Exploring the trans effect of the NH₃ ligand in platinum halide complexes Pt(NH₃)ClX₂⁻ (X = Cl, Br, I) using cryogenic photoelectron spectroscopy and quantum chemical calculations

Cryogenic anion photoelectron spectroscopy, combined with quantum chemical calculations, was employed to investigate PtClX₂⁻, Pt(NH₃)ClX₂⁻ (X = Cl, Br, I), and their isomers. Photoelectron spectra recorded at 193 nm, supported by B3LYP-D3(BJ)/aug-cc-pVTZ(-pp) calculations, provided adiabatic (ADEs) and vertical detachment energies (VDEs) with excellent agreement between experimental and theoretical results. Coordination of the NH₃ ligand to PtClX₂⁻ reduced the electron binding energy, and substantially elongated trans Pt-halogen bonds. Further computational analyses, including natural population analysis (NPA), frontier molecular orbital (FMO) studies, and dissociation energy calculations, all revealed significant changes in charge distributions and stability of trans halogen ligands. The results demonstrated that NH₃ coordination notably elevated FMO orbital energies, with the extent of this elevation correlating strongly with the trans halogen's orbital contributions. These findings provide new insights into ligand-induced electronic and structural transformation in platinum halide systems and establish a theoretical foundation for understanding the underlying molecular mechanisms that dictate the activities of platinum-based anticancer drugs.