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Structural disorder is observed due to the fact that protonation, as well as oxidation, of the N atoms of 2,2′-bi­pyridine occurs either at either of the N atoms. The disorder extends to the remainder of the cation, with a refined occupancy rate of 0.717 (4) for the major moiety., The title compound 2,2′-bipyridin-1′-ium 1-oxide bromide crystallizes as a monohydrate, C10H9N2 +·Br−·H2O. Structural disorder is observed due to the fact that protonation, as well as oxidation, of the N atoms of 2,2′-bi­pyridine occurs at either of the N atoms. The disorder extends to the remainder of the cation, with a refined occupancy rate of 0.717 (4) for the major moiety. An intra­molecular N—H⋯O hydrogen bond forces the bi­pyridine unit into an s-cis conformation. Each pair of neighbouring 2,2′-bipyridin-1′-ium ions forms a dimeric aggregate by hydrogen bonds between their respective N—O and the N—H functions. These dimers and hydrogen-bonding inter­actions with bromide ions and the water mol­ecule give rise to a complex supra­molecular arrangement.

A series of (H2O)5Cr(X-pyO)3+ ions (pyO = pyridine N-oxide, X = H, 3-CH3, 4-CH3, 4-OCH3, 4-NO2) were prepared by the reduction of the corresponding pyridine N-oxide adducts of diperoxochromium(VI) species with acidic ferrous perchlorate. The (H2O)5Cr(X-pyO)3+ complexes undergo aquation to yield Cr(H2O)6 3+ and X-pyO according to the rate law k_obs = k_o+k_– 1[H+]– 1. The values of the rate constants extrapolated to 298 K at 1.0 M ionic strength are: k0 = 2.80×10– 6 s– 1, k– 1 =1.86×10– 8 Ms– 1 (X = 4-NO2) ; 7.80×10– 8, 6.27×10– 10 (H) ; 4.80×10– 8, 3.20×10– 10 (3-CH3) ; 3.05×10– 8, 1.60×10– 10 (4-CH3) ; and 2.37×10– 9, 4.76×10– 11 (4-OCH3). The reaction of the 4-OCH3 complex exhibits two additional terms in the rate law, k1[H+]+k– 2[H+]– 2. The binding of 4-OCH3– pyO to chromium is suggested to take place through the methoxy group.