The series of 4-center unsaturated chelate ligands A=B-C=D with redox activity to yield (-)A-B=C-D- in two steps has been complemented by two new combinations RNNC(R')E, E = O or S, R = R' = Ph. The ligands N-benzoyl-N'-phenyldiazene = L-O, and N-thiobenzoyl-N'-phenyldiazene = L-S, (obtained in situ) form structurally characterized compounds [(acac)(2)Ru(L)], 1 with L = L-O, and 3 with L = L-S, and [(bpy)(2)Ru(L)](PF6), 2(PF6) with L = L-O, and 4(PF6) with L = L-S (acac(-) = 2,4-pentanedionato; bpy = 2,2'-bipyridine). According to spectroscopy and the N-N distances around 1.35 angstrom and N-C bond lengths of about 1.33 angstrom, all complexes involve the monoanionic (radical) ligand form. For 1 and 3, the antiferromagnetic spin spin coupling with electron transfer-generated Rum leads to diamagnetic ground states of the neutral complexes, whereas the cations 2(+). and 4(+) are EPR-active radical ligand complexes of Ru-II. The complexes are reduced and oxidized in reversible one-electron steps. Electron paramagnetic resonance (EPR) and UV-vis-NIR spectroelectrochemistry in conjunction with time-dependent density functional theory (TD-DFT) calculations allowed us to assign the electronic transitions in the redox series, revealing mostly ligand-centered electron transfer: [(acac)(2)Ru-III(L-0)](+) reversible arrow [(acac)(2)Ru-III(L center dot-)] reversible arrow [(acac)(2)Ru-III(L2-)](-)/[(acac)(2)Ru-II(L center dot-)](-), and [(bpy)(2)Ru-III(L center dot-)](2+)/[(bpy)(2)Ru-II(L-0)](2+) reversible arrow [(bpy)(2)Ru-II(L center dot-)](+) reversible arrow [(bpy)(2)Ru-II(L2-)](0). The differences between the O and S containing compounds are rather small in comparison to the effects of the ancillary ligands, acac(-) versus bpy.