Cobalt-59 nuclear magnetic resonance studies of the kinetics and mechanism of deuteration for tris(ethylenediamine) cobalt chloride

Abstract

Dissolution of [Co(en)₃]Cl₃ in D₂O gives spectra which are time-dependent because of the progressive replacement of protons by deuterons. The isotopic chemical shift is 5.0 ppm per ¹H →²H substitution and all thirteen isotopomers can be detected, even using a low-field spectrometer. Isotopic distributions have been obtained by bandshape-fitting. The exchange rate has been measured under a variety of conditions and is found to depend on the [¹H]/[²H] concentration ratio and on the pH. A simple model for the exchange is proposed and tested. An isotopic effect influences the rate constants. Four separate values for rate constants have been extracted from the data. These values, which range from 8 × 10⁶ to 1 × 10⁶ dm³ mol^–1 s^–1, indicate that the rate of removal of D by OD^– > H by OD^– > H by OH^– > D by OH^–.