The diruthenium(III) complex [(acac)(2)Ru(mu-L2-)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 (Delta Delta,Lambda Lambda) diastereomer. The (RuRuIII)-Ru-III configuration in 1 (d(Ru-au) = 6.799 angstrom) results in a triplet ground state (mu = 2.82/2 mu(B) at 300 K) with a density functional theory (DFT) calculated triplet-singlet gap of 10840 cm-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 H-1 NMR signals with delta values ranging from -10 to -65 -65 ppm and an anisotropic electron paramagnetic resonance (EPR), spectrum (< g > = 2.133, g(1) -g(3) = Delta g = 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 Run Rum mixed-valent state 1(-) (K-c approximate to 10(7)). The intermediate 1(-) shows a weak, broad Ru-II -> Ru-III intervalence charge transfer (IVCT) band at about 1040 nm (epsilon = 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 frequencies of the uncoordinated C=O groups of L2- in 1" by IR spectroelectrochemistry suggests valence averaging ((RuRu2.5)-Ru-2.5) in 1(-) on the vibrational time scale. The mixed-valent 1(-) displays a rhombic EPR signal (< g > = 2.239 and Delta g = 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 1(n) (n = 0,-,2-) have been assigned as charge transfer processes with the support of TD-DFT analysis.