We employed UV/Vis spectroscopy to monitor the absorption changes upon titrating FeCl2 or CuSO4 to the respective DpTs. It has been demonstrated that DpTs form 2:1 complexes with Fe(II), which is also consistent with our spectroscopic data, as we observed saturation of absorption after titrating 0.5 equiv. of Fe(II) (Figure ). The DpT-Cu(II) complexes, on the other hand, have been isolated in both 1:1 and 2:1 binding modes using different reaction conditions. Richardson et al. have inferred from EPR data that the Cu(II) complex prefers the 1:1 binding mode in solution. In line with these observations, our titrations with Cu(II) showed a saturation of absorption after titrating 1.0 equiv. of Cu (II), indicating a 1:1 stoichiometry (Figure ). However, the UV/Vis spectra for 3 and 4 initially showed an absorption maximum at 447 nm (Figures 4 and S51) at 0.4 equiv. of metal, which shifted to 427 nm as the titration progressed.