Modulated Ligand-Ligand Exchange Coupling and Elusive Spinmerism in a Bis(verdazyl)iron(II) Complex

1,5-Dimethyl-3-(2,2'-bipyridine-6-yl)-6-oxoverda-zyl (bipyvd) was used as a tridentate radical ligand toward iron(II) perchlorate to yield bis(1,5-dimethyl-3-(2,2'-bipyridine-6-yl)-6-oxoverdazyl)iron(II) perchlorate, [Fe(bipyvd)₂] (ClO₄)₂. The single-crystal X-ray Fe-N distances are compatible with a low spin (S_Fe = 0) iron(II) center surrounded by two bipyvd radical ligands. A similar conclusion is reached from difference dedicated configuration interaction (DDCI) wave function calculations, which support the coupling of radicals through a closed-shell metal ion bridge. The singlet-triplet splitting is ferromagnetic, with a calculated exchange coupling constant J_LL^S_Fe = 0 = +120 cm^-1. Higher in energy, the calculated spin states split into two subsets successively characterized by pure local spin states S_Fe = 2 and S_Fe = 1. In contrast, the coordination sphere exhibits either a pure local spin state or a superposition of S_L = 1 and S_L = 0 local spin states, a phenomenon previously coined into spinmerism. A marked modification of the ligand-ligand ferromagnetic coupling is evaluated, with J_LL^S_Fe = 2 = +89 cm^-1 and J_LL^S_Fe = 1 = +140 cm^-1, whereas the metal-ligand interactions remain almost constant at ca. +290 cm^-1. The control of the magnetic interactions between organic radicals through a spin-crossover ion revisits the traditional picture dominated by the metal ions and stresses the manifestation of spin entanglement between the coordination sphere and the metal center environment.