Hydrogen-1 nuclear magnetic resonance studies of the ion association of some unsymmetrically substituted tetra-alkylammonium tribromo(triphenylphosphine)cobaltate(II) complexes in various solvents

The concentration association constant of [NMe(n-C₈H₁₇)₃][CoBr₃(PPh ₃)] in nitrobenzene at 34°C has been determined by n.m.r. spectrometry. On comparing this result with those obtained earlier for [NRBuⁿ₃][CoBr₃(PPh₃)] (R = Me and Buⁿ), it is concluded that the symmetry of the cation plays a very important role in determining the extent of ion association for this type of ion-pair complex. There is a linear correlation between the observed difference in isotropic shift of the N-methyl and N-methylene protons and the dielectric constant (ε) of the solvents for ε ≤ 10, but not at higher values of ε. This is interpreted in terms of the preferred orientation of the N-methyl group towards the anion, the decrease in interionic distance with decreasing ε, the formation of micelles in solvents of low dielectric constant, and the 1:1 ion-pair formation in solvents of higher dielectric constant. The problem of analyzing the concentration dependence of observed isotropic shifts in solvents of low dielectric constant is discussed.