Based on the NBO calculations, we can also confirm the Fe-N eq (calc) Fe-N eq (exp) Fe-N ax (calc) Fe-N ax (exp) Löwdin atomic charges (a.u.) ED groups increase the electron density, leading to a more negative charge, while EW groups have the opposite effect. This is also reflected in the charge on the Fe, although the variations are smaller. The charge on the Neq atoms on the side rings are practically unaffected. These also account for the subtituent-dependent variations seen in the Mössbauer spectra. The lines are guides to the eye. The top panel displays the assignment of colours to the compounds, which will be maintained throughout the figures in this paper.