The rotational correlation times of [La(EDTA)](-) and [Co(en)(3)](3+) were measured as a function of solution viscosity using deuterium NMR relaxation measurements with labeled compounds. The solution viscosity was varied using either sucrose or glycerol as a cosolute at a constant temperature of 298 K. The nuclear electric quadrupole coupling constants were obtained from both solid state and Liquid phase measurements. However, because of uncertainties in the measurements and analysis, it is not possible to conclude that the solution phase and solid phase values are significantly different. The dependence of the rotational correlation time on solution viscosity shows large deviation from that predicted by the Stokes-Einstein model. Use of a modified Stokes-Einstein equation to correct for differences between the microdynamic viscosity and the bulk viscosity requires that the solute-solvent interaction parameter be dependent on the cosolute concentration. For the metal ion complexes studied, glycerol affects the rotational correlation times more than sucrose at the same bulk viscosity; however, the rotational correlation time as a function of cosolute concentration below 50% sucrose or glycerol is similar for both solvent systems.