The first two complexes of the trioxodinitrate anion were prepared by reaction of Zn(bipy)Cl(2).1.5H(2)O (1.1.5H(2)O) with Na(2)N(2)O(3).H(2)O in water to produce Zn(II)(bipy)(H(2)O)(N(2)O(3)) (2) and an analogous reaction of Co(bipy)(2)Cl(2).2.5H(2)O (3.2.5H(2)O) to produce Co(II)(bipy)(2)(N(2)O(3)) (4). Both complexes are air-sensitive and crystallize in the monoclinic space group P2(1)/n. For 2.H(2)O T = 24-26 degrees C, Z = 4, a = 7.7595(6) Å, b = 9.4798(7) Å, c = 17.197(1) Å, beta = 98.142(7) degrees, and V = 1252.3(2) Å(3). The final residuals were R1 = 0.0452, wR2 = 0.1172, and quality-of-fit = 1.057. The final model consisted of the main structure and a minor, 4% disordered component. For 4.4H(2)O T = -123 degrees C, Z = 4, a = 9.0950(9) Å, b = 17.924(3) Å, c = 13.695(2) Å, beta = 90.32(1) degrees, and V = 2232.5(6) Å(3). The final least-squares residuals were R1 = 0.0859, wR2 = 0.1975, and quality-of-fit = 0.917. A network of hydrogen bonds involving the crystal water molecules and the coordinated trioxodinitrate anion was found in each case; in 4, a series of fused four- and eight-membered rings were found to be a dominant feature in the extended structure. The structural data and the IR spectrum indicate that the N-N bond of trioxodinitrate, 1.264(5) Å in the free anion, is longer for the coordinated anion (1.280(5) Å in 2 and 1.333(11) Å in 4).