Further insights into the electronic structure variations are revealed from 15 N NMR chemical shifts, which were also determined from HMBC measurements. The signals for the two types of nitrogens in the results are displayed in Figure . Over the full R + range, δ for Nax increases by 70 ppm (the correlation between δ(Nax) and R + is outstandingly high, with R 2 = 0.97), while it decreases only by 10 ppm for Neq. Thus the previously realized predictive power of N NMR chemical shifts on the SCO behaviour of iron(II) complexes were also shown in our systems. The large shift of the chemical shift indicates the large change in the charge density on the Naxwith the substitution. Actually, the behaviours displayed in the figure resemble closely the calculated Löwdin atomic charges on the N atoms in Figure . Furthermore, calculations on the 15 N NMR shifts describe well the chemical shifts and its variations with R + , only the Neqshifts are displaced by a constant of c.a. 10 ppm from the experimental results, which also supports the aforementioned observations.