To understand the effect of protonation on the spin-state, magnetic measurements were performed on both complexes 15 and 16. The T vs. T plot shows a similar gradual SCO behavior for both 15 and 16, with no effect of the protonation level on SCO. Around room temperature, the T value for complex 15 is 0.62 cm 3 mol -1 K, whereas 0.75 cm 3 mol -1 K for complex 16, both are much less than the expected value for a HS Co(II) ion (2.7 cm 3 mol -1 K, S=3/2, g=2.4). The T value decreases to 0.41 cm 3 mol -1 K, reaching a plateau of around 150 K, matching the expected value for one Co(II) LS ion (0.45 cm 3 mol -1 K,S= ½, g=2. space group (Fig. ). The average Fe-N bond distance is 1.97 Å, suggesting the presence of Fe(II) LS state. At 220 K, the crystal retains the same space group; however, a distinct change is observed in the average Fe-N bond distance, i.e., 2.17 Å, typical for Fe(II) HS ion, along with increased distortion in the coordination sphere and 5% increase in the unit cell volume. These results agreed well with the solid-state magnetic measurements, which reveals an abrupt and complete SCO with a T1/2 of 207 K (Fig. ). At 300 K, the T value of 3.45 cm 3 mol -1 K (g=2) is typical for the 5 T2g HS ground state. After increasing the temperature to 400 K, the T1/2 slightly shifts higher to 210 K due to the loss of solvent methanol molecules. The SCO in the solution state was also studied via variable temperature Evans 1 H-NMR method in CD3CN from 298 to 343 K and fitted by regular solution model. The T1/2 for the SCO in solution state was found to be 357 K (Fig. ).