The synthesis, structure, and solid-state UV–vis–NIR spectroscopy of four new f-element squarates, M2(C4O4)3(H2O)4 (M = Eu, Am, Cf) and Sm­(C4O4)­(C4O3OH)­(H2O)2·0.5H2O, four new cationic lanthanide squarate chlorides, [M4(C4O4)5(H2O)12]­Cl2·5H2O (M = Eu, Dy, Ho Er), and two new actinide squarate oxalates, M2(C4O4)2(C2O4)­(H2O)4 (M = Am, Cf), are presented. All of the metal centers are trivalent. Single-crystal X-ray diffraction analysis reveals that M2(C4O4)3(H2O)4 and Sm­(C4O4)­(C4O3OH)­(H2O)2·0.5H2O have a two-dimensional sheet structure constructed from MO7(H2O)2 monocapped square-antiprismatic (coordination number (CN) = 9) metal centers and SmO6(H2O)2 square-antiprismatic (CN = 8) metal centers, respectively, whereas M2(C4O4)2(C2O4)­(H2O)4 have a three-dimensional (3D) structure constructed from MO7(H2O)2 monocapped square-antiprismatic (CN = 9) metal centers. Additionally, the cationic framework materials [M4(C4O4)5(H2O)12]­Cl2·5H2O have a 3D structure constructed from two crystallographically unique MO5(H2O)3 square-antiprismatic (CN = 8) metal centers. In these structures, the squarate ligands bind to the metal centers with varying coordination modes and denticities. The results of this study provide another example of the nonparallel chemistry between the lanthanides and transplutonium elements. From the crystallographic data for the isotypic series M2(C4O4)3(H2O)4 (M = La–Nd, Sm, Eu) and the linear regression fit to a plot of the unit cell volume as a function of the cube of the ionic radius, the nine-coordinate ionic radius of Cf 3+ was determined to be 1.127 ± 0.003 Å. Finally, computational analysis of the americium and californium complexes M2(C4O4)3(H2O)4 and M2(C4O4)2(C2O4)­(H2O)4 reveals three important attributes: (i) the 5f orbitals are nonbonding in all cases, with the bonding differences occurring with the empty 6d orbitals; (ii) the Cf complexes exhibit more covalent character than their Am counterparts; and (iii) there is more covalent character in the squarate-oxalate complexes than in the squarate complexes.