## Materials and Methods
### [Pt(bpy\({}^{\prime\prime}\)E1-H)(Me)(Pph\({}_{3}\))][Br\({}_{4}\)], 8a-Bf\({}_{4}\)
It should be noted that in 6-substituted bipyridines, only the substituted pyridine ring undergoes C-H activation, both with 6-CH\({}_{3}\) and with 6-CF\({}_{3}\) substituents. Only in 5-substituted 2,2\({}^{\prime}\)-bipyridines a different behavior of the substituted and unsubstituted ring has been reported [51]. Previous data show an acceleration of the process due to the bulkiness of the substituent (e.g., 6-_tert_-butyl-2,2\({}^{\prime}\)-bipyridine vs. 6-Me-bipyridine [51]). With the intention to measure the steric \begin{table}
\begin{tabular}{l l l l l} \hline \hline \multicolumn{1}{c}{**R a**} & **Proton Affinity 69** & **Basicity 69** & **pKa Pyridinium 70** & \(\chi\)**Angle (°)** \\ \hline H & 922.2 & 889.5 & 5.17 & 98.8 \\ \hline
2-CH\({}_{3}\) (6-CH\({}_{3}\)) & 936.0 & 903.3 & 5.97 & 125.1 \\
3-CH\({}_{3}\) (5-CH\({}_{3}\)) & 932.2 & 899.6 & 5.68 & \\
4-CH\({}_{3}\) & 936.0 & 903.3 & 6.02 & \\ \hline
2-C\({}_{2}\)H\({}_{6}\) (6-C\({}_{2}\)H\({}_{5}\)) & 941.0 & 908.3 & 5.97 & 157.1 (max), 125.1 (min) \\
3-C\({}_{2}\)H\({}_{5}\) (6-C\({}_{2}\)H\({}_{5}\)) & 936.8 & 904.2 & 5.70 & \\
4-C\({}_{2}\)H\({}_{5}\) & 939.7 & 907.1 & 6.02 & \\ \hline
2-CF\({}_{3}\) (6-CF\({}_{3}\)) & 885.3 & 852.7 & - & 137.6 \\
3-CF\({}_{3}\) (6-CF\({}_{3}\)) & 890.4 & 857.7 & 3.36 & \\
4-CF\({}_{3}\) & 891.6 & 859.0 & 3.59 & 71 & \\ \hline
2-OCH\({}_{3}\) (6-OCH\({}_{3}\)) & 925.9 & 893.3 & 3.28 & 152.3 (max), 109.3 (min) \\
3-OCH\({}_{3}\) (6-OCH\({}_{3}\)) & 930.9 & 898.3 & 4.88 & \\
4-OCH\({}_{3}\) & 948.9 & 916.3 & 6.62 & \\ \hline \multicolumn{5}{l}{Proton affinity and basicity values in the gas phase (KJ/mol) of substituted pyridines and pKa data of substituted pyridinium ions in solution. \(\chi\)angle (°) for 6-R-2,2\({}^{\prime}\)-bipyridines [68; 69; 70]. a Pyridine numbering; bipyridine numbering in parentheses.} \\ \end{tabular}
\end{table} 
Table 1: Stereoelectronic properties of substituted pyridines and 2,2\({}^{\prime}\)-bipyidines. contribution to the reaction, we have recently reported the \(\zeta\) angle of substituted bipyridines [51], which evaluated the steric hindrance of a bipyridine ligand. The angle \(\zeta\) is defined as the angle between the bisector of the N\(-\)Pt\(-\)N angle and the tangent to the remotest van der Waals surface of the substituent, passing through the metal center, as depicted in Figure 2 for [Pt(bpy\({}^{6\text{Me}}\))(CH\({}_{3}\))\({}_{2}\)]. These data show that unsubstituted 2,2'-bipyridine occupies only 8.8\({}^{\circ}\) above the theoretical 90\({}^{\circ}\) available for a square planar configuration. A methyl substituent in position 6, with a \(\zeta\) angle of ca 125\({}^{\circ}\) exceeds the available space by 35\({}^{\circ}\). For the two ligands compared in this paper, bpy\({}^{6\text{Et}}\) and bpy\({}^{6\text{Mo}}\), it is possible to identify a minimum and a maximum angle due to substituent flexibility, whereas the maximum is similar for both the ligands (157\({}^{\circ}\) vs. 152\({}^{\circ}\), respectively) and the minimum hindrance of bpy\({}^{6\text{Mo}}\) is definitely smaller (109\({}^{\circ}\) vs. 125\({}^{\circ}\)). Putting together all these data, it is expected that, in comparison with the unsubstituted 2,2'-bipyridine, 6-ethyl-2,2'-bipyridine, bpy\({}^{6\text{Et}}\) possesses a better nitrogen donor in the substituted ring, whereas bpy\({}^{6\text{Mo}}\) has a less basic one. Both the substituted ligands are expected to have a favorable detachment of the second nitrogen (first act of the rollover process: from A to B in Scheme 1) for steric reasons (bpy\({}^{6\text{Et}}\) favored in respect to bpy\({}^{6\text{Mo}}\)) whereas bpy\({}^{6\text{Mo}}\) will decoordinate easily the nitrogen donor due to its electronic properties. As for the remote C-H bond activation, (Scheme 1, from B to C) bpy\({}^{6\text{Et}}\) will be favored, with the ethyl substituent being a worst electron-donor in the _para_ position than the methoxy one.