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(Cr-53), A(N-14), and A(C-13) and of g were determined from the EPR spectra at room temperature and at 66 K. The values of A(perpendicular to) 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)(H2O)(5)(2+), Cr(NO)(H2O)(5)(2+), Cr(NS)(H2O)(5)(2+), and Cr(NSe)(H2O)(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 Cr3+. The sigma donating ability was found in the order N3->> NO < NS approximate to NSe and the pi accepting ability in the order NO > NS approximate to NSe. Time dependent DFT calculations gave in all four complexes a d(x2-y2) <- d(xy) transition energy around 17 500 cm(-1).