ИСТИНА

Spin crossover in transition metal complexes (TMC) is a multiplicity change of electronic states of the metal ions under external perturbation (e.g. a variation in temperature, pressure). The majority of so-called spin-active compounds being exhibited this phenomenon are based on Fe(II) and aromatic ligands with donor nitrogen atoms. Theoretical organic chemistry says that the substituents on the aromatic ring alter the reactivity of the ring positions due to the redistribution of the electron density in the π-system. One can expect similar changes in the complexing ability manifested by the strength of the crystal field induced by the ligands. Indeed, the alternation of the diazine moiety from pyrazine to pyrimidine in the TMC pair of type FeL3 (L = 2-(1H-imidazole-2-yl)diazine) leads to variation of absorption spectra and appearance of spin transition at 100 K [1]. To study the influence of the non-coordinating nitrogen position in relation to the donor nitrogen atom on the spin crossover phenomenon we applied the effective Hamiltonian crystal field method (EHCF) [2]. We calculate dependence of the splitting parameter 10Dq from the distance between the iron ion and the donor nitrogen atoms. Moreover, we obtain a slice of the potential energy surface using a simplified scheme of a hybrid approach ECF/MM that combines EHCF and molecular mechanics approaches [3]. It turned out that the positions of the singlet and quintet profile minima for the two studied complexes lie at approximately the same interatomic distances Fe – N. However, the difference between the minima energy, which is responsible for the spin crossover ability, is significantly less in the case of the TMC with a pyrimidine fragment. This result is in agreement with experiment. Obtained dependence of splitting parameter from the relative position of nitrogen atoms in the diazine moiety is compared with eσ parameters of the angular overlap model [4] and the charge on the donor atoms. The detailed mechanism of transfer of the inactive nitrogen atom influence on the donor atom is determined. This process includes the charge redistribution in the π-system of diazine fragment and the energy shift of σ-molecular orbitals, which leads to the observed alternation of eσ and 10Dq parameters. References: 1. Domingo A., Arruda E.G.R., Phung Q., Formiga A., Pierloot K., Book of abstracts of the 15-th V.A. Fock meeting on quantum and computational chemistry. 2. Soudakov A.V., Tchougreeff A.L., Misurkin I.A., Theor. Chim. Acta 83, 389 (1992). 3. Darkhovskii M.B., Pletnev I.V., Tchougreeff A.L., J. Comp. Chem. 24, 1703 (2003). 4. Darkhovskii M.B., Tchougreeff A.L., Zh. Fiz. Khim. 74, 360 (2000).