## Materials and Methods
### Reactivity with Acids
Under mild reaction conditions complex **4a** reacts only slightly with [H\({}_{3}\)O-18-crown-6] [BF\({}_{4}\)], showing that the free nitrogen is not available for protonation and the Pt-C (sp\({}^{2}\)) bond is fairly stable. No signs of N-H protons are visible in the spectra and only a few amounts of a secondary species can be detected by NMR spectroscopy. Analysis of the signals indicate that the trace species is not a protonated complex, but we were not able to characterize the secondary compound. In contrast, the chloride derivative **4b**, in line with the behavior of the corresponding methyl complex **1b**, is easily protonated at the uncoordinated nitrogen, to give also in this case the mesoionic cationic species [Pt(bpy\({}^{6\text{Et+}}\))(Cl)(DMSO)]\({}^{+}\), **5b**, (isolated in the solid state as its BF\({}_{4}\) salt). Also in this case, the \({}^{1}\)H NMR confirm the protonation showing a broad signal at 13.3 ppm. All the other signals are downfield shifted. In particular, the diagnostic signals of the H\({}_{4}\) and H\({}_{3^{\prime}}\) protons are shifted downfield to 0.90 and 0.39 ppm, respectively. The stereoelectronic properties of the starting rollover complexes **1a-b** can be modified by substitution of the ancillary ligands. Starting from complexes **1a-b** and **4a-b**, subsequent substitution reactions with PPh\({}_{3}\) yields the corresponding phosphine complexes: [Pt(bpy\({}^{6\text{OMe}}\)-H)(Me)(PPh\({}_{3}\))], **6a**, previously reported by some of us [71], [Pt(bpy\({}^{6\text{Et}}\)-H)(Me)(PPh\({}_{3}\))], **6b**, [Pt(bpy\({}^{6\text{OMe}}\)-H)(Cl)(PPh\({}_{3}\))], **7a**, and [Pt(bpy\({}^{6\text{Et}}\)-H)(Cl)(PPh\({}_{3}\))], **7b**. The \({}^{31}\)P NMR of complexes **6** and **7** clearly demonstrate the substitution of DMSO with PPh\({}_{3}\). The coordination of PPh\({}_{3}\) reflects the position of DMSO in the parent complex, i.e., in _trans_ position to C(3) in complexes **6a-b** and _trans_ to N in complexes **7a-b**, as shown by the \({}^{31}\)P NMR spectra (e.g., **6a:**\(\delta\) = 32.59 ppm, J\({}_{\text{Pt-P}}\) = 2257 Hz, **6b**: \(\delta\) = 32.71 ppm, J\({}_{\text{Pt-P}}\) = 2221 Hz, in agreement with P-_trans_-C; **7a:**\(\delta\) = 22.31 ppm, J\({}_{\text{Pt-P}}\) = 4245 Hz, **7b**: \(\delta\) = 22.61 ppm, J\({}_{\text{Pt-P}}\) = 4297 Hz, in agreement with P-_trans_-N). Notably, the Pt-P coupling constant is greater in **6a** then in **6b**, whereas the opposite happens for **7a** and **7b**, showing an interesting combination of _cis_ and _trans_ influences. In addition to these data, the \({}^{1}\)H NMR spectra of **6a** and **6b** show a doublet with satellite for the coordinated methyl protons, due to coupling with \({}^{31}\)P and \({}^{195}\)Pt nuclei (e.g., **6a:**\(\delta\) 0.73 ppm, \({}^{2}\)J\({}_{\text{Pt-H}}\) = 82.8 Hz, \({}^{3}\)J\({}_{\text{Pt-H}}\) = 7.8 Hz). Furthermore, the H\({}_{\text{6}^{\prime}}\) in **6a-b** and the H\({}_{4}\) in **7a-b** appear strongly shielded, due to the anisotropic effect of the adjacent phenyl rings of PPh\({}_{3}\). The phosphine complexes **6a-b** and **7a-b** were reacted with acids in order to analyze the influence of PPh\({}_{3}\) on the reactivity. Due to the better donor properties of PPh\({}_{3}\) compared to DMSO, compounds **6** and **7** are expected to have a higher electron density on the Pt center than the corresponding species **1** and **4**. First, the reaction with [H\({}_{3}\)O-18-crown-6][BF\({}_{4}\)] was studied. In contrast to complex **1a**, **6a** reacts in a clean way with the acid to give the cationic adduct [Pt(bpy\({}^{6\text{OMe}}\))(Me)(PPh\({}_{3}\))]\({}^{+}\). This reaction consists in a \"retro-rollover\" process, where the C\({}_{3}\)-Pt bond is attacked by the acid and, after rotation of the internal bipyridine C-C bond, a \(\kappa^{2}\)-N,N complex is formed. It is worth noting that the analogous DMSO complex **1a** reacts on the other Pt-C bond, the Pt-CH\({}_{3}\) one, releasing methane.