Complexation of the ligand 3-diphenylamino-4-hydroxycyclobut-3-ene-1,2-dione (diphenylaminosquarate) with metal salts of La, Eu, Gd and Tb produced the first set of lanthanide complexes with a monosubstituted aminosquarate ligand. The lanthanum complex La[(C6H5)(2)NC4O3](3)(H2O)(6). 5H(2)O (1) is monomeric and crystallizes in the triclinic space group P (1) over bar, with a nine-coordinate La center bonded to three pendant diphenylaminosquarate groups and six aqua ligands. The Eu, Gd, and Tb complexes [M(mu-(C6H5)(2)NC4O3)((C6H5)(2)NC4O3)(NO3)(OH2)(4)](2). 4H(2)O (2-4) are isomorphous (also P (1) over bar), each metal atom being nine-coordinate and bridged to its neighbor by two diphenylaminosquarate ligands in a mu-1,2-fashion; the coordination polyhedron around each metal is completed by a pendant diphenylaminosquarate ligand, four aqua ligands, and a chelating nitrate ion. The resistance of the diphenylamino substituent to hydrolysis, vis-a-vis the hydrolysis of dialkylamino substituents, is discussed.