Mixed Valence Aspects of Diruthenium Complexes [{(L)ClRu}<sub>2</sub>(<i>μ</i>-tppz)]<i><sup>n</sup></i><sup>+</sup> Incorporating 2-(2-Pyridyl)azoles (L) as Ancillary Functions and 2,3,5,6-Tetrakis(2-pyridyl)pyrazine (Tppz) as Bis-Tridentate Bridging Ligand

Tppz [2,3,5,6-tetrakis(2-pyridyl)pyrazine]-bridged complexes [{(L)ClRu}<sub>2</sub>(<i>μ</i>-tppz)]<i><sup>n</sup></i><sup>+</sup> with structurally similar but electronically different ancillary ligands, 2-(2-pyridyl)azoles (L), were synthesized as diruthenium(II) species. Cyclic voltammetry, EPR of paramagnetic states, and UV−vis−NIR spectroelectrochemistry show that the first two reduction processes occur at the tppz bridge and that oxidation involves mainly the metal centers. The mixed valent intermediates from one-electron oxidation exhibit moderate comproportionation constants 10<sup>4</sup> < <i>K</i><sub>c</sub> < 10<sup>5</sup> but appear to be valence-averaged according to the Hush criterion. Redox potentials, EPR, and UV−vis−NIR results show the effect of increasing donor strength of the ancillary ligands along the sequence L<sup>1</sup> < L<sup>2</sup> < L<sup>4</sup> ≪ L<sup>3</sup>, L<sup>1</sup> = 2-(2-pyridyl)benzoxazole, L<sup>2</sup> = 2-(2-pyridyl)benzthiazole, L<sup>3</sup> = 2-(2-pyridyl)benzimidazolate, L<sup>4</sup> = 1-methyl-2-(2-pyridyl)-1<i>H-</i>benzimidazole. Whereas the mixed valent complexes with L<sup>1</sup> and L<sup>2</sup> remain EPR silent at 4 K, the analogues with L<sup>4</sup> and L<sup>3</sup> exhibit typical ruthenium(III) EPR signals, albeit with some noticeable ligand contribution in the case of the L<sup>3</sup>-containing complex. Intervalence charge transfer (IVCT) bands were found in the visible spectrum for the complex with L<sup>3</sup> but in the near-infrared range (at ca. 1500 nm) for the other systems.