Modifications of Tanabe-Sugano d$^6$ diagram induced by radical ligand field: ab initio inspection of a Fe(II)-verdazyl molecular complex

Quantum entanglement between the spin states of a metal centre and radical ligands is suggested in an iron(II) [Fe(dipyvd)$_2$]$^{2+}$ compound (dipyvd = 1-isopropyl-3,5-dipyridil-6-oxoverdazyl). Wavefunction \textit{ab initio} (Difference Dedicated Configuration Interaction, DDCI) inspections were carried out to stress the versatility of local spin states. We named this phenonmenon \textit{excited state spinmerism}, in reference to our previous work (see Roseiro et. al., ChemPhysChem 2022, e202200478) where we introduced the concept of spinmerism as an extension of mesomerism to spin degrees of freedom. The construction of localized molecular orbitals allows for a reading of the wavefunctions and projections onto the local spin states. The low-energy spectrum is well-depicted by a Heisenberg picture. A 60 cm$^{-1}$ ferromagnetic interaction is calculated between the radical ligands with the $S_{total} = 0$ and $1$ states largely dominated by a local low-spin $S_{Fe} = 0$. In contrast, the higher-lying $S_{total} = 2$ states are superpositions of the local $S_{Fe} = 1$ (17%, 62%) and $S_{Fe} = 2$ (72%, 21%) spin states. Such mixing extends the traditional picture of a high-field $d^6$ Tanabe-Sugano diagram. Even in the absence of spin-orbit coupling, the avoided crossing between different local spin states is triggered by the field generated by radical ligands. This puzzling scenario emerges from versatile local spin states in compounds which extend the traditional views in molecular magnetism.