Molecular and electronic structure and IR spectra of mononuclear dinitrosyl iron complex Fe(SC2H3N3)(SC2H2N3)(NO)2]: a theoretical study

The molecular and electronic structures of different isomers of a mononuclear dinitrosyl iron complex [Fe(SC2H3N3)(SC2H2N3)(NO)2] were calculated by the B3LYP and PBE density functional methods. Both theoretical approaches provide good agreement between the calculated and experimental geometry of the lowest-lying isomer (bond lengths differ by 0.02–0.04 A and bond angles differ by 2–3° in terms of root-mean square values). A feature of the complex is an intramolecular hydrogen bond N-H...N between the thiolate and thione ligands, which causes equalization of the Fe-S and S-C bond lengths. The ground state of the system has a spin of 1/2 and exists at antiparallel orientation of the spin (S = 3/2) of the Fe atom with formal electron configuration d7 and two local spins (S = 1/2) of the NO ligands. Although each NO group has a small negative charge, which is mainly localized on the O atom, the Fe-NO bond can be treated as similar to homeopolar one. This corresponds to the effective trivalent state of Fe with an oxidation state of 1+. Both theoretical methods correctly reproduce the experimental structure of the IR spectrum, but the PBE functional provides a better description of absolute positions of spectral lines, whereas the B3LYP functional gives a somewhat better description of the relative intensities of spectral components. In spite of similar geometric parameters of coordination of two NO groups, the splitting between the NO stretching bands is rather large (58 cm−1); this value is satisfactorily reproduced in theoretical calculations. A strong intramolecular hydrogen bond causes a large frequency shift of the N-H stretching vibrations corresponding to a broad absorption band in the region 2300–2600 cm−1.