A NEW POLYMORPH OF MALEIC HYDRAZIDE

1,2-Dihydropyridazine-3,6-dione, C4H4N2O2, M(r) = 112.09, monoclinic, P2(1)/c, a = 6.891 (2), b = 9.674 (2), c = 6.946 (1) angstrom, beta = 100.07 (2)-degrees, V = 455.9 (2) angstrom3, Z = 4, D(x) = 1.63 (1) g cm-3, lambda(Cu Kalpha) = 1.54178 angstrom, mu = 10.31 cm-1, F(000) = 232, m.p. above 306-308 K (temperature of decomposition), T = 292 K, final R = 0.043 for 715 observed reflections. In the crystalline state the molecules of maleic hydrazide are present in the monolactim form (i.e. 3-hydroxy-6-pyridazinone). They are connected head-to-tail into planar chains along a by hydrogen bonds in which the O(3)...O(6') distance is 2.605 (1) angstrom. The chains are linked into pairs by hydrogen bonds N(1)...O(6) of 2.925 (1) angstrom. The geometry of the hydrogen bonds and the pattern formed by the molecules within such pairs of chains are very similar in the title structure and the previously reported triclinic polymorph of maleic hydrazide; however, the arrangements of these pairs of chains in the crystal lattices are markedly different.