| Summary: | Typical FeII spin crossover SCO complexes exhibit {N6} coordination which offers a stronger ligand field than the {N4S2} ligand set. Recently in [1] the synthesis, crystal structures, magnetic and spectroscopic studies on the family of neutral mononuclear complexes, [FebpteNCE2] where E=S, Se, or BH3 with mixed nitrogen-sulfur ligand surrounding of the FeII 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 FeII ion four nitrogen ligands creating a crystal field of intermediate strength and two sulfur ones giving a much weaker field. The aim of the present communication is to give an explanation of the SCO phenomena in ironII complexes with N4S2 coordination environment [1]. nbsp;Since of the weaker crystal field acting on the ironII ion in the [FebpteNCE2] complexes it has been assumed that besides the ironII states with the spin values in the spin conversion the states with also participate. The calculations of the energies of the complex for the states of the FeII ion with spin values 0, 1 and 2 have been performed within the frames of the density functional theory DFT using the program package ORCA 4.012 [2]. In the work the B3LYP functional [3] together with Grimmeacute;s dispersion correction have been employed. The def2-TZVP [4] basis together with the auxiliary SARC/J and def2-TZVP/C basis sets [5] have been used. Calculations labeled RIJCOSX [6] used the COSX approximation for the exchange terms in conjunction with the RI-J approximation. For the COSX approximation the numerical accuracy has been set by the Gridx5 and FinalGrid6 keywords. To control the target precision of the energies and the wave functions for all clusters the ORCA keyword ―VeryTightSCF‖ SCF stands for the self-consistent field was used, as a result the ground state energies were converged to 10-9 hartree. The performed calculations demonstrated that the complex [FebpteNCS2] is in the state with the spin value S=2 at temperatures T=90 and 230 ...
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