Complexes with halide ligands, such as [Re(CO)3(Pyta)Br] + , are thermally and photochemically robust under low-energy ultraviolet (UV) light (λ > 320 nm). In contrast, complexes with phosphorus ligands, such as [Re(CO)3(Pyta)(TPP)] + , are photoactive and undergo photochemical ligand substitution (PLS) reactions, where the axial CO ligand is substituted by a solvent molecule (as shown in Scheme2). The PLS is a typical reaction that proceeds via the 3 MC state of d 6 metal complexes, which can be qualitatively studied using conventional ligand-field theory. O. Ishitani and coworkers have shown in a computational comparative study on CO-dissociation in two complexes fac-[Re(CO)3(bpy)Cl] + and fac-[Re(CO)3(bpy)P(OMe)3] + in which they propose the later to be more efficient in releasing CO. They propose that for the complex with a phosphine ancillary ligand, the 3 MC state is energetically accessible from the 3 MLCT state, leading to selective dissociation of the axial CO in the PLS reaction. However, the study does not delve into the specific geometry of the dissociative 3MC state or detail the formation of the final stable photoproduct. From an experimental perspective, S. Ferry Forgues and co-workers have recently demonstrated the potential of the [Re(CO) 3 (Pyta)(TPP))] + complex as an effective PhotoCORM. While their work offers valuable insights, exploring the mechanism behind the CO-ligand dissociation could further enhance our understanding and aid in the design of novel PhotoCORMs that can be photoactivated in the visible region. In this work, we go beyond the discussion of CO ligand dissociation pathway of [Re(CO)3(Pyta)(TPP)] + , as the main process in PLS reactions, and discuss the complete reaction mechanism of the formation of the stable photoproduct formation after the CO has been replaced with solvent. This was explored using state-of-the-art quantum chemical calculations. Geometry optimizations were performed for ground-state (GS), triplet-excited-state ( 3 ES) as well as for minimum energy crossing point of triplet and singlet multiplicities ( 1/3 MECP). We discuss the transition from 3 MLCT state to the 3 MC state, the barrier in the 3 MC state, the dissociative 3 MC among the other MCs, and the formation of the photoproduct. This is the first in-silico investigation of the complete mechanism behind the photoproduct formation of the rhenium(I) pyta tricarbonyl complex with a phosphorus ligand.