Reaction of 3,6-diaryl-1,2,4,5-tetrazines (aryl = R = phenyl, 2-furyl or 2-thienyl) with 2 equiv of Ru(acac)2(CH3-CN)2 results in reductive tetrazine ring opening to yield diruthenium complexes [(acac)2RuIII(dih-R2-)RuIII(acac) 2] bridged by the new 1,2-diiminohydrazido(2-) (dih-R2- = HNC(R)NNC(R)NH2-) ligands. rac/meso diastereoisomers could be detected and separated for the compounds with R = phenyl and 2-thienyl, all species are diamagnetic and were characterized by 1H NMR spectroscopy. Crystal structure determination of the meso isomers with R = phenyl and 2-thienyl confirmed the 1,2-diiminohydrazido formulation through long N-N (≈1.40 Å) and short C= N(H) bonds (≈1.31 Å), implying two bridged ruthenium(III) centers at about 4.765 Å distance with strong antiferromagnetic coupling. The complexes undergo two reversible and well-separated one-electron reduction and oxidation processes, respectively. EPR Spectroscopy of the paramagnetic intermediates with comproportionation constants Kc > 1012 and UV-vis-NIR spectroelectrochemistry were used to identify the accessible redox states as [(acac)2Ru II(dih-R2-)RuII(acac)2] 2-,[(acac)2RuII(dih-R2-)Ru III(acac)2]-,[(acac)2Ru III(dih-R2-)RuIII(acac)2], [(acac)2RuIII-(dih-R•-)Ru III(acac)2]+, and [(acac)2Ru III(dih-R)RuIII(acac)2]2+. While the UV-vis-NIR spectroscopic response of [(acac)2Ru(dih-R)Ru(acac) 2]0/-/2- is very similar to that of [(bpy) 2Ru(adc-R)Ru(bpy)2]4+/3+/2+, adc-R2- = 1,2-diacylhydrazido(2-), the EPR result indicating ligand-centered spin for [(acac)2RuII(dih-R•-)Ru II(acac)2]- despite deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures. © 2006 American Chemical Society.