In fact, comparing the lowest triplet and quintet energetics is very revealing. The relative energies of the different spin states are plotted in Figure for the complexes studied. From the figure it is obvious that the S = 1 triplet state is barely affected by the substitution (the energies of 10 out of the 13 cases actually fall within a narrow range of 20 meV), as this is the state where the d x 2 -y 2 atomic orbital based e * g orbital is populated. (This would be affected by substitutions on the side pyridine rings.) However, in the quintet state the atomic d z 2 -based e * g orbital becomes populated, too. As this is oriented along the axial direction towards the substituted central rings, the substituent effects create large shifts in the quintet energy.