## Materials and Methods
### Reactivity with Acids
The reaction with the adduct H\({}_{3}\)O\({}^{+}\)-crown ether was finally studied also in the case of complexes **7a** and **7b**, [Pt(bpy\({}^{\text{6 OMe}}\)-H)(Cl)(PPh\({}_{3}\))] and [Pt(bpy\({}^{\text{6 Et}}\)-H)(Cl)(PPh\({}_{3}\))]. Even in this case, the behavior of the two complexes is different; complex **7a** does not react with the acid, likely due to the scarce basicity  
Figure 8: On the left: compounds **8a** and 8a-**iso**; on the right: optimized geometry of **8a** (hydrogens were omitted for clarity). of the free nitrogen atom and the platinum atom. The Pt-C(sp\({}^{2}\)) bond is stable under the reaction conditions studied and complex **7a** is recovered unreacted. In contrast, the free nitrogen in complex **7b** is rapidly protonated to give the cationic complex [Pt(bpy\({}^{\text{6 Et}}\)*)(Cl)(PPh\({}_{3}\))]\({}^{+}\), **10b** (\({}^{1}\)H NMR: \(\delta\) 13.3 ppm, broad). As expected, after protonation the H\({}_{4}\) and H\({}_{3^{\prime}}\) signals appear desshielded by ca 0.8 and 0.5 ppm, respectively. In addition, the \({}^{31}\)P NMR signal moves from 22.61 to 19.59 ppm, with a reduction of the \({}^{195}\)Pt-\({}^{31}\)P coupling constant value (from 4297 to 4139 Hz). The complex is stable both in solution and in the solid state. In general, all the bpy\({}^{\text{6 Et}}\) rollover complexes were easily protonated by reaction with [H\({}_{3}\)O\(\cdot\)18-crown-6] [BF\({}_{4}\)], whereas none of the bpy\({}^{\text{6 OMe}}\) complexes show any trace of protonation, with the possible exception of complex **6a**, which, follows a rapid retro-rollover reaction. With the intention to shed light into this behavior, we analyzed by means of DFT calculations complexes **1a**-**b**, **4a**-**b**, **6a**-**b**, **7a**-**b** and their nitrogen-protonated counterparts **3a**-**b**, **5a**-**b**, **9a**-**b**, **10a**-**b** (obtained bpy\({}^{\text{6 Et}}\) and not obtained bpy\({}^{\text{6 OMe}}\)) (see Supplementary Materials). In addition, as a comparison, the analogous unsubstituted 2,2\({}^{\prime}\)-bpy complexes **1e**, **4e**, **6e**, **7e**, **3e**, **5e**, **9e**, **10e**, were analyzed as well (see Scheme 2). **Scheme 2. Rollover neutral and protonated complexes analyzed by means of DFT calculations. **Proton affinity values (P.A.) were calculated as the reverse enthalpy changes of the reaction reported in Scheme 2, according to ref [81]. It is worth noting that free 2,2'-bipyridines adopt the more stable N,N _transoid_ coplanar conformation [82] (Chart 1, B) found in rollover cyclometalated complexes, in place of the less stable _cisoid_ coplanar configuration (Chart 1, A), adopted in chelated complexes. In order to compare similar conformations in free ligands and rollover complexes, we comment here only P.A. data for the _transoid_ neutral and protonated species (Chart 1 B \(\rightarrow\) C; Table 4, first line of data). In the protonated ligands, however, the _cisoid_ configuration (D or E) is the most stable one, due to the presence of a weak N-H--N interaction (N--H distances 1.96-1.98 A). Table 4 (second line of data) also reports _true_ P.A. data, i.e., those relative to the stable species D (\(\text{bpy}^{\text{GE}}\)) or E (\(\text{bpy}^{\text{6 OMe}}\)). P.A. data limited to _transoid_ conformations (Chart 1, B \(\rightarrow\) C) reported in Table 4, show that bpy\({}^{\text{6 Et}}\) complexes have systematically higher values (ca 34-35 KJ/mol) than corresponding bpy\({}^{\text{6 OMe}}\) ones. Unsubstituted bipyridine complexes (**1e-10e**) lie in the middle. In contrast, when the free ligands are considered, 2,2'-bipyridine has a lower P.A. value (948.5 KJ/mol) than free bpy\({}^{\text{6 OMe}}\) (952.4 KJ/mol), even considering the _cisoid_ conformations (982.7 vs. 985.0 KJ/mol). Taking into account only _transoid_ conformations, in the ligands the higher P.A. value is shown, as expected, by uncoordinated bpy\({}^{\text{6 Et}}\) (968.3 KJ/mol), only 16 KJ/mol above bpy\({}^{\text{6 OMe}}\) and less than half of the difference found in corresponding complexes.