Kinetics and mechanism of base hydrolysis of mer-[Cr(pic)3]0 and [Cr(ox)2(pic)]2− (pic = picolinate, ox = oxalate)

Abstract

Mer-[Cr(pic) 3 ] 0 and [Cr(ox) 2 (pic)] 2-undergo successive base hydrolysis to give chromates(III). Dissociation of the first ligand, pic from [Cr(pic) 3 ] 0 and ox from [Cr(ox) 2 (pic)] 2-, proceeds in two stages, namely initial chelate-ring opening followed by slower liberation of the monodentate ligand. Kinetics of both the stages were studied spectrophotometrically in 0.2-0.9 M NaOH solution, under pseudo-first-order conditions. The calculated values of k obs were independent of [OH -]. A mechanism is proposed, where the formation of intermediates in the hydroxo form prevents the monodentate ligand from undergoing chelate-ring closure. Evidence for the formation of an intermediate with O-bonded picolinate is given. The effects of pH and the complex composition on the reactivity are discussed.

Oxidation in vitro of chromium(III) dietary supplements mer-[Cr(pic)3] and trans(S,S)-[Cr(Cys)2]− by hydrogen peroxide

“…First, the value of the rate constant for decay of the starting complex, at 0.26 9 10 -3 s -1 , is not much different than that for the CrO 4 2-anion formation (0.41 9 10 -3 s -1 ). Values of the rate constant for the picolinato chelate ring opening determined before, extrapolated to 298 K, vary between 0.7-1.0 9 10 -3 s -1 at 0.2-0.9 M Na [26] and are very close to those shown in Table 1. Thus, the rates of Cr III ?…”

“…In the presence of HCO 3 -, peroxocarbonates are formed. Secondly, in strongly alkaline media the equilibrium between the complex with the open and closed chelate ring is shifted to the latter, which retards an inner sphere electron transfer [25,26].…”

“…The mechanism of base hydrolysis of mer-[Cr(pic) 3 ] 0 was presented in our previous work [26], in which subsequent entry of a water molecule (not OH -ion) into the chromium(III) coordination sphere precedes formation of the hydroxido complexes. In the next steps mono-and dihydroxido derivatives of the starting complex, S-OH and S-(OH) 2 , are transformed into hydroperoxidochromium(III) intermediates, I-HO 2 and I-OH-HO 2 .…”