## Materials and Methods
### Reactivity with Acids
A closer look at the optimized geometries verified that 8a is less sterically strained even if the PPh\({}_{3}\) is in _cis_ position to the OMe substituent of the bpy, which could seem counterintuitive in terms of the \"classical\" Tolman cone angle. The phenyl rings of the PPh\({}_{3}\) can rotate to accommodate the OMe substituent between them, thus leading to an overall decrease in geometrical strain. The sum of the internal angles of the metallacycle in 8a is 538.02\({}^{\circ}\), closer to the theoretical 540\({}^{\circ}\) for a regular pentagon than 8a-iso, which shows, for the same parameter, 534.46\({}^{\circ}\). The angle between the best planes calculated through the metallacycle (Pt-N\({}_{1}\)-C\({}_{2}\)-C\({}_{2^{\prime}}\)-N\({}_{1^{\prime}}\)) and the one through the coordination plane (Pt-P-C-N\({}_{1}\)-N\({}_{1^{\prime}}\)) is very similar for both isomers but lower for 8a, 12.04 vs. 12.80. It is interesting to observe that the two pyridine rings are bent 12 degrees from the metallacycle plane in 8a-iso while only 7.6\({}^{\circ}\) in 8a. DFT data suggest that the lower stability of 8a-iso is due to a mixture of steric and electronic factors: the less flexible methyl group _cis_ to the OMe substituent of the bpy\({}^{6\text{OMe}}\) creates a greater distortion in the structure and, being the group with the highest _trans_-influence in the coordination sphere, is positioned _trans_ to the pyridinic nitrogen which has a stronger _trans_-influence compared to the one of the substituted ring. Figure 7: \({}^{1}\)H NOE contacts in complex 8a. In the case of **8a**, the PPh\({}_{3}\) accommodates the OMe substituent better and the methyl group places itself in _trans_ to the group that has the lowest _trans_-influence, in agreement with the _trans_-influence rule in square planar Pt(II) complexes. Furthermore, it should also be noted that complex **8a** needs an isomerization to be formed, having the unsubstituted pyridine ring in _cis_ to the coordinated methyl group, whereas the starting complex **6a** has a _cis_ PPh\({}_{3}\)-unsubstituted pyridine configuration. As a conclusion, PPh\({}_{3}\) is usually considered a bulky ligand, having a 145\({}^{\circ}\) cone angle, on the basis of this value isomer **8a-iso** is expected to be the stable isomer. However, as shown by the optimized geometry of **8a**, reported in Figure 8, the cone angle should not be considered as a \"solid cone\" but in some cases can make room inside to host a substituent. This result is interesting and can lead to a revisitation of ligand \"bulkiness\" and \"cone angle\" concepts. When the reaction is performed on the second complex, **6b**, [Pt(bpy\({}^{\text{6 Et}}\)-H)(Me)(PPh\({}_{3}\))], the results are, also in this case, different. In this case the nitrogen is easily protonated to give the mesoionic cationic complex [Pt(bpy\({}^{\text{6 Et}}\))(Me)(PPh\({}_{3}\))][BF\({}_{4}\)], **9b-BF\({}_{4}\)**, with high yields. Also in this case, the NMR spectra confirm the proposed formulation; in particular, the NH proton appears as a broad singlet at 13.2 and the \({}^{31}\)P NMR spectrum is in agreement with a P-Pt-C _trans_ coordination (\(\delta\) = 31.2 ppm; JPt-P = 2400 Hz). Comparison with the parent complex **6b** (\(\delta\) = 32,7 ppm), JPt-P = 2221 Hz) show an increase in the Pt-P coupling constant, indicative of a stronger Pt-P bond. The diagnostic H\({}_{4}\) and H\({}_{3^{\prime}}\) protons, as expected, are strongly deshielded with respect to the parent complex **6b**: H\({}_{4}\)\(\delta\) = 0.79 ppm, H\({}_{3^{\prime}}\), \(\delta\) = 0.39 ppm). Interestingly, complex **9b** is fairly stable in solution not giving signs of retro-rollover conversion, whereas the analogous complex [Pt(pyqui*)(Me)(PPh\({}_{3}\))]\({}^{+}\) in 2 days completely converts in its \(\kappa^{2}\)-N,N isomer.