Delta/lambda-(delta/lambda-1,1’-Biisoquinoline)bis(2,2’-bipyridine)ruthenium(II) bis(hexafluorophosphate) (2) exists as an similar to 3:1 mixture of its two diastereomeric forms in acetone solutions at 25 degrees C. The major isomer, (Delta,delta/Delta lambda)-2, crystallizes in the monoclinic space group C2/c with Z = 8, a = 29.12(1), b = 18.593(7), and c = 17.85(1) Angstrom, beta = 127.81(4)degrees, R = 0.053, and R(W) = 0.062 at 25 degrees C. As expected, the 1,1’-biisoquinoline ligand is nonplanar, which is a result of a transannular steric interaction between H-8 and H-8’. Diastereomerically pure samples of 2 were found to isomerize rapidly in solution at room temperature in the absence of light to give a thermodynamic mixture of the two diastereomers. The rate data for the latter equilibrium at 80 degrees C are K = 2.89, k(6a(maj)-->6a(min)) = 12.7(3) s(-1), and k(6a,(min)-->6a(maj)) = 36.6(9) s(-1). The activation parameters were determined in the temperature range of 50-90 degrees C : Delta H-double dagger (maj-->min) = 68.7 kJ mol(-1), Delta S-double dagger(maj-->min) = -21 J K-1 mol(-1), Delta H-double dagger(min-maj) = 66.1 kJ mol(-1), and Delta S-double dagger(min-->maj) = -38 J K-1 mol(-1). Spin saturation transfer (SST), spin inversion transfer (SIT), and two-dimensional exchange spectroscopy (2D EXSY) NMR experiments using 2 and its 2,2’-bipyridine-d(8) analogue demonstrate that the interconversion of the two diastereomers is the result of an intramolecular process of C-2 symmetry that does not change the cis/trans relationship between the 1,1’-biisoquinoline and 2,2’-bipyridine ligands. Irregular mechanisms that involve breaking just one of the ruthenium-isoquinoline bonds have been ruled out because the rate of isomerization of a water-soluble derivative of 2, Delta/lambda-(delta/lambda-1,1’-biisoquinoline) bis(2,2’-bipyridine)ruthenium(II) dichloride, is essentially the same in D2O containing 1 M LiCl (k(6a(maj)-6a(min)) = 5.7(2) s(-1)) and 1 M DCl (k(6a(maj)-->6a(min)) = 7.1(1) s(-1)) at 80 degrees C. We therefore conclude that interconversion of the two diastereoisomers of 2 takes place by a regular mechanism that involves atropisomerization of the eta(2)-1,1’-biisoquinoline ligand via a syn transition state.