Rates and activation parameters for dimethyl sulphoxide exchange with hexakis(dimethyl sulphoxide)-nickel(II), -cobalt(II), and -iron(III) ions

Abstract

Natural-abundance ¹⁷O n.m.r. at 54.24 MHz has been used to measure the rates of dimethyl sulphoxide (dmso) exchange with [M(dmso)₆]²⁺(M = Co or Ni) ions, and 90-MHz stopped-flow ¹H Fouriertransform n.m.r., and 22.63-MHz ¹³C n.m.r. line broadening, to measure the rate of dmso exchange with [Fe(dmso)₆]³⁺ ion. At 298.2 K the rate constants and associated activation parameters for the exchange of a single solvent molecule are: 10^–3k_ex^I/s^–1= 445 ± 12, 16.5 ± 0.3, and 0.026 ± 0.002; ΔH^‡/kJ mol^–1= 49.0 ± 1.5, 49.2 ± 0.4, and 52.1 ± 0.7; ΔS^‡/J K^–1 mol^–1= 28 ± 5, 0.9 ± 1.1, and –43 ± 3 for Co^II, Ni^II, and Fe^III respectively. A systematic method is proposed for adjusting the relative weights of data sets when simultaneously fitting ln(T_2r^–1) and Δω_r, data to the Swift–Connick equations.