## Materials and Methods
### [Pt(bpy\({}^{\prime\prime}\)E1-H)(Me)(Pph\({}_{3}\))][Br\({}_{4}\)], 8a-Bf\({}_{4}\)
Stereoelectronic Properties of Substituted 2,2\({}^{\prime}\)-bipyridines_  
The coordinative behavior of substituted bipyridines is strictly dependent on the electronic and steric properties given by their substituents. In the particular case of rollover cyclometallation, two points should be considered: coordination/decoordination of the labile nitrogen donor atom, and activation of a remote C-H bond. It is generally agreed that after chelation, the first step of the process is nitrogen detachment (see Scheme 1), therefore a weaker M-N bond will facilitate the rollover reaction; in contrast, good donor nitrogen atoms will stabilize chelated adducts and contrast rollover metalation. A weak M-N bond can be achieved electronically, by reducing the electron density at the nitrogen (e.g., introducing an electron-withdrawing substituent in the ring), or sterically, employing a bulky substituent in proximity of the nitrogen atom [8]. The last step of the process, i.e., the remote C-H bond activation (see Scheme 1, from D to E), in the case of platinum (II) likely involves a nucleophilic behavior of the metal [7,8]; in this case, electron withdrawing groups are expected to facilitate the process. Owing to these considerations, the CF\({}_{3}\) substituent in 6-CF\({}_{3}\)-2,2\({}^{\prime}\)-bipyridine (bpy\({}^{6}\)CF3) favors all of these steps, decreasing the electronic density of the nearby nitrogen, increasing the steric congestion around it and activating the C(3)-H bond towards nucleophilic attack of the metal. As a consequence, rollover cyclometalation of bpy\({}^{6}\)CF3 is very fast even at room temperature [67]. It is also worth noting that starting from electron-rich Pt(II) complexes, such as [PtMe\({}_{2}\)(DMSO)\({}_{2}\)], the process is irreversible due to methane formation, so kinetic factors will rule the reaction outcome. We report here our investigation on two 6-substituted-2,2\({}^{\prime}\)-bipyridines (bpy\({}^{6}\)R): 6-ethyl-2,2\({}^{\prime}\)-bipyridine (bpy\({}^{6}\)Et, R = CH\({}_{2}\)CH\({}_{3}\)) and 6-methoxy-2,2\({}^{\prime}\)-bipyridine (bpy\({}^{6}\)OMe, R = OCH\({}_{3}\)), in comparison with previously studied 2,2\({}^{\prime}\)-bipyridine (bpy, R = H), 6-methyl-2,2\({}^{\prime}\)-bipyridine (bpy\({}^{6}\)Me, R = CH\({}_{3}\)), and 6-trifluoromethyl-2,2\({}^{\prime}\)-bipyridine (bpy\({}^{6}\)CF3, R = CF\({}_{3}\)) [67]. Evaluation of the donor capabilities of the nitrogen atoms in 2,2\({}^{\prime}\)-bipyridines may be extrapolated by analyzing proton affinity and basicity data of the corresponding substituted pyridines and pyridinium ions (see Table 1). Data reported in Table 1 confirms that methyl and ethyl substituents have a marked electron-donor effect in all positions, although to a smaller extent in position 3, while the opposite is true for CF\({}_{3}\). In contrast, the methoxy substituent is not so straightforward in its contribution, having inductive electron withdrawing effect (-I effect) but mesomeric electron donating effect (+M effect) [71]. In the gas phase, proton affinity of methoxypyridines is only slightly higher than that of pyridine, but in solution the pKa values show that 2-methoxy-pyridine is a worse donor with respect to pyridine by two pKa units (pKa = 3.1-3.2 vs. 5.2), likely due to solvation effects. In contrast, the pKa value of 4-methoxy-pyridinium ion (6.62) shows that a methoxy substituent in the _para_ position makes the nitrogen a better \(\sigma\)-donor. This value suggests that for a MeO- group in the _para_ position, the mesomeric effect is predominant (the nitrogen is more basic and electron-rich than that in 4-Me-pyridine). By extrapolation, a _para_ C-H bond will have a higher electron density and will be less prone to electrophilic activation. A second important element in rollover metalation is given by steric factors, which play a significant role in the regioselectivity of the process.