## 1 Introduction  
Cyclometalated complexes constitute an important class of organometallic compounds owing to their intrinsic stability and the wide range of their potential applications [1; 2; 3]. Among the ample variety of cyclometalated compounds, the so-called rollover complexes constitute a special and intriguing case due to their peculiar properties [4; 5; 6].  
The differences between the classical and rollover cyclometalated compounds at a first sight may appear trivial; however, they are, as a matter of fact, substantial. The first difference which connotes the rollover subclass resides in the C-H bond activation process which generates the complexes: classical cyclometallation needs simple coordination of a monodentate ligand, followed by C-H bond activation and metalation; at variance, the rollover process needs a bidentate heterocyclic ligand, which may \"choose\" between a classical chelation or cyclometallation (see Scheme 1). Usually the first act is the formation of a chelated complex, followed by decoordination of one of the donor atoms, internal rotation of the heteroaromatic rings and, finally, a \"remote\" C-H bond activation (Scheme 1) [7,8]. As reported by [PERSON], [PERSON] and [PERSON] in their precursor work on rollover cyclometalation (where they coined the term \"roll-over cyclometalation\") [7], metalation is faster for Pt(II) electron-rich starting complexes. An oxidative addition of the C(3)-H bond followed by reductive elimination of methane is a likely reaction pathway.  
2-Phenylpyridine and 2,2\({}^{\prime}\)-bipyridine are common examples of ligands able to give classical and rollover cyclometalated complexes, as depicted in Figure 1.  
A first consequence of the difference between the two classes of organometallic species is that the rollover process is reversible in nature by means of protonation, leading to noteworthy applications, e.g., in catalysis [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].  
A second important difference between the two subclasses is given by the uncoordinated donor atom, usually a nitrogen, able to act as a Lewis base and hence influence the reactivity and properties of the complex, by means of coordination, protonation, interactions and, as stated above, reversibility of the rollover process. For such reasons, these \"rollover\" ligands belong to the interesting category of \"ligands with multiple personalities\" [20]. protonation of the uncoordinated nitrogen transforms the rollover-cyclometalated 2,2\({}^{\prime}\)-bipyridine, formally an anionic ligand, into a neutral mesoionic ligand, an abnormal-remote pyridylene ligand [21, 22, 23, 24, 25].  
Figure 1: Coordination modes of 2-Phenylpyridine and 2,2\({}^{\prime}\)-bipyridine.  
Due to their intriguing peculiarities, rollover complexes are attracting a growing interest in recent years, as demonstrated by the rising number of papers, among which some have recent reviews [4; 26; 27]. The interest is focused on the reactivity of these species [28; 29; 30; 31] and to applications in organic synthesis [32; 33; 34; 35] and catalysis [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19], as well as chemosensors [36], luminescent devices [37], and potent antitumor agents [38; 39; 40; 41; 42; 43].  
Even though a large number of studies have been dedicated to 2,2\({}^{\prime}\)-bipyridine and its derivatives, the reaction is not so restricted and has a general interest being applicable to an ample variety of bidentate heterocyclic donors [44; 45].  
As a part of the ongoing work in our laboratory exploring this field [46; 47; 48; 49; 50] we report here the comparison between platinum(II) rollover derivatives of 6-ethyl- and 6-methoxy-2,2\({}^{\prime}\)-bipyridine, where the different properties of the substituents deeply influence the reactivity of the corresponding cyclometalated complexes.