Reduction of Th((OC6H2Bu2)-Bu-t-2,6-Me-4)(4) using either KC8 or Li in THF forms a new example of a crystallographically characterizable Th(III) complex in the salts [K(THF)(5)(Et2O)][Th((OC6H2Bu2)-Bu-t-2,6-Me-4)(4)] and [Li(THF)(4)] [Th((OC6H2Bu2)-Bu-t-2,6-Me-4)(4)]. Surprisingly, in each structure the four aryloxide ligands are arranged in a square-planar geometry, the first example of this coordination mode for an f element complex. The Th(III) ion and four oxygen donor atoms are coplanar to within 0.05 angstrom with O-Th-O angles of 89.27(8) to 92.02(8)degrees between cis ligands. The ligands have Th-O-C(ipso) angles of 173.9(2) to 178.6(4)degrees, and the aryl rings make angles of 58.5 to 65.1 degrees with the ThO4 plane. The effect of the eight tert-butyl substituents in generating the unusual structure through packing and/or dispersion forces is discussed. EPR spectroscopy reveals an axial signal consistent with a metal-based radical in a planar complex. DFT calculations yield a C-4-symmetric structure that accommodates a low-lying SOMO of 6d(z2) character with 7s Rydberg admixture.