First-order rate constants from 75 000 to 150000 s™1 are measured by the pulsed-accelerated-flow technique for the rate of loss of IC12™ in neutral and basic solutions. The rate varies inversely with the Cl™ concentration because ICl(aq) is the reactive form. The hydrolysis of ICl(aq) to give HOI and Cl™ is extremely rapid (A:H;0 = 2.4 X 106 s'1 at 25.0 °C, µ = 0.50). Hydroxide ion accelerates the rate of hydrolysis. The second-order rate constant for the reaction of OH' with ICl(aq) is very large (& = 4.5 X 109 M™1 s'1). Reactions of ICl(aq) with H2P04™/HP042™ and with HC03"/C032™ buffers are also very rapid and are first order in the total buffer concentration, with resulting second-order rate constants, (1.3 ± 0.2) X 109 M'1 s™1, that do not depend on the basicity of the buffer or the ratio of buffer species. Two ICl(aq) species are proposed to be in rapid equilibria: H2OICl (82%) and IC1 (18%). Kinetic data indicate that IC1 (the less hydrated species) is so reactive that it is unselective and reacts with H2P04™, HP042', HC03™, C032™, and OH™ at the diffusion limit (7 X 109 M'1 s'1). The more hydrated species, H20IC1, also reacts rapidly with OH™ (3.9 X 109 M™1 s™1) by proton transfer to form H20 and HOIC1™, which rapidly loses Cl™ to give HOI. Spectrophotometric measurements of the equilibrium constants at 25.0 °C give ([lCl(aq)][Cl™])/[ICl2™] = 0.013 (µ = 1.0 and 0.5) and ([HOI][Cl™]2[H+])/[ICl2™] = 1.06 X 10™6 3 (µ = 0.5).Kinetics of the hydrolysis of Cl2, Br2, and I2 have been de-

A pulsed-accelerated-flow spectrophotometer with UV-visible capability is described that permits measurement of pseudo-first-order rate constants as large as 500 000 s(-)(1) (t(1/2) = 1.4 μs). Chemical rate processes are resolved from physical mixing rate processes by variation of flow velocities under conditions of turbulent flow. Two mixing processes are found in the mixing/observation tube. One mixing rate constant, valid for the full length of the tube, is directly proportional to the flow velocity. The other mixing behavior, proportional to the square of the flow velocity, is found only in the immediate vicinity of the 10 inlet reactant streams that collide with one another in the middle of the observation tube. Contributions from the latter mixing become more important for very fast reactions that take place close to the inlet jets. These mixing models and improved signal/noise permit the measurement of rate constants for very fast reactions. Applications of the PAF method to electron-transfer, proton-transfer, hydrolysis, and non-metal redox reactions are reported for pseudo-first-order and second-order reactions.

The estimate for the Ru(NH3)5pz2+ ion is modestly higher than the 0.20 electron (22% ligand character to the ground-state MO),21,22 which has been estimated from the ground-and excited-state pAfa's. Protonation is sure to alter the solvation in the resultant Ru(NH3)5(pzH)3+ complex, and this could lead to a small error in the estimate of the extent of back-donation from Ru(II) by the pKa method. 19 The results here are in good agreement with the estimates of Zwickel and Creutz based on Ru(NH3)4L22+ spectra.18 The result that CO and the coordinated acetylenic unit occurs with more significant charge transfer than the N-heterocycles is in keeping with prior studies.10,23'24 This effect has been attributed to the synergistic stabilization ofaccepting/a-donating with small molecules with triple bonds (e.g. CO, CN", R2C2, N2, NO+).23 Cook et al. estimated 0.8 ± 0.2 electron transferred from Pt(PPh3)2Pt(C2H4) and 1.8 ± 0.2 for 02 as the acceptor.1 Assuming less good -donation from a d6 Ru(II) center compared with a d10 Pt(0) center, one might anticipate that C2H4 or dmad would receive a somewhat smaller (21) Taube, H.

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