Uninegative chelating

S-Diketones, by virtue of their keto-enol tautomerism (Scheme 21), are weak acids and in most cases bind nickel(II) in the deprotonated enol form, acting as uninegative chelating ligands. 

Finally there are many types of uninegative A.A chelate ligands,138 examples being (9-XXIXa) and (9-XXIXb) the former is similar to acetylacetonates while the latter is analogous to salicylaldiminato ligands. 

With neutral ligands, cationic species such as [Ag(py)4]2+, [Ag(bipy)2]2+, and [Ag(phen)2]2+ form crystalline salts, whereas with uninegative chelating ligands such as 2-pyridinecarboxylate, neutral species such as (18-I-IX) are obtained. In acid solution the [Ag(bipy)2]2+ ion is in equilibrium with the [Ag(bipy)]2+ ion, and it is the latter that oxidizes H202 in acid solution. 

A further investigation " deals with the anation of cw-[Co(en)2(H20)2] by quinolinic acid. At pH 4.05 quinolinic acid (H2A) behaves as a uninegative and bidentate (N, O) donor. A two-step reaction is observed the first step, is the replacement of one water and the second is the slower chelation step. Rate constants for both steps have been obtained and the ion-pair constant for the first step evaluated. 

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