Kinetics and mechanism of iron(III)-nitrilotriacetate complex reactions with phosphate and acetohydroxamic acid

The kinetics and mechanism of the substitution of coordinated water in nitrilotriacetate complexes of iron(III) (Fe(NTA)(OH2)(2) and Fe(NTA)(OH2)(OH)(-)) by phosphate (H2PO4- and HPO42-) and acetohydroxamic acid (CH3C(O)N(OH)H) were investigated. The phosphate reactions were found to be pH dependent in the range of 4-8. Phosphate substitution rates are independent of the degree of phosphate protonation, and pH dependence is due to the difference in reactivity of Fe(NTA)(OH2)(2) (k = 3.6 x 10(5) M-1 s(-1)) and Fe(NTA)(OH2)(OH)(-) (k = 2.4 x 10(4) M-1 s(-1)). Substitution by acetohydroxamic acid is insensitive to pH in the range of 4-5.2, and Fe(NTA)(OH2)(2) and Fe(NTA)(OH2)(OH)(-) react at equivalent rates (k = 4.2 x 10(4) and 3.8 x 10(4) M-1 s(-1), respectively). Evidence for acid-dependent and acid-independent back-reactions was obtained for both the phosphate and acetohydroxamate complexes. Reactivity patterns were analyzed in the context of NTA labilization of coordinated water, and outer-sphere electrostatic and H-bonding influences were analyzed in the precursor complex (K-os).