2,2'-dipyridylketone (dpk) as ancillary acceptor and reporter ligand in complexes [(dpk)(Cl)Ru(μ -tppz)Ru(Cl)(dpk)]<SUP>n+</SUP> where tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine

The tppz-bridged diruthenium(II) complex [(dpk)(Cl)Ru<SUP>II</SUP>(μ -tppz)Ru<SUP>II</SUP>(Cl)(dpk)](ClO<SUB>4</SUB>)<SUB>2</SUB>, [2](ClO<SUB>4</SUB>)<SUB>2</SUB>, and mononuclear [(dpk)(Cl)Ru<SUP>II</SUP>(tppz)](ClO<SUB>4</SUB>), [1](ClO<SUB>4</SUB>) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, dpk = 2,2'-dipyridylketone], have been synthesized. The 260 mV separation between successive one-electron oxidation couples in [2]<SUP>2+</SUP> translates to a relatively small comproportionation constant, K<SUB>c</SUB>, of 2.5 × 10<SUP>4</SUP> for the intermediate. It is shown how electrochemical data (E<SUB>ox</SUB>, E<SUB>red</SUB>, K<SUB>c</SUB>) reflect the donor/acceptor effects of ancillary ligands L in a series of systems [(L)ClRu(μ -tppz)RuCl(L)]<SUP>n</SUP>, particularly the competition between L and tppz for electron density from the metal. According to EPR (g<SUB>1</SUB> = 2.470, g<SUB>2</SUB> = 2.195, and g<SUB>3</SUB> = 1.873 at 4 κ) the intermediate [2]<SUP>3+</SUP> is a mixed-valent Ru<SUP>II</SUP>Ru<SUP>III</SUP> species which shows a rather narrow intervalence charge transfer (IVCT) band at 1800 nm (ε = 1500 M<SUP>-1</SUP> cm<SUP>-1</SUP>). The width at half-height (Δν<SUB>½</SUB> ) of 700 cm<SUP>-1</SUP> of the IVCT band is much smaller than the calculated value of 3584 cm<SUP>-1</SUP>, obtained by using the Hush formula Δν<SUB>½</SUB> = (2310E<SUB>op</SUB>)<SUB>½</SUB> (E<SUB>op</SUB> = 5556 cm<SUP>-1</SUP>, energy of the IVCT transition) which would be applicable to localized (Class II) mixed-valent Ru<SUP>II</SUP>Ru<SUP>III </SUP>systems. Valence delocalization in [2]<SUP>3+</SUP> is supported by the uniform shift of the ν<SUB>C=O</SUB> band of the N,N'-coordinated dpk ligands from 1676 cm<SUP>-1</SUP> in the RuIIRuII precursor to 1690 cm<SUP>-1</SUP> in the Ru<SUP>2.5</SUP>Ru<SUP>2.5</SUP> form, illustrating the use of the dpk acceptor to act as reporter ligand via the free but π-conjugated organic carbonyl group. The apparent contradiction between the moderate value of K<SUB>c</SUB> and the narrow IVCT band is being discussed considering "borderline" or "hybrid" "Class II-III" concepts of mixed-valency, as well as coordination aspects, i.e., the bis-tridentate nature of the π-acceptor bridging ligand. Altogether, the complex ions [1]<SUP>+</SUP> and [2]<SUP>2+</SUP> display four and five successive reduction processes, respectively, involving both tppz<SUP>-</SUP> and dpk-based unoccupied π orbitals. The one-electron reduced form [2]<SUP>+</SUP> has been assigned as a tppz<SUP>·-</SUP> radical-anion-containing species which exhibits a free-radical-type EPR signal at 4κ (g<SUB>|| </SUB>= 2.002, g<SUB>⊥</SUB> = 1.994) and one moderately intense ligand-based low-energy band at 965 nm (ε = 1100 M<SUP>-1</SUP> cm<SUP>-1</SUP>)