Hydroxamate formation constants

Narula and Gupta76 have measured the thermodynamic stepwise stability constants for the formation of complexes of dialkyltin(TV) ions, R2Sn2+ (R = Me or Bu ), with hydroxamic acids, RCONHOH (R = Ph, Me2CH, Me) potentiometrically in 75% dioxane-water media. This has been followed by determination of formation constant of Af-arylhydroxamic acids-dialkyltin(IV) ions.77... 

In contrast to the hydroxamate siderophores, little or nothing is known about the stability constant for the catechol siderophore, entero-bactin. Prior to determining the formation constant of enterobactin (for which hydrolysis of the ligand presents special problems), the reaction of catechol itself with ferric ion has been investigated (31). 

Hydroxamate- or catecholate-containing siderophores are strongly absorbing species with characteristic spectra (see Table 1) which can be utilized for spectrophotometric determination of the complex formation constant. Iron(III) hydroxamates absorb in the visible region, producing a broad absorption band in the 420-440 nm region. Iron(III) catecholates exhibit pH-dependent absorption maxima. Unfortunately, the overall Fe + ion complex formation constants cannot be determined directly at neutral pH, because the extremely high stability of siderophore complexes precludes direct measurements of the equilibrium of interest, which would yield the desired formation constant for a tris-bidentate siderophore complex, /3no (equation (2)). ... 

Hydroxamates, cupferron, and related ligands. As anionic oxygen donor ligands, hydroxamates have a strong affinity for the oxophilic tetravalent actinides, with solution complex formation constants generally greatest for Pu and decreasing as follows Pu > Np > > U. ... 

The overall ferric ion complex formation constants of siderophores cannot be determined directly at neutral pH, because the strong iron binding pulls the equilibrium (Eq. 1) to the right, exhibiting (in the case of hydroxamates) no appreciable dissociation into free ligand and free iron above pH 2. 

Formation Constants of Iron Complexes of Hydroxamic Acid Polymers (12)... 

Table 4. Formation constants for some actinide (IV) hydroxamates and catecholates...

Table II. Formation Constants for Some Actinide(IV) Hydroxamates ...

Formation constants of 3d metal ions with A-m-tolyl-p-substituted benzohydroxamic acids and of rare earths with thenoylhydroxamic acid have been determined. Formation constants of proton and metal complexes of iV-phenyl-2-thenoyl- and A-p-tolyl-2-thenoyl-hydroxamic acids have also been determined. In addition, study has been made of the mixed ligand complexes involving nicotine- and isonicotino-hydroxamic acids. A method of extraction and spectrophotometric determination of vanadium with chlorophenylmethylbenzohydroxamic acid has also been published. It may be mentioned that hydroxamic acids (in particular, the A-phenylbenzohydroxamic acid) have been widely used as analytical reagents for metal ions. Solvent extraction of titanium by benzo- or salicyl-hydroxamic acid in the presence of trioctylamine in the form of coloured complexes has been reported. A-w-Tolyl-p-methoxybenzohydroxamic acid has been used for extraction and spectrophotometric determination of Mo and W from hydrochloric acid media containing thiocyanate. 

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