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In this study, the insertion of different monodentate co-ligands on Pt(II) complexes bearing a monoanionic C^N*N luminophore as a tridentate chelator was achieved beyond the previously reported chlorido- ([<b>PtCl(L)</b>]) and cyanido-decorated ([<b>PtCN(L)</b>]) analogues. To investigate the impact of the auxiliary ligand on the photophysical properties, we introduced a neutral carbonyl-ligand and observed a lower photoluminescence quantum yield (<i>Φ</i>_L) than with a cyanido moiety. However, the direct substitution of the chlorido co-ligand by a NO-related derivative was not successful. Interestingly, the attempted reduction of the successfully inserted nitrito-<i>N</i>-ligand in [<b>PtNO₂(L)</b>] resulted in the oxidation of the Pt(II)-center to Pt(IV), as demonstrated by X-ray diffractometry. For comparison, the trifluoroacetato Pt(II) and chlorido Pt(IV) complexes ([<b>PtTFA(L)</b>] and [<b>PtCl₃(L)</b>], respectively) were also synthesized. The photophysical characterization revealed similar photoluminescence profiles for all complexes, indicating a weak effect of the co-ligand on the excited state; in fact, all complexes display emission from metal-perturbed ligand-centered states (even the Pt(IV) species). Nonetheless, longer excited state lifetimes (<i>τ</i>_av) suggest a reduced thermally-activated radiationless deactivation via metal-centered states upon exchange of the chlorido units for other monodentate entities, yet without significantly improving the overall <i>Φ</i>_L at room temperature. The irreversible oxidation waves (measured via cyclic voltammetry) mostly stem from the Pt(II)-center; the co-ligand-related drop of these potentials correlates with the increasing <i>σ</i>-donating capacities of the ancillary ligand. In summary, an enhanced <i>π</i>-acceptor capacity does not necessarily improve the <i>Φ</i>_L and can even impair radiative rates by compromising the perturbative participation of the metal center on the emissive triplet state; in addition, strong <i>σ</i>-donor abilities improve the phosphorescence efficiencies by hampering the thermal population of dissociative electronic configurations related to the participation of antibonding <i>d</i>*-orbitals at the metal center.