[Ru-II(hedta)(D2O)](-), hedta(3-) = N-(hydroxyethyl)ethylenediaminetriacetate, reacts with pyrazine in D2O at 25 degrees C to yield several isomers and less than or equal to 20% of the pyrazine-bridged binuclear complex. Two isomers of 56.2% combined abundance have differentiated alpha (near) and beta (remote) H-1 NMR pyrazine resonances at 9.09 ppm (alpha or H2, H6 pair) and 8.33 ppm (beta or H3, H5 pair). The other isomer of ca. 24% abundance exhibits only a singlet at 8.76 ppm, indicative of fluxional pyrazine movement from N-1 to N-4. This is believed to be the cis-polar isomer. Within 24 h the differentiated isomers convert to the fluxional isomer, which remains fluxional in 50% D2O/50% CD3OD down to 237 K. The fluxional isomer has all equivalent C-13 NMR resonances at 152.44 ppm, 5.18 ppm downfield of free pyrazine. Comparison with the bridged binuclear ion {[Ru(hedta)](2)(pz)}(2-) revealed fortuitously similar shifts; the H-1 NMR spectrum shows a singlet at 8.76 ppm, and the C-13 NMR spectrum, a singlet at 152.40 ppm. These species have different electrochemical signatures, however, with the 1:1 fluxional complex having a Ru-II/III wave at 0.20 V that shifts to 0.35 V upon protonation of the N-4 position, whereas the binuclear complex has two waves at 0.18 and 0.33 V which are independent of pH. H-1 NMR indicates stereochemically rigid coordination of 2-methylpyrazine (2-CH(3)pz) at N-4 with the 2-CH3 position remote in the major species (65.5%) and at N-2 with the 2-CH3 site adjacent (34.4%) in the lesser isomer. The proton resonances are as follows. Remote isomer (N-4) : H2, 8.22 ppm; H3, 8.97 ppm; H5, 8.88 ppm; CH3, 2.53 ppm. Adjacent (N-1) isomer : H2, 8.73 ppm; H3, 8.42 ppm, H5, 8.50 ppm; CH3, 2.45 ppm. A slow conversion of the strained adjacent isomer to the remote isomer is observed. Remote and adjacent isomers were also prepared for [(NH3)(5)Ru(2-CH(3)pz)](2+) in 87.7% and 12.3% yield. Protonation of [Ru(hedta)(pz)](-) yields nonfluxional complexes : a major species (63%) bound eta(2)(1,2) With four complicated resonance patterns at 8.72, 8.57, 8.00, and 7.91 ppm, each signal having across-the-ring couplings which require couplings to at least two different ring protons and a minor N-1-coordinated N-4 protonated species (16%) with alpha and beta proton pairs resonating at 8.81 and 8.25 ppm. An eta(2)-(2,3) isomer is also detectable (20%) which has four types of H-1 resonances at 9.51, 9.01, 8.85, and 8.15 ppm. The weakened sigma bonding from pzH(+) and enhanced pi-acceptor capacity power of pzH(+) combine to induce a switch in coordination to either eta(2)(1,2) or eta(2)(2,3).