Manganese and cadmium complexes of singly and doubly deprotonated 2,3-pyrazinedicarboxylic acid (pyzdcH(2)) have been prepared and studied. Crystal data: [H3O](2x)[Mn(pyzdc)(2)](x), 1, monoclinic, C2/(c) (no. 15), a = 14.372(1) Angstrom, b = 8.475(2) Angstrom, c = 13.0771(9) Angstrom, beta = 114.753(4)degrees, Z = 4; [H3O](2x)[Cd(pyzdc)(2)](x), 2, monoclinic, C2/c (no. 15), a = 14.4857(9) Angstrom, b = 8.502(1) Angstrom, c = 14.9803(7) Angstrom, beta = 126.951(2)degrees Z = 4; [Mn(pyzdc)(H2O)(2)](x) . 2xH(2)O, 3, monoclinic, P2(1)/c (no. 14), a = 6.5523(7) Angstrom, b = 7.6702(7) Angstrom, c = 20.5815(6) Angstrom, beta = 93.605(5)degrees; Z = 4; [Cd(pyzdc)(H2O)(3)](x) . xH(2)O, 4, monoclinic, Cc (no. 9), a = 6.088(1) Angstrom, b = 11.5410(9) Angstrom, c = 15.0624(7) Angstrom, beta = 96.532(8)degrees, Z = 4. The structures were solved by Patterson (1, 4) or direct (2, 3) methods and were refined by full-matrix least-squares procedures to R = 0.036, 0.025, 0.034, and 0.022 (R(w) = 0.032, 0.025, 0.036, and 0.023) for 2825, 4961, 1672, and 2641 reflections with I greater than or equal to 3 sigma(F-2), respectively. The structures of 1 and 2 consist of extended anionic chains of metal ions that are linked by double-bridging pyzdc(2-) groups, with electrical neutrality maintained by the presence of H3O+ cations in the lattice. In 3 double chains of manganese ions are cross-linked by pyzdc(2-) ligands forming a ''ladder-like'' motif. The structure of 4 consists of extended chains of cadmium ions in which single pyzdc(2-) groups bridge metal centres. Magnetic susceptibility studies indicate, at most, only weak antiferromagnetic exchange in 1 but moderately strong antiferromagnetic interactions in 3, mediated by bridging carboxylate groups (J = -0.27 cm(-1)).