## Materials and Methods
### Synthesis and Characterization  
In our experience, the best starting complex for Pt(II) rollover cyclometallation is the electron-rich derivative [PtMe\({}_{2}\)(DMSO)\({}_{2}\)]. With this starting complex, the reaction proceeds even at room temperature for both ligands studied in this paper. In order to obtain reasonable conversions, the syntheses may be optimized operating in acetone at 40-50 \({}^{\circ}\)C to give the rollover complexes [Pt(bpy\({}^{6\text{Mo}}\)-H)(Me)(DMSO)], **1a**, and ([Pt(bpy\({}^{6\text{Et}}\)-H)(Me)(DMSO)], **1b**. Activation of bpy\({}^{6\text{Me}}\) has been recently reported by some of us [66] and occurs easily, whereas activation of bpy\({}^{6\text{Et}}\) proceeds slowly at room temperature. Bpy\({}^{6\text{Et}}\) has a slightly greater  
Figure 2: (**A**) Stereoelectronic factors and numbering scheme in 6-substituted 2,2’-bipyridines (6-R-2,2’-bipyridine, bpy\({}^{6\text{R}}\): R = H, CH\({}_{3}\), OCH\({}_{3}\), CH\({}_{2}\)CH\({}_{3}\), F, CF\({}_{3}\), Ph, CR\({}_{2}\)Ph, etc. ); (**B**) \(\zeta\) angle for [Pt(bpy\({}^{6\text{Me}}\))(CH\({}_{3}\))\({}_{2}\)]. steric hindrance than bpy\({}^{6\text{OMe}}\), hence the easier C-H bond activation in bpy\({}^{6\text{OMe}}\) should not be ascribed to steric factors but may be due to the scarce donor ability of the nitrogen atom close to the OMe substituent. The rollover metalation is clearly confirmed, in the \({}^{1}\)H NMR spectrum, by (i) the absence of the H\({}_{3}\) proton; (ii) the AB system with satellites given by the remaining H4 and H5 protons in the substituted pyridine ring; (iii) coordination of the Me and DMSO ligands (confirmed by \({}^{195}\)Pt satellites), (iv) coordination shift and \({}^{195}\)Pt satellites of the H6\({}^{\prime}\) proton. In order to evaluate differences in the properties of rollover complexes due to the substituent, we compared the NMR data of a series of substituted 2,2\({}^{\prime}\)-bipyridines obtained in our laboratory (6-R-2,2\({}^{\prime}\)-bpy, R=OMe, Et, Me, CH\({}_{3}\), Cr\({}_{3}\), Ph, H). Comparison of \({}^{1}\)H NMR data of complexes **1a**-**1e** (Table 2) shows, in general, minor differences. An interesting trend is displayed by the H\({}_{4}\) proton, whose coupling constant with \({}^{195}\)Pt (\(\bar{3}\))(\({}^{195}\)Pt-H\({}_{4}\)) increases with the electron-withdrawing ability of the substituent (50, 53, 54 and 56 Hz, for bpy\({}^{6\text{OMe}}\), bpy\({}^{6\text{OMe}}\), bpy and bpy\({}^{6\text{C}\  
The voltammetric response of **1a** shows an anodic, irreversible process located at 1.29 V (Figure 3). The evaluation of the onset potential allowed the determination of the energy value of HOMO (highest occupied molecular orbital) (-5.59 eV). The comparison with literature data [74] suggests that the anodic process is due to a mono-electronic EC (electrochemical-chemical) transfer. The main reason for the irreversibility of the process is the instability of the formally Pt(III) derivative. Indeed, usually anodic process on Pt(II) derivatives are attributed mainly to the metal center [75]. Nevertheless, according to our previous evaluation on the parent derivative [Pt(bpy-H)(Me)DMSO) **1e**[76], we can hypothesize that the anodic process involves mainly the bpy ligand and, to a minor extent, the metal center. The voltammetric response of **1b** (Figure 3) shows an irreversible, broad anodic process located at more positive potential values (between 1.4 V and 1.5 V), characterized by an evidently poor reproducibility. As a consequence, in the case of 1b it is possible to approximately estimate the energy of the HOMO (about \(-5.4\) eV). The scarce reproducibility of the response of **1b** does not allow accurate comparison between the two complexes and the parent unsubstituted-bpy derivative. In either case, we can observe (Table 3) that the bpy\({}^{6\text{Et}}\) cyclometalated complex appears more stable than the corresponding bpy\({}^{6\text{OMe}}\) derivative, which in turn is more stable than the bpy one.