Quantum Chemical Modeling of Pressure-Induced Spin Crossover in Octahedral Metal-Ligand Complexes

Spin state switching on external stimuli is a phenomenon with wide applicability ranging from molecular electronics to gas activation in nanoporous frameworks. Here we model spin crossover as a function of hydrostatic pressure in octahedrally coordinated transition metal centers by applying a field of effective nuclear forces that compress the molecule towards its centroid. For spin crossover in first-row transition metals coordinated by hydrogen, nitrogen, and carbon monoxide, we find the pressure required for spin transition to be a function of ligand position in the spectrochemical sequence. While pressures on the order of 1 GPa are required to flip spins in homogeneously ligated octahedral sites, we demonstrate a five-fold decrease in spin transition pressure for the archetypal strong field ligand carbon monoxide in octahedrally coordinated Fe²⁺ in [Fe(II)(NH₃)₅CO]²⁺.