We can thus see that the use of the Hammett parameters to describe the electron-donating character of the substituents and its effect on tuning T 1/2 holds to a great extent, but some points are clearly out of the trend. To understand the origin of such deviations, we collected more data on the low-spin state systems. In particular, NEVPT2 calculations analyzed using the ab initio ligand field theory (AILFT) framework were used to get the energies of the five d-based molecular orbitals, and a Hirshfeld charge analysis was done to calculate the total charge of the Cp 1-R rings for each R. Numerical data can be found in the SI.