The diruthenium(III) complex [(acac)2Ru(μ-L 2-)Ru(acac)2] (1) with acac- = acetylacetonato = 2,4-pentanedionato and a 2,5-pyrazine-dicarboxylato bridge, L2-, has been obtained and structurally characterized as the rac (ΔΔ, ΛΛ) diastereomer. The RuIIIRuIII configuration in 1 (dRu-Ru = 6.799 Å) results in a triplet ground state (μ = 2.82 μB at 300 K) with a density functional theory (DFT) calculated triplet-singlet gap of 10840 cm-1 and the metal ions as the primary spin-bearing centers (Mulliken spin densities: Ru, 1.711; L, 0.105; acac, 0.184). The paramagnetic 1 exhibits broad, upfield shifted 1H NMR signals with δ values ranging from -10 to -65 ppm and an anisotropic electron paramagnetic resonance (EPR) spectrum (〈g〉 = 2.133, g1 - g3 = Δg = 0.512), accompanied by a weak half-field signal at g = 4.420 in glassy frozen acetonitrile at 4 K. Compound 1 displays two closely spaced oxidation steps to yield labile cations. In contrast, two well separated reversible reduction steps of 1 signify appreciable electrochemical metal-metal interaction in the Ru IIRuIII mixed-valent state 1- (Kc ≈ 107). The intermediate 1- shows a weak, broad Ru II→RuIII intervalence charge transfer (IVCT) band at about 1040 nm (ε = 380 M-1 cm-1); the DFT approach for 1- yielded Mulliken spin densities of 0.460 and 0.685 for the two metal centers. The monitoring of the νC=O frequencies of the uncoordinated C=O groups of L2- in 1n by IR spectroelectrochemistry suggests valence averaging (Ru2.5Ru 2.5) in 1- on the vibrational time scale. The mixed-valent 1- displays a rhombic EPR signal (〈g〉 = 2.239 and Δg = 0.32) which reveals non-negligible contributions from the bridging ligand, reflecting a partial hole-transfer mechanism and being confirmed by the DFT-calculated spin distribution (Mulliken spin density of -0.241 for L in 1-). The major low energy electronic transitions in 1n (n = 0,-,2-) have been assigned as charge transfer processes with the support of TD-DFT analysis. © 2011 American Chemical Society.