KINETICS AND MECHANISM OF THE FORMATION AND REACTIVITY OF U-SULFITO-BIS[BIS(1,2-DIAMINOETHANE)CHROMIUM(III)]

The kinetics of formation of [(en)2Cr(mu-OH)(mu-SO,)Cr(en)2]3+ cation from [(en)2Cr(mu-OH)2Cr(en)2]4+ (en = 1,2-diaminoethane) in aqueous bu ffered sulfite solution has been studied spectrophotometrically. This formation consisted of two steps; first a rapid uptake of SO, by the r ing-cleaved mono-mu-hydroxo species [(OH)(en)2Cr(p-OH)Cr(en)2(OH)]3+ t o form a terminal O-bonded sulfito intermediate. This reaction was stu died at 600 nm in the ranges 3.78 less-than-or-equal-to pH less-than-o r-equal-to 5.47, 20 less-than-or-equal-to theta less-than-or-equal-to 30-degrees-C and 0.03 less-than-or-equal-to [S]T less-than-or-equal-to 0.05 mol dm-3 ([S]T = [SO2] + [HSO3-] + [SO32-]). The rate constant f or uptake of SO2 at 25-degrees-C was found to be (8.7 +/- 0.1) x 10(7) dm3 Mol-1 s-1 with DELTAH(double dagger) = 6.5 +/- 0.9 kJ mol-1 and D ELTAS(double dagger) = -71 +/- 2 J K-1 mol-1. The second step in which the sulfito intermediate undergoes a ring closure to form the (mu-OH) (mu-SO3) complex with the elimination of H2O (or OH-) was studied in t he ranges 2.46 less-than-or-equal-to pH less-than-or-equal-to 4.46, 19 .9 less-than-or-equal-to theta less-than-or-equal-to 29.9-degrees-C an d 0.03 less-than-or-equal-to [S]T less-than-or-equal-to 0.05 mol dm-3, and the rate constant for this process, k3, was found to be 0.68 s-1 at 25.2-degrees-C with DELTAH(double dagger) = 28 +/- 1 kJ mol-1 and D ELTAS(double dagger) = -155 +/- 3 J K-1 mol-1. The rate constant for r ing closure with the loss of OH- was found to be 0.15 s-1 at 25.2-degr ees-C with DELTAH(double dagger) = 66 +/- 7 kJ mol-1 and DELTAS(double dagger) = -40 +/- 24 J K-1 mol-1. The (mu-OH)(mu-SO2) complex decompo ses in acid to give free sulfite with a rate constant of (7.8 +/- 0.1) x 10(-5) s-1 at 25-degrees-C which is independent of [H+] over the ra nge 0.01-1.0 mol dm-3 HClO4. It undergoes additional sulfite uptake (k = 3.01 x 10(-4) s-1, 25-degrees-C, DELTAH(double dagger) = 91 +/- 8 k J mol-1, DELTAS(double dagger) = -8.7 +/- 29.6 J K-1 mol-1) to form a possible disulfito product. The reaction was found to be independent o f [S]T but dependent on [H+] over the ranges 2.11 less-than-or-equal-t o pH less-than-or-equal-to 7.28, 0.03 less-than-or-equal-to [S]T less- than-or-equal-to 0.05 mol dm-3 and 20 less-than-or-equal-to theta less -than-or-equal-to 30-degrees-C.