K-a band ESEEM spectroscopy was used to determine the hyperfine (hfi) and nuclear quadrupole (nqi) interaction parameters for the oxo-O-17 ligand in [(MOO)-O-17(SPh)(4)](-), a spectroscopic model of the oxo-Mo(V) centers of enzymes. The isotropic hfi constant of 6.5 MHz found for the oxo-O-17 is much smaller than the values of similar to 20-40 MHz typical for the 170 nucleus of an equatorial OH(2) ligand in molybdenum enzymes. The 17 0 nqi parameter (e(2)qQ/h = 1.45 MHz, eta approximate to 0) is the first to be obtained for an oxc, group in a metal complex. The parameters of the oxo-O-17 ligand, as well as other magnetic resonance parameters of [(MoO)-O-17(SPh)(4)](-) predicted by quasi-relativistic DFT calculations, were in good agreement with those obtained in experiment. From the electronic structure of the complex revealed by DFT, it follows that the SOMO is almost entirely molybdenum cl, and sulfur p, while the spin density on the oxo-O-17 is negative, determined by spin polarization mechanisms. The results of this work will enable direct experimental identification of the oxo ligand in a variety of chemical and biological systems.