Four valence tautomeric complexes with the general composition [Co(3,5-DTBSQ)(2)(NN)] have been investigated, where 3,5-DTBSQ(-) is the semiquinonate form of 3,5-di-tert-butyl-o-quinone. The diiminium ligand NN is one of the following Ligands : phen (1,10-phenanthroline, complex 1); bpy (2,2’-bipyridine, complex 2); dmbpy (4,4’-dimethyl-2,2’-bipyridine, complex 3); dpbpy (4,4’-diphenyl-2,2’-bipyridine, complex 4). All four of the complexes undergo a valence tautomerism converting from low-spin (Is) [Co-III(3,5-DTBSQ)(3,5-DTBCAT)-(NN)] at low temperatures to high-spin (hs) rCo"(3,5-DTBSQ)2(NN)] at high temperatures, where the 3,5-DTBCAT(2-) ligand is the catecholate form of the o-quinone ligand. Variable temperature (150-340 K) magnetic susceptibility data are presented for toluene solutions of all four complexes. These data were least-squares fit to give Delta h and Delta S values that characterize the valence tautomerism equilibrium. The Delta H values range from 21.33 to 38.36 kT mol(-1), whereas the Delta S values range from 60.6 to 133.8 J mol(-1) K-1. Analogous to Fe-II spin crossover complexes, the cobalt valence tautomerism is an entropy driven process. : phen (1,10-phenanthroline, complex 1); bpy (2,2’-bipyridine, complex 2); dmbpy (4,4’-dimethyl-2,2’-bipyridine, complex 3); dpbpy (4,4’-diphenyl-2,2’-bipyridine, complex 4). All four of the complexes undergo a valence tautomerism converting from low-spin (Is) [Co-III(3,5-DTBSQ)(3,5-DTBCAT)-(NN)] at low temperatures to high-spin (hs) rCo"(3,5-DTBSQ)2(NN)] at high temperatures, where the 3,5-DTBCAT(2-) ligand is the catecholate form of the o-quinone ligand. Variable temperature (150-340 K) magnetic susceptibility data are presented for toluene solutions of all four complexes. These data were least-squares fit to give Delta h and Delta S values that characterize the valence tautomerism equilibrium. The Delta H values range from 21.33 to 38.36 kT mol(-1), whereas the Delta S values range from 60.6 to 133.8 J mol(-1) K-1. Analogous to Fe-II spin crossover complexes, the cobalt valence tautomerism is an entropy driven process. : phen (1,10-phenanthroline, complex 1); bpy (2,2’-bipyridine, complex 2); dmbpy (4,4’-dimethyl-2,2’-bipyridine, complex 3); dpbpy (4,4’-diphenyl-2,2’-bipyridine, complex 4). All four of the complexes undergo a valence tautomerism converting from low-spin (Is) [Co-III(3,5-DTBSQ)(3,5-DTBCAT)-(NN)] at low temperatures to high-spin (hs) rCo"(3,5-DTBSQ)2(NN)] at high temperatures, where the 3,5-DTBCAT(2-) ligand is the catecholate form of the o-quinone ligand. Variable temperature (150-340 K) magnetic susceptibility data are presented for toluene solutions of all four complexes. These data were least-squares fit to give Delta h and Delta S values that characterize the valence tautomerism equilibrium. The Delta H values range from 21.33 to 38.36 kT mol(-1), whereas the Delta S values range from 60.6 to 133.8 J mol(-1) K-1. Analogous to Fe-II spin crossover complexes, the cobalt valence tautomerism is an entropy driven process.