Modeling of Spin Crossover in Iron(Ii) Complexes with N4s2 Coordination

Typical Fell spin crossover (SCO) complexes exhibit {N-6} coordination, which offers a stronger ligand field than the {N4S2} ligand set. Recently, the synthesis, crystal structures, and magnetic and spectroscopic studies on the family of neutral mononuclear complexes, [Fe(bpte)(NCE)(2)] (where E = S, Se, or BH3) with mixed nitrogen-sulfur ligand surrounding of the iron(II) ions have been reported. A priori, it is not clear what is the origin of SCO in these complexes, which contain in the nearest surrounding of the Fe-II ion four nitrogen ligands, creating a crystal field of intermediate strength, and two sulfur ligands, giving a much weaker field. The present paper looks at how on the basis of density functional theory calculations an explanation can be given of the SCO phenomenon in the iron(II) complexes with mixed nitrogen-sulfur coordination environment. It is demonstrated that in compounds [Fe(bpte)(NCE)(2)] (E = S, BH3), the classical scenario of spin crossover with the participation of iron(II) states with spin values S = 0 and 2 takes place. In the alpha-polymorph of the [Fe(bpte)(NCSe)(2)] compound, two types of Fe-II ions in the unit cell demonstrate different behavior: the ions of one type remain in the high-spin state in the whole temperature range, while the other type of ions manifest spin crossover. The spin transitions demonstrated by the two types of iron(II) ions in the gamma-polymorph of the [Fe(bpte)(NCSe)(2)] compound occur at different temperatures.