Kinetics and mechanism of the oxidation of nitrilotris(methylenephosphonato)chromium(III) by periodate

ISSN： 0340-4285

Source： Transition Metal Chemistry, Vol.26, Iss.6, 2001-12, pp. : 695-699

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Abstract

The kinetics of oxidation of nitrilotris(methylenephosphonato)chromium(III), Cr^IIINTMP, by periodate to yield Cr^VI have been studied spectrophotometrically over the 5.80–6.85 pH range at 22–33 °C. The reaction rate, which is first-order with respect to [Cr^IIINTMP] and [IO^−_4] and inversely dependent on [H^+], obeys the rate law:$${{-\hbox{d}[\hbox{Cr}^{\rm III}\hbox{NTMP}]} \over {\hbox{d}t}} ={{kKK_h[\hbox{IO}^-_4] [\hbox{Cr}^{\rm III}]_{\rm T}}\over {K_h+ [\hbox{H}^+] +KK_h[\hbox{IO}^-_4]}}}$$The values of the intramolecular electron transfer, k, and the formation constant of the intermediate complex, K, were determined at various temperatures. The hydrolysis constant for Cr^IIINTMP, K, was determined spectrophotometrically and is in agreement with the value estimated from the kinetic data. The activation parameters were calculated from the temperature dependence of the specific rate constants. A mechanism is proposed in which the hydroxo complex, [CrHNTMP(OH)]^3−, is the reactive species. The results support a mechanism where intramolecular electron transfer is the rate-determining step.