In contrast, in the case of [Co( )](ClO4)2, the χT vs T data point to an incomplete spin-crossover (SCO) behavior, with a transition from a mixture of states at low T to a progressive conversion to the HS state at T > 100 K, suggesting that the LS configuration is lower in energy. Although this contradicts the modelling data, which suggests that the HS states is favored, it should be noted that, when starting from the experimentally derived geometry, the calculated energy gap HS-LS is small (2.7 kJ/mol) and was obtained in the gas phase and is thus inconclusive. Although many 4-and 6-coodinate Co II complexes are known to undergo a SCO transition, with an interesting reversible switch between SCO and SIM behavior reported recently, 5-coordinate Co II spin-crossover compounds are rare, making [Co( )](ClO4)2 only the second spin-crossover complex characterized by a neutral CoN5 coordination sphere reported. The decrease of the susceptibility observed at low temperatures for all complexes was attributed to the presence of significant magnetic anisotropy rather than antiferromagnetic impurities or interactions between the spins (in agreement with the conclusions from the EPR data), as the field dependent magnetization data collected at 100 K show a perfectly linear trend for all complexes (Figure ). The reduced magnetization plots (Figure ) also indicate magnetic anisotropy due to the absence of a single master curve. In order to determine the magnetic parameters of [Co(1-3)](ClO4)2, the χT vs T plots (Figure ) and the magnetization vs H at multiple temperatures (Figure ) were simultaneously fitted with PHI. The best fits yielded gaverage of 2.33, 2.31 and 2.33 and D values of -78, -59 and -95 cm -1 for complexes [Co(1-3)](ClO4)2, respectively (Table , see also Figure for fits with different values of D). These values are considerably larger than those obtained for other pentacoordinate mononuclear cobalt(II) complexes (Table ). Because of the change in the spin population as a function of temperature for [Co(1)](ClO4)2, only the fit of the magnetization was used to estimate the parameters (i.e. the χT vs T plot was not fitted for [Co(1)](ClO4)2 in Figure ), and the best fits were obtained by including a contribution of the LS species at 0.45, 0.05 and 0.1 for [Co(1-3)](ClO4)2, respectively (Table ). As already indicated by comparison of the solution and solid-state EPR measurements, further supported by the magnetization data at 100 K as well as solution susceptibility values obtained from the Evan's method (2.05, 2.56 and 2.29 cm 3 •K•mol -1 for [Co(1-3)](ClO4)2, respectively), no J coupling needs to be considered in fitting the solid-state magnetic data. In contrast to EPR, the inclusion of the rhombic zero-field parameter E in the magnetic data did not lead to a better fit, which is not unexpected due to the lower sensitivity of SQUID measurements compared with EPR. The large negative zero-field splitting terms revealed by DC measurements for all complexes [Co(1-3)](ClO4)2 (Table ), which are in good agreement with the values obtained from X-band and HFEPR simulations, and the variability of D with the rotaxane framework thus justify their further investigation as potential 'tuneable' SIMs.