Reaction of 3-methyl-4-hydroxycyclobut-3-ene-1,2-dione (methylsquarate) with LaCl3. 7H(2)O and M(NO3)(3). xH(2)O (M = Eu, Gd, and Tb) in alcoholic solutions afforded the polymeric complexes {La[mu (3)-(CH3C4O3)][(H-2)(6)]- [Cl-2].H2O}(n) (1), {Eu[mu-(CH3C4O3)(3)][(H2O)(2)]}(n) (3), {Gd[mu-(CH3C4O3)(2)][(H2O)(4)][NO3].H2O}(n) (4), and {Tb[mu-(CH3C4O3)(2)][(H2O)(4)] [NO3].H2O}(n) (5), respectively and with La(NO3)(3 .)6H(2)O, the monomer La(CH3C4O3)(3)(H2O)(6). H2O (2). Complex 1 crystallizes in the orthorhombic space group Pca2(1), with a = 13.239(2) Angstrom, b = 7.836(1) Angstrom, c = 14.209(2) Angstrom, and Z = 4. The lanthanum atom is nine-coordinate adopting a monocapped square antiprismatic geometry, being bonded to three mu (3)-bridging methylsquarate and six pendant aqua ligands and forming cationic polymer sheets which an intercalated by chloride ions and water of crystallization. Complex 3 is a 3-dimensional neutral network polymer in which the methylsquarate ligands are mu -1,3-bridging and the europium atoms adopt a bicapped trigonal prismatic geometry. 3 crystallizes in the orthorhombic space group Pbna with a = 8.713(1) Angstrom, b = 13.115(1) Angstrom, c = 15.253(1) Angstrom, and Z = 4. The gadolinium and terbium complexes, 4 and 5, are isomorphous cationic sheet polymers with the methylsquarate ligand again adopting a mu -1,3-bridging role and with the metal having square antiprismatic geometry. Adjacent sheets are arranged so as to form channels within which the nitrate anions and water molecules are located. Complex 4 crystallizes in the monoclinic space group P2(1)/c, with a = 14.543(2) Angstrom, b = 8.396(1) Angstrom, c 15.251(2) Angstrom, beta = 109.37(1)degrees, and Z = 4. The monomer 2 crystallizes in the space group P2(1)/c with a = 10.438(2) Angstrom, b = 14.813(2) Angstrom, c = 15.151(2) Angstrom, beta = 108.21(1)degrees, and Z = 4. The lanthanum atom is nine-coordinate adopting a monocapped square antiprismatic geometry with each metal atom being coordinated to three methylsquarate and six aqua ligands. In all of the polymeric species there is a pattern of delocalization which extends to include adjacent metal centers.