Reactions of sulfur dioxide with pentaaquoorganochromium(III) complexes and trispolypyridinechromium(II) complexes
The well-characterized series of pentaaquoorganochromium(III) complexes, abbreviated (H[subscript]2O)[subscript]5CrR[superscript]2+, is known to react with various electrophiles, such as Br[subscript]2 and Hg[superscript]2+. The rate law is first order in each reactant. In this work, the kinetics of the reaction of SO[subscript]2 indicated an electrophilic mechanism also. The inorganic products, Cr(H[subscript]2O)[subscript]6[superscript]3+ and (H[subscript]2O)CrSO[subscript]2R[superscript]2+, which occur in a 80% ot 20% ratio, are attributed to a post-transition state rearrangement of the intermediates;The photochemical excitation of the Cr(NN)[subscript]3[superscript]3+ series of compounds (where NN is bipyridine, phenanthroline or their substituted derivatives) yields the long-lived [superscript]2T/[superscript]2E [superscript]*Cr(NN)[subscript]3[superscript]3+ excited states. These excited states are strong oxidants, and undergo chemical reactions with reductants to yield Cr(NN)[subscript]3[superscript]2+. Both the formation of Cr(NN)[subscript]3[superscript]2+ and its reactions with other chemical species can be conveniently monitored spectrophotometrically;The kinetics and thermodynamics of these outer sphere redox reactions are correlated by Marcus theory, which predicts the rate constant for a reaction based on the free energy change, size and charge of the reactants, and their self-exchange rate constants. The self-exchange reaction is the transfer of an electron from a reduced complex to the oxidized form of the same complex and vice versa, for example D[superscript]- + D → D + D[superscript]-. In a reaction between two different chemical species, Marcus theory may be used to calculate any unknown parameter provided that all the others are known;In this work, the self-exchange rate constant for SO[subscript]2/SO[subscript]2[superscript]- was calculated by applying Marcus theory to reactions of Cr(NN)[subscript]3[superscript]2+ with SO[subscript]2 in acidic aqueous solution. ftn[superscript] aDOE Report IS-T-1366. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.