A series of Co(II) complexes of the tropocoronand ligand system [TC-n,m](2-) (n,m = 3,3; 3,4; 4,4; 4,5; 5,5; 6,6; 7,7)have been prepared and fully characterized. X-ray structural analyses have been carried out on [Co(TC-3,3)], [Co(TC-4,4)], [Co(TC-4,5)], [Co(TC-5,5)], [Co(TC-6,6)], and the dinuclear complex [Co-2(mu-OAc)(mu-OH)(TC-7,7)]. In the mononuclear series, the coordination geometries vary from square-planar for [Co(TC-3,3)] to tetrahedral for [Co(TC-6,6)]. The twist angle, defined as the dihedral angle between the two five-membered aminotroponiminate chelate rings, increases from 9.0 to 84.5 degrees as a result of increasing torsional strain within the polymethylene linker chains of the tropocoronand macrocycle. The change in geometry from square-planar to tetrahedral, accompanied by a change in spin state from S = 1/2 to S = 3/2, occurs abruptly between [Co(TC-4,4)] and [Co(TC-4,5)] as determined by X-ray crystallographic and solid state magnetic studies. This dependence of geometry and spin state on ligand size is similar to that observed previously in the Ni(II) system, where the low spin to high spin transformation occurred at the [Ni(TC-4,5)]/[Ni(TC-5,5)] junction. The shift in the position of this transformation with metal ion is an indication of the greater preference of Co(n) for tetrahedral geometry compared with Ni(II).