## Materials and Methods
### Reactivity with Acids  
One of the most important features of rollover complexes derived from 2,2'-bipyridines is the presence of an uncoordinated nitrogen atom, able to act as a Lewis base. As reported by us and by other groups [37; 78; 79], the reactivity of Pt(II) rollover complexes with electrophiles may be complex, due to  
Figure 4: Frontier molecular orbitals of neutral complexes **1a**, **1b** and **1e** and corresponding protonated complexes **3a**, **3b** and **3e**. the presence of several centers able to act as nucleophiles: the uncoordinated nitrogen, the electron-rich platinum center, and the two Pt-C bonds. In the case of nitrogen protonation, the resulting neutral bipyridine ligand, reported in this paper as bpy*, is an isomer of the corresponding bipyridine, and should be considered either as a mesoionic ligand or an abnormal remote carbene, being a 3-pyridylene ligand (Figure 5) [21; 22; 23; 24; 25]. Two different situations were studied: reaction with a strong acid bearing a weakly coordinating anion, [H\({}_{3}\)O-18-crown-6] [BF\({}_{4}\)], and reaction with HCl, having, at variance, a strongly coordinating anion. As expected, the reaction proceeded differently in the two cases, also depending on the nature of the neutral and anionic ligands (PPh\({}_{3}\) vs. DMSO and Me vs. Cl) as well as the substituent of the bipyridine ligand. The protonation reactions were performed under mild conditions, e.g., acetone at room temperature. The behavior towards acids of complexes **1a** and **1b** is very different. Complex **1b** is easily protonated at the free nitrogen to give the stable cationic pyridylene complex [Pt(bpy\({}^{6\text{Et*}}\))(Me)(DMSO)]\({}^{+}\), **3b**, which can be isolated in the solid state as its BF\({}_{4}\) salt 3b-BF\({}_{4}\) (Figure 6). The neutral ligand bpy\({}^{6\text{Et*}}\) is both a mesoionic isomer of free bpy\({}^{6\text{Et}}\) and an abnormal 3-pyridylene [21; 22; 23; 24; 25]. This behavior brings rollover-cyclometalated 6-ethyl-2,2\({}^{\prime}\)-bipyridine ligand into the class of \"ligands with multiple personalities\", i.e., ligands that, far from being simple spectators in reactions, can enhance or modify the reactivity of the metal center after protonation, deprotonation or other simple acts [20]. Complex **3b** has been characterized by means of microanalysis and \({}^{1}\)H NMR spectroscopy. The \({}^{1}\)H NMR spectrum clearly shows the N-H proton as a broad singlet at \(\delta\) = 13.3 which disappears after treatment with D\({}_{2}\)O. In addition, all the aromatic signals are shifted to lower fields with respect to **1b** (e.g., 3b: H\({}_{4}\), \(\delta\) = 8.62 ppm, \({}^{3}\)J\({}_{\text{Pt-H}}\) = 57 Hz; H\({}_{6^{\prime}}\), \(\delta\) = 9.90 ppm, \({}^{3}\)J\({}_{\text{Pt-H}}\) = 12.5 Hz; **1b**: H\({}_{4}\), \(\delta\) = 7.88 ppm, \({}^{3}\)J\({}_{\text{Pt-H}}\) = 53 Hz; H\({}_{6^{\prime}}\), \(\delta\) = 9.67 ppm, \({}^{3}\)J\({}_{\text{Pt-H}}\) = 14 Hz). The noteworthy low field shift of the H\({}_{4}\) proton (+0.74 ppm) is also a good indicator of protonation. Its coupling constant with the \({}^{195}\)Pt nucleus (\({}^{3}\)J\({}_{\text{Pt-H}}\) = 57 Hz) is also larger than that in the parent complex **1b** (\({}^{3}\)J\({}_{\text{Pt-H}}\) = 53 Hz) as expected for a decreased  
Figure 5: Some resonance forms of a pyridylene complex [Pt(bpy\({}^{6\text{R*}}\))(CH\({}_{3}\))L]\({}^{+}\). Figure 6: Protonation of complex **1b** to give the abnormal-remote pyridylene complex **3b**. Chemical shifts and coupling constants (in parenthesis) are reported for notable protons. donor ability of the metalated carbon. The low field shift of the H\({}_{3^{\prime}}\) proton (from 8.34 ppm in **1b** to 8.74 ppm in **3b**, \(\Delta\delta\) = +0.40 ppm) may be linked to the presence of the nearby N-H proton.