Electron paramagnetic resonance studies of nitrosyl and thionitrosyl and density functional theory studies of nitrido, nitrosyl, thionitrosyl, and selenonitrosyl complexes of chromium.

The novel S = 1/2 thionitrosyl complexes Cr(NS)(CN)(5)(3-), Cr(NS)(dmso)(5)(2+), and Cr(NS)(nmf)(5)(2+) (dmso = dimethyl sulfoxide, nmf = N-methylformamide) have been prepared, and their optical and electron paramagnetic resonance (EPR) spectra were studied. The values of the isotropic and anisotropic hyperfine and superhyperfine coupling constants A((53)Cr), A((14)N), and A((13)C) and of g were determined from the EPR spectra at room temperature and at 66 K. The values of A(⊥) and A(iso) in the thionitrosyl complexes were slightly higher than in the analogous nitrosyl complexes. A common feature in the optical absorption spectra of the thionitrosyl complexes in solution at 298 K is an absorption band around 600 nm with a vibronic structure whereas such a band is located around 450 nm in the analogous nitrosyl complexes. Density functional theory (DFT) studies of the series of complexes Cr(N)(H(2)O)(5)(2+), Cr(NO)(H(2)O)(5)(2+), Cr(NS)(H(2)O)(5)(2+), and Cr(NSe)(H(2)O)(5)(2+) show that the unpaired electron resides in a metal-based d(xy) orbital and that the electronic structure in the equatorial plane is similar in all four complexes and similar to Cr(3+). The σ donating ability was found in the order N(3-) ≫ NO < NS ≈ NSe and the π accepting ability in the order NO > NS ≈ NSe. Time dependent DFT calculations gave in all four complexes a d(x(2)-y(2)) ← d(xy) transition energy around 17 500 cm(-1).