligand complexes formed by Cu(II), Ni(II), Zn(II), ethylenediamine ten), 2,2'-bipyridine (bpy), glycinate (Gly), disodium salt of 4,5-dihydroxybenzene 1,3-disulfonate (Tiron), diethylenetriamine (dien) or 2,2':6,2 "-terpyridine (terpy)( = ligand B) and acetohydroxamate (Aha), N-methylacetohydroxamate (MeAha) or N-phenylacetohydroxamate (PhAha) ( = ligand A) were determined in water (25 degrees C, I = 0.2 M KCl) by pH-metric, spectrophotometric, EPR and calorimetric methods. Mixed-ligand complexes with typical hydroxamate type chelation mode involving the NHO- moiety are formed in all systems. However, further copper(II) induced deprotonation of the NHO- moiety of Aha in the presence of en or bpy results in the formation of mixed-ligand complexes with hydroximato chelates at high pH. The results show the favoured coordination of a hydroxamate to metal(II)-en and especially to a metal(II)-bpy moiety. If ligand B is Gly, the increase of stability of the mixed-ligand complexes is as expected on statistical basis, whereas the formation of complexes involving O,O-coordinated hydroxamate and O,O-coordinated Tiron is unfavoured. The tridentate coordination of dien or terpy results in five-coordinated mixed-ligand copper(II) complexes in which, most probably, the hydroxamate moiety adopts an equatorial-axial coordination mode. This quite unstable hydroxamate chelate can not hinder the hydrolysis of the complex above pH 8. Under very basic conditions acetohydroximato moieties (-CONO2-) displace the rigid terpy ligand from the coordination sphere and complexes, [Cu(AhaH(-1))(2)](2-) involving hydroximato chelates are formed. (C) 2000 Elsevier Science Ltd. All rights reserved.