STEPWISE AQUATION OF 1,6-DIBROMO-2-AQUO-3-AMMINEETHYLENEDIAMINECHROMIUM(3) CATION TO TETRAAQUOETHYLENEDIAMINECHROMIUM(3) CATION

The kinetics of the successive aquation steps of green Cr(en)(NH3)(OH2)Br2+ to Cr(en)(OH2)43 + have been investigated by spectrophotometry and also by Br release for the first two equation steps. The first step produces magenta Cr(en)- (NH3)(OH2)2Br2+, and was studied in 0.1-2 F HClO4 (μ = 0.3-2 M, NaClO4,) from 20 to 35°. At 25° the first-order rate constant is k1 = 4.1 X 10-4 sec-1 (0.1-0.3 FHClO4; μ = 0.3 M, NaClO4), Ea = 20.7 ± 0.3 kcal mol-1, and log PZ(sec-1) = 11.81 ± 0.23. Aquation of the monobromo product appears to give a single pink isomer of Cr(en)(NH3)(OH2)33+. In 0.15-1 F HClO4(μ = 1 M, NaClO4) at 34-53° the observed hydrolysis rate constant k2.obsd has the form k2.obsd = k2 + (k2′/[H+]), where k2 is the first-order rate constant for aquation of magenta Cr(en)(NH3)(OH2)2Br3+ and k2 has been interpreted as k2h-Ka, where k2h, is the first-order rate constant for aquation of Cr(en)(NH3)(OH2)(OH)Br+ and ka′ is the first acid dissociation constant of magenta Cr(en)(NH3)(OH2)2Br2+. At 25° k2 = 4 X 10-5 sec-1 (by extrapolation), Ea = 23.2 ± 0.1 kcal mol-1, and log PZ(sec-1) = 11.57 ± 0.09. At 34° and μ = 1 M, k2h, ≈ 0.01 sec-1. Aquation of pink Cr(en)(NH3)(OH2)33+ to Cr(en)(OH2)43+ was studied in 0,15-3 F HClO4 from 45 to 60° found to be first order. In 0.15-1 F HClO4 (μ = 1.0 M), k3 = 5.7 X 10-5 sec-1 at 52.50° ca. 1 X 10-6 sec-1 at 25° by extrapolation), Ea = 27.5 ± 0.3 kcal mol-1, and log PZ(sec-1) = 14.08 ± 0.17. There is no evidence for Cr-N bond rupture involving the ethylenediamine ligand in any of these reactions. The kinetic results confirm the trans-dibromo and trans-bromoaquo configurations spectrophotometrically assigned earlier to green Cr(en)(NH3)(OH2)Br2+ and magenta Cr(en)(NH3)(OH2)2Br2+. © 1969, American Chemical Society. All rights reserved.