Kinetics and mechanisms of the reductions of thallium(III) and chromium(III) by uranium(III) in perchlorate media

The kinetics and mechanisms of the reaction of U(III) with Tl(III) and Cr(III) have been examined in perchlorate media. The Tl(III) reaction was found to obey the rate law -d[U(III)]/ dt = [U3+][Tl3+] [k/sub -1/K/(H + K)] in which k/sub -1/ has a value of (3.6 +- 0.2) x 105 M-1 s-1 at 200C and an ionic strength of 2.0 M, whereas the Cr(III) reaction was found to exhibit a mechanism dominated by a reaction path having an inverse second-order concentration dependence. The rate constants are compared with those obtained for other similar systems; it is suggested, that the acid concentration independent and the inverse acid concentration dependent reaction paths of many electron-transfer reactions involving metal ions exhibit a limiting rate of 106 to 107 M-1 s-1. Plausible reasons for this limiting value are discussed