Abstract
Water exchange from the oxo-centered rhodium(III) trimer, [Rh3(μ3-O)(μ-O2CCH3)6(OH2)3]+, was investigated using variable-temperature (272.8−281.6 K) and variable-pressure (0.1−200 MPa) 17O NMR spectroscopy. The exchange reaction was also monitored at three different acidities (pH = 1.8, 2.9, and 5.7) in which the molecule is in the fully protonated form (pK a = 8.3 (±0.2), I = 0.1 M, T = 298 K). The temperature dependence of the pseudo-first-order rate coefficient for water exchange yields the following kinetic parameters: k ex 298 = 5 × 10-3 s-1, ΔH ⧧ = 99 (±3) kJ mol-1, and ΔS ⧧ = 43 (±10) J K-1mol-1. The enhanced reactivity of the terminal waters, some 6 orders of magnitude faster than water exchange from Rh(H2O)6 3+, is likely due to trans-labilization from the central oxide ion. Also, another contributing factor is the low average charge on the metal ions (+0.33/Rh). Variation of reaction rate with pressure results in a ΔV ⧧ = +5.3 (±0.4) cm3 mol-1, indicative of an interchange-dissociative (I d ) pathway. These results are consistent with those published by Sasaki et al. who proposed that water substitution from rhodium(III) and ruthenium(III) oxo-centered trimers follows a dissociative mechanism based on highly positive activation parameters (Sasaki, Y.; Nagasawa, A.; Tokiwa-Yamanoto, A.; Ito, T. Inorg. Chim. Acta 1993, 212, 175−182).