ELECTRON-TRANSFER .30. CHROMIUM(III)-BOUND PYRAZINE RADICALS

The pyrazine greens are strongly absorbing species [FORMULA OMMITED] formed by the action of Cr2+on substituted pyrazines in aqueous acidic solution. The reactions of one of the most stable of these, derived from pyrazinecarboxamide, with a number of (NH3)5CoIII complexes yield Co2+, together with the same Cr(III) product as is formed in reduction by Cr2+ itself, but rates are several orders of magnitude lower. Such reactions are further inhibited by excess amide. Kinetic data support sequence 2, in which the green radical cation, formulated as CrIIIPz., dissociates (k1) to the parent pyrazine and Cr2+, which, in turn, may react with Co(III) (k2) or return to the radical cation (fe_i). Values of k1/k-1 obtained from measurements on different Co(III) systems are in agreement, and values for the reactions of fluoro- and bromopentaamminecobalt(III) complexes with Cr2+ are consistent with literature rates. The calculated rate of dissociation of the green ion to Cr2+ is 1010—1011 times lower than the acceped range for substitution reactions at Cr(II) centers but several or ers of magnitude above the heterolysis rates of the usual Cr(III) complexes in water, suggesting that the rate of dissociation is determined by the rate of internal electron transfer within the radical cation. The equilibrium constant for the conversion of CrIIIPz. to Cr2+ is found to be one-tenth of that estimated from the reduction potentials of Cr3+ and pyrazinecarboxamide, indicating that CrIIIPz. is 10 times as stable toward aquation as is the pyrazinecarboxamide complex of Cr(II). © 1978, American Chemical Society. All rights reserved.