Reaction of 1-methylcyclobutenedione with M(NO3)(2). xH(2)O [M = Co (1) and Ni (2)] produces isomorphous polymeric complexes of formula M(CH3C4O3)(NO3)(H2O)(4). 2H(2)O that crystallize in the monoclinic space group P2(1)/m (No. 11) with, for 1, a = 6.409(5) Angstrom, b = 14.717(10) Angstrom, c = 7.259(6) Angstrom, beta = 114.73(4)degrees, and Z = 2 (the metal atom being positioned on a center of symmetry). In both cases, adjacent metal centers are bridged mu-1,3 by the 1-methylcyclobutenedione ligand, which exhibits distinct multiple bond localization. The positive charges on the polymer chains are in each case balanced by nitrate ions. The metal atoms are six-coordinate with trans-oriented 1-methylcyclobutenedione ligands, and with four aqua ligands completing the coordination sphere. Reduction of the crystallization temperature, however, resulted, with Co, in the formation of the monomeric complex CO(CH3C4O3)(2). 4H(2)O (4) Isomorphous Mn (3), Cu (5), and Zn (6) analogues have also been synthesized. These crystallize in the monoclinic space group P2(1)/n (No. 14) with, for 3, a 8.8258(8) Angstrom, b = 16.1527(14) Angstrom, c = 9.9084(8) Angstrom, beta = 93.230(7)degrees, and Z = 4. The central metal atom in each of these compounds is six-coordinate, being bound to two trans-oriented 1-methylcyclobutenedione ligands and four aqua ligands, the former coordinating in a monodentate fashion via the oxygen atom trans to the methyl substituent. In both the polymeric and monomeric complexes extensive three-dimensional hydrogen-bonded networks are formed in the solid state.