Our working hypothesis is that, in principle, one can tune the ligand field around the metal center by making the ring more negatively charged, as is experimentally observed when moving from [Mn(Cp 1-Me ) 2 ], an SCO molecule, to the fully methylated system [Mn(Cp*) 2 ], a low-spin system. Following this idea, different substituents with more or less electron withdrawing effects have been tested in the [Mn(Cp 1-R ) 2 ] system. Although a trend is observed, the results are far from showing a clear correlation between the computed T 1/2 and the Hammett parameter  p . However, a look at figure 1 allows us to identify several outliers that greatly deviate from the trend.