Experimental and theoretical studies of the structure and IR spectra of neutral diamagnetic binuclear iron nitrosyl complexes Fe₂(µ-SC_6−n H_5−n N_n)2(NO)₄ (n = 0, 1, 2)

Abstract

Geometric and electronic structures of the iron thiolate tetranitrosyl complexes Fe₂(µ-SC_6−n H_5−n N_n)₂(NO)₄, n = 0, 1, 2) were calculated using the B3LYP and PBE density functional methods. The geometric structures of the complexes found by the both theoretical approaches well agree with experiment, and a divergence of bond lengths is at most several hundredths of angstrom. According to the experimental data, the ground state of the system is diamagnetic, with the antiparallel orientation of the local spins 1/2 of the Fe(NO)₂ fragments. The NO group bears a small negative charge mostly concentrated on the O atom, and the Fe-NO bond should be considered as homopolar. The formal electronic configuration of Fe is d⁷, which corresponds to the oxidation state 1+. However, in view of the presence of two homopolar bonds, the Fe atoms in the complexes should be trivalent. The both methods used well reproduce the experimental structure of the IR spectrum, but the PBE functional gives somewhat better description of the absolute position of the lines and the B3LYP functional better describes their relative intensities. The shifts of the vibrational bands of the thiolate ligand in the spectra of the complexes to both long-and short-wavelength regions are theoretically reproduced both qualitatively and semiquantitatively. Comparison of the calculated and measured splittings of the doublet of the stretching NO vibrations made it possible to attribute the appreciable disorder of splitting (15–55 cm⁻¹) to the structural nonequivalence of the NO groups in the complexes. Due to peculiarities of NO binding, even a small difference in the N-O and Fe-N distances (∼0.01 Å) affects the frequency of the NO vibrations.