Nuclear Magnetic Relaxation of 19F in Transition-Metal Complexes. II. Complexes with Mn(II), Co(II), Fe(II), and Ni(II)

Abstract

The effect of Mn(II), Fe(II), Co(II), and Ni(II) on T1 and T2 of aqueous 19F has been measured from pH 0–6 at 25°C as a function of fluoride concentration and magnetic field, similar to previous work with Cu(II) [J. Chem. Phys. 48, 4445 (1968)]. Equilibrium constants for the first coordination complex MF+ were obtained from the data and ranged from 1 to 5; there was no evidence of higher complexes. The principal results were the association and dissociation rates of the complex. The latter is found to be of the form kd = τm−1 = ka + kb(H+) + kx(F−) + ky(HF) + kz(HF2−), where τm is the lifetime of the complex. These rates are given in the following table: Mn(II)Fe(II)Co(II)Ni(II)logka5.95.85.03.3logkb8.78.88.45.8logkx6.2···5.23.8logky7.1······4.7logkz6.9···6.3···The electron relaxation time T1e was >1 × 10−8 sec for Mn(II) and <5 × 10−12 sec for the other three metals, similar to results found for the aquo complex. The isotropic hyperfine interaction constants Ai for FeF+ and CoF+ were 8.7 × 108 and 2.1 × 108 rad/sec. The temperature dependence of T1 and T2 for Ni19F+ was similar to that of proton relaxation in the aquo complex.