## Materials and Methods
### Reactivity with Acids
In the more electron-rich complex **6a**, at variance, the Pt-C(sp\({}^{2}\)) bond is attached. No evidence of nitrogen protonation was achieved also in this case. The reaction proceeds very fast; as previously reported by some of us, the retro-rollover reaction of similar complexes is faster with better electron-donors (PCy\({}_{3}\)\(>\) PPh\({}_{3}\)\(>\) P(OPh)\({}_{3}\)) [76]. In this case, the combined donor properties of PPh\({}_{3}\) and bpy\({}^{6\text{OMe}}\) may be responsible for the reaction outcome. The retro-rollover process should lead in principle to two geometric isomers, one with the OMe substituent in _cis_ position to the Me and the other one in _trans_. Only one isomer, however, is formed during the reaction and can be isolated in the solid state. An extended 1D and 2D NMR investigation, by means of \({}^{1}\)H, \({}^{31}\)P, \({}^{1}\)H COSY and \({}^{1}\)H NOESY spectra, indicate that the isolated species correspond to the less sterically favored isomer, i.e., that with the bulky PPh\({}_{3}\) in _cis_ to the methoxy substituent. The \({}^{31}\)P NMR shows a singlet with satellites in line with a P-_trans_-N arrangement (\(\delta\) = 17.40 ppm, \({}^{1}\)J\({}_{\text{Pt-P}}\) = 4510 Hz). The \({}^{1}\)H NMR shows all the expected aromatic protons and the \({}^{1}\)H NOESY shows correlations between OCH\({}_{3}\) and H\({}_{\text{ortho}}\) of PPh\({}_{3}\) (signals at 2.61 and 7.76 ppm) as well as with H\({}_{5}\) (6.79 ppm). Furthermore, the coordinated methyl at 0.64 ppm shows contacts with the H\({}_{\text{ortho}}\) PPh\({}_{3}\) protons and H\({}_{\text{6}^{\prime}}\) (7.76 and 8.86 ppm, respectively, Figure 7). An analogous retro-rollover process was observed for rollover complexes derived from 2-(2'-pyridyl)quinoline (pyqui). In that case, however, two geometric isomers of the cationic adduct [Pt(pyqui)(Me)(PPh\({}_{3}\))]\({}^{+}\) were observed in solution and isolated in the solid state as BF\({}_{4}\) salts [76]. The behavior of rollover complexes derived from pyqui is, once again, different from that of bpy\({}^{6\text{OMe}}\) and bpy\({}^{6\text{Et}}\): the DMSO complex [Pt(pyqui-H)(DMSO)Me], **1d**, is easily nitrogen-protonated to give the stable mesoionic species [Pt(pyqui\({}^{*}\))(Me)(DMSO)]\({}^{+}\), whereas [Pt(pyqui-H)(Me)(PPh\({}_{3}\))],6d, is protonated as well, to give [Pt(pyqui*)(Me)(PPh\({}_{3}\))]\({}^{+}\), stable and isolable in the solid state as BF\({}_{4}\) salt. In solution [Pt(pyqui*)(Me)(PPh\({}_{3}\))]\({}^{+}\) slowly (2 days) follows a retro-rollover process to give [Pt(pyqui)(Me)(PPh\({}_{3}\))]\({}^{+}\) as a couple of isomers [76]. The isolation of only one isomer in the case of bpy\({}^{6\text{OMe}}\) may be attributed to a different reaction pathway or to a higher stability of the observed isomer due to electronic factors. It may be assumed that the methoxy substituent influences the adjacent nitrogen reducing its donor properties and, as a consequence, the ligand with the highest _trans_-influence, i.e., the methyl, places itself in _trans_ to the donor with the worst _trans_-influence, i.e., the nitrogen of the substituted pyridine ring, to give the most stable isomer. In agreement, the \({}^{3}\)J\({}_{\text{Pt-H}}\) coupling constant of the coordinated methyl protons (74 Hz) is larger than that found in the analogous 2,2\({}^{\prime}\)-bipyridine complexed (69 Hz), suggesting a stronger Pt-CH\({}_{3}\) bond. In order to shed more light on the observed outcome of the reaction we performed a series of DFT calculations. The two potential isomers, i.e., PPh\({}_{3}\)_cis_ (8a) or _trans_ (8a-iso) to the substituted pyridine ring, were optimized and in agreement with the experimental data we note that 8a is more stable than its corresponding isomer by 10.6 KJ/mol considering only the optimization energy. Following the frequency analysis, we evaluated the ZPE corrected \(\Delta\)H and \(\Delta\)G, these values confirm that 8a is more stable by 10.86 and 10.06 KJ/mol, respectively.