Results for such calculations are summarized in Table . Although these substituents have a similar effect on the [Cr(Cp 1-R ) 2 ] as the one observed in [Mn(Cp 1-R ) 2 ], i. e. they reduce the spin-state energy gap, the corresponding T 1/2 does not experience a significant decrease. To understand that behaviour, we must analyze the corresponding entropy change in both families. While for the [Mn(Cp 1-Me ) 2 ] S = 19.87 cal K -1 mol -1 , for the chromium analog [Cr(Cp 1-Me ) 2 ] this value is only S = 11.61 cal K -1 mol -1 . This reduction of the entropy change is largely due to the electronic contribution to the entropy change, S elec , that can be calculated