Thus, ∆∆E Rn X measures the relative reactivity of reaction Rn for a complex with ligand X compared to that same reaction with the bpy ligand. For instance, if the ∆∆E Rn X is posi-tive, reaction Rn with ligand X is less favorable (i.e., less likely to form the intermediate) compared to the same reaction with the bpy ligand. Conversely, if the ∆∆E Rn X is negative, then this reaction is more favorable (i.e., more likely to form the intermediate) than the corresponding bpy complex. For the simplified model complexes in Figures and, we also use the bpy complex as the reference for each reaction. This puts the ∆∆E R Y values [Y= pm, pb, px, mae, N(c 3 ), P(c 3 ), N(c 4 ), P(c 4 )] of the models on the same scale as the ∆∆E R X values of the full complexes. Note that, by construction: