We present an in-depth investigation of the influence of chloro-to-iodo exchange on the thermally activated delayed fluorescence (TADF) of a dinuclear platinum(II) complex featuring monodentate halide ancillary ligands. The complexes are constructed using a ditopic bis-N^C^N-chelating ligand of the form (N^C^N–N^C^N)Pt2X2. The initially formed chloro complex (X = Cl) is readily transformed into the iodo analogue (X = I). The change is found to increase the radiative decay rate constant kr by around 3-fold to 3–4 × 105 s−1. This remarkably high value is comparable with the state-of-the-art iridium(III) organometallic phosphors. The improved luminescence properties of the iodo compound are shown to be due to a smaller singlet–triplet energy gap ΔEST compared to the chloro analogue. Iodide reduces the HOMO–LUMO overlap in a multiple resonance-like orbital structure of the complex. Iodination is therefore the first practical strategy shown to improve the TADF properties of diplatinum(II) complexes. The analogous monoplatinum(II) phosphorescent complex is studied in parallel in order to shed light more generally on the effect of iodo ligands on the triplet and singlet states and on spin–orbit coupling (SOC) in platinum(II) complexes.