EPR spectroscopy was carried out on powders and frozen-solutions of [Co(1-3)](ClO4)2 to evaluate their spin configurations and provide an estimate of D. X-band EPR spectra of polycrystalline samples at 10 K (Fig. ) display features characteristic of both LS (dotted signals at ca. ~300 mT) and HS (solid black lines) configurations. This signal assignment is confirmed by the persistence of the LS signal at temperatures as high as 100 K (Figure ) at which the HS species relaxes too fast to be detectable. Moreover, at this temperature the difference in the signal intensities among the three complexes becomes noticeable, unambiguously pointing to an effect of the ligand on the spin state population. In keeping with the calculated HS-LS energy gap, while in [Co( )](ClO4)2) the LS signal is barely observable, the spectra acquired at 100 K show that the number of ions in this configuration is much more substantial in the other two complexes, particularly in 1. )](ClO4)2 at 4.5 K. Simulations for the HS states in (A) and (C) were performed with PHI with parameters listed in Table . Simulation parameters for the LS states (B) are given in the ESI (Table ) Table . Experimental and calculated (SA-5-CASSCF [7,5]) parameters for the HS states of Co(1-3)](ClO4)2.