A series of new complexes belonging to the [Co(4-terpyridone)(2)]X-p center dot nS family (4-terpyridone)2,6-bis(2-pyridyl)4(1H)-pyridone) have been synthesized and characterized, using X-ray single crystal determination and magnetic susceptibility studies, to be X = [BF4](-) (p = 2) and S = H2O for polymorphs 1 and 2, X = [BF4](-) (p = 1) and [SiF6](2-) (p = 0.5 and S = CH3OH for 3, X = [SiF6](2-) (p = 1) and S = 3CH(3)OH and H2O for 4, X = [Co(NCS)(4)](2-) (p = 1) and S = 0.5CH(3)OH for 5, X = l(-) (p = 2) and S = 5H(2)O for 6, X = [PF6](-) (p = 1) for 7, and X = [NO3](-) (p = 2) for 8. Compounds 1-7 can be grouped in three sets according to the space group in which they crystallize: (i) P(1) over bar triclinic (1, 3), (ii) P2(1) monoclinic (2), and (iii) P(2)1/c monoclinic (4-7). The tridentate 4-terpyridone ligands coordinate the Co(II) ions in a mer fashion defining essentially tetragonally compressed [CoN6] octahedrons. The Co-N axial bonds involving the pyridone rings are markedly shorter than the Co-N equatorial bonds collectively denoted as Co-N-central and Co-N-distal, respectively. The differences in the average Co-N-central or Co-N-distal distances observed for 1 - 7 reflect the different spin states of Co( II). Complexes 7 and 4' are fully high spin (HS), while 5 and 6 are low spin (LS). However, the counterion [Co(NCS)(4)](2-) in complex 5 is high spin. Complexes 1, 2, 3, and 8 exhibit spin-crossover behavior in the 400 - 100 K temperature region. Compounds 1 and 2 are polymorphs, and interestingly, 1 irreversibly transforms into 2 above 340 K because of a crystallographic phase transition which involves a drastic modification of the crystal packing. The relevant thermodynamic parameters associated with the spin transition of polymorph 2 have been estimated using the regular solution theory leading to Delta H = 3.04 kJ mol(-1), Delta S = 20 J K-1 mol(-1), and Gamma 0.95 kJ mol(-1).