MECHANISM OF ELECTRON TRANSFER FOR THE OXIDATION OF TERNARY COMPLEX OF CHROMIUM(III) INVOLVING GUANOSINE AND DL-ASPARTIC ACID WITH PERIODATE

The oxidation of the ternary complex, [Cr(Ill)(Gu)(Asp)(H2O)2](2+) (Gu = guanosine and Asp = DL-aspartic acid) by periodate in aqueous solution to chromium(VI) has been studied spectrophotometrically over the 25-45 degrees C range. The reaction is first order with respect to both [1041 and [Cr(Ill)], and increases with pH over the 2.38-3.68 range. The experimental rate law is consistent with the derived mechanism and the reaction follows the rate law: d[Cr(III)]/dt = (k(1)K(5)+ k(2)K(6)K(1/)[H+])[IO4-][Cr(111)](T)/{1 + K-1/[H+] + (K-5+ K1K6/[H+])[IO4-]), where K-1, K-5 and K-6 are the deprotonation of [Cr(III)(Gu)(Asp)(H2O)(2)](2+) and pre-equilibrium formation constants for [Cr(III)(Gu)(Asp)(H2O)(OIO3)](+) and [Cr(III)(Gu) (Asp)(OH)OIO3] precursor complexes, respectively. The reaction may follow a 2-step inner-sphere electron transfer mechanism via coordination of IO4- to chromium(III). Thermodynamic activation parameters have been calculated.