Spin Crossover in Iron(II) Porphyrazine Induced by Noncovalent Interactions Combined with Hybridization of Iron(II) Porphyrazine and Ligand’s Orbitals: CASPT2, CCSD(T), and DFT Studies

Spin crossover processes invoked by external stimuli, e.g., temperature, pressure, irradiation with light, electric or magnetic field, as well as formation of dative bond, are well-known phenomena. The effect of weak noncovalent interactions on spin crossover is, however, much less explored. To improve understanding of the origin of spin crossover processes driven by noncovalent interactions, the multireference and density functional theory calculations are performed on iron(II) porphyrazine complexes with CO, NO, benzene, and pyridinyl radicals. The electron structure analyses are performed to discuss the differences in the nature of covalent and noncovalent bonding of CO and NO in axial and parallel orientations, respectively, as well as noncovalent bonding of benzene and pyridinyl radical with iron(II) porphyrazine. It is shown that the weak intermixing between the d orbitals of the metal center and suitably oriented p-orbitals of N-containing ligands are responsible for their stronger influence on the...