. For Mn II , there is a large change in the total spin between high-spin and low-spin states (S HS = 5/2, S LS = 1/2), but for Cr II , the situation is much softer. Actually, the computed S elec for Mn II and Cr II are, respectively, 5.96 and 3.31 cal K -1 mol -1 . Because T 1/2 = H/S, for the chromium systems, the reduction of the spin-state energy gap is largely compensated by the decrease in the entropy change due to the Cr II electron configuration. Nevertheless, if we can reduce the H even more for a [Cr(Cp 1-R ) 2 ] system, it should, potentially, exhibit SCO behaviour.