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Kinetic studies of the reactions of Ru(NH3)5pz2+ (pz = pyrazine), Ru(NH3)4bpy2+ (bpy = 2,2'-bipyridine), Ru(NH3)5pzMe3+ (pzMe+ = JV-methylpyrazinium), Ru2(NH3)10pz5+, RuRh(NH3)10pz5+, and Ru2(NH3)10pz4+ with S2Og1 2" are interpreted on the basis of a mechanism involving ion-pair formation between the ruthenium complexes and S2Os2" followed by one-electron transfer from Ru(II) to S2Os2". The dependence of the rate constants upon the standard free energy changes for the above reactions as

Single-crystal EPR experiments have been performed on orthorhombic [(NH3)5Ru(pz)Ru(NH3)5]Cl5'5H20 at 3 K. Two resonance lines appear in the crystallographic ab plane, one each from the two structurally equivalent binuclear ions with different orientation. The resonances are described by g tensors that have their principal axes collinear to the molecular axes. Both gx components are parallel to the crystallographic c axis, which is perpendicular to the plane of the pyrazine rings. The angle between the Z axes (Ru-Ru line) of the two magnetically inequivalent complexes is 86 (1)°i n agreement with X-ray diffraction results (85.5°). The components of the g tensor are gx = 1.346 (3), gY -2.799 (3), and gz = 2.487 (3). No Ru hyperfine structure could be observed owing to the large line width in the undiluted single crystal. The EPR spectrum was simulated by diagonalization of the 6 X 6 perturbation matrix using a one-center basis set in the hole formalism with the parameters D/\ -2.317 (20), E/\ = 0.745 (20), and K = 0.981 (5). Both an axial and a rhombic component of the ligand field must be considered to reproduce the g tensor anisotropy. The orbital reduction factor k within the t2g5 configuration is estimated to be 0.79. The unpaired electron is predominantly in an orbital perpendicular to the plane of the pyrazine ring. Delocalization of the odd electron over the two ruthenium ions via the tr* system of the pyrazine ring is therefore possible.

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