The physical properties of organometallic ionic liquids containing cationic ruthenium sandwich complexes {[Ru(C5H5)-(C6H5R)]X
R = OCH(C4H9)2, OCH(C2H5)(C6H13)
X = N(SO2CF3)2, N(SO2F)2} are dependent on the substituents. To obtain detailed information about the motional states and configurations of the constituent ions of these ionic liquids, we performed one-dimensional (1D) heteronuclear Overhauser enhancement spectroscopy (HOESY) NMR experiments and the standard relaxation rate (1/T1 and 1/T2) measurements. We determined the intermolecular cross-relaxation rates between some cation blocks and the anion in each ionic liquid by analyzing the experimental data based on the extended Solomon differential equations. As a result, we were able to estimate several intermolecular distances between the cations and anions in the ionic liquids. This is the first time that NMR has been applied to the determination of the intermolecular crossrelaxation rates and the estimation of intermolecular distances of highly viscous ionic liquids consisting of heteronuclear multiple-spin systems without the support of any X-ray data. The viscosities of these ionic liquids depended largely on the motional correlation times of the cations rather than on the intermolecular distances between the cations and anions.