Russian Chemical Bulletin, International Edition, Vol. 56, No. 7, pp. 1289—1297, July, 2007 1289 Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1244—1252, July, 2007. 1066-5285/07/5607-1289 © 2007 Springer Science+Business Media, Inc. Molecular and electronic structure and IR spectra of mononuclear dinitrosyl iron complex [Fe(SC 2 H 3 N 3 )(SC 2 H 2 N 3 )(NO) 2 ]: a theoretical study A. F. Shestakov,  Yu. M. Shul´ga, N. S. Emel´yanova, N. A. Sanina, and S. M. Aldoshin Institute of Problems of Chemical Physics, Russian Academy of Sciences, 1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax: +7 (496) 515 5420. E-mail: a.s@icp.ac.ru The molecular and electronic structures of different isomers of a mononuclear dinitrosyl iron complex [Fe(SC 2 H 3 N 3 )(SC 2 H 2 N 3 )(NO) 2 ] were calculated by the B3LYP and PBE den- sity 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 Å 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 d 7 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 . Key words: sulfur-nitrosyl iron complexes, nitrogen monoxide donors, IR spectroscopy, quantum chemical calculations, density functional, B3LYP, PBE, electronic structure. Dinitrosyl iron complexes with sulfur-containing ligands are biomimetic models for natural donors of ni- trogen monoxide. 1 Recently, they have been intensively studied in relation with possible use in medicine as NO donors. 2,3 Mononuclear dinitrosyl iron complexes with sulfur-containing ligands exhibit a characteristic EPR signal with g ≈ 2.03 in solutions. Only a few crystal structures of this type of complexes are available at the moment. 4—8 To search for new complexes, we proposed to use aza-heterocyclic thiols (in particular, substituted 1,2,4-triazol-3-thiols having a great coordination poten- tial and showing a broad spectrum of biological activi- ties) 9,10 as sulfur-containing ligands. We synthesized a new neutral mononuclear iron complex with 1,2,4-triazol- 3-thiolyl ligand and carried out room-temperature 11 and low-temperature 12 X-ray studies of the species. In this work report on a quantum chemical study of the molecu- lar and electronic structures and IR spectra of this complex. Experimental Density functional theoretical studies of isolated complex [Fe 2 (SC 2 H 3 N 3 )(SC 2 H 2 N 3 )(NO) 2 ] (1) and its possible isomers were carried out by the B3LYP method with the 6-31G* basis set and the GAUSSIAN-98 program 13 and by the PBE method with the SBK pseudopotential and an ex- tended basis set using the PRIRODA program (see Ref. 14). The energies of the optimized structures were compared with inclusion of zero-point vibrational ener- gies. The B3LYP calculated total energies were obtained using the 6-311++G** basis set. IR spectra of complex 1 synthesized following a known proce- dure 11 were recorded on a Perkin—Elmer/Spectrum BX Fourier- Transform spectrophotometer. Samples were prepared as KBr pellets (1 mg of the compound under study per 300 mg of KBr).