Spin-state bifurcation between hydrogenation and oxygen activation in [Fe(Cp)(CO)₂]: a DFT study toward a spin-aware molecular d-manifold descriptor.

CONTEXT: Spin-state effects can reshape the thermodynamic accessibility of molecular iron intermediates, but their incorporation into descriptor-based catalyst models remains challenging because molecular complexes exhibit discrete, spin-dependent orbital manifolds rather than continuous d-bands. Here, [Fe(Cp)(CO)₂] is examined by DFT as a spin-state-dependent thermodynamic network connecting H₂-derived and O₂-derived basins. The calculations show that CO dissociation exposes a strongly quartet-compatible FeCp(CO) fragment, whereas the σ-H₂ basin is consistently doublet-favored. O₂-derived η1-O₂ and η2-peroxo structures are best described, more conservatively, as spin-flexible open-shell basins, because their ordering is functional-dependent and several formally doublet solutions exhibit substantial spin contamination. The descriptor is therefore not presented as a kinetic predictor, but as a computed spin-resolved Fe-3d orbital metric that rationalizes the electronic distinction between the hydrogenation and oxygen-activation basins. METHODS: Geometry optimizations and harmonic frequency calculations were performed primarily at the unrestricted ωB97X-D/def2-TZVP level using Q-Chem. Gibbs energies were reconstructed from the Q-Chem thermochemical output and used only for stoichiometry-normalized thermodynamic pathway mapping. Functional sensitivity was evaluated by PBE0-D3(BJ), TPSSh-D3(BJ), and B3LYP-D3(BJ) single-point calculations for FeCp(CO), σ-H₂, η1-O₂, and η2-peroxo spin gaps. Additional NBO-based population analyses and molecular-orbital coefficient analyses were used to extract Fe natural charges, Fe spin populations, Fe 3d occupancies, and spin-resolved Fe-3d molecular-orbital centroids. No transition-state or IRC calculations are reported; consequently, all mechanistic language is limited to thermodynamic basin mapping.