Powder pattern electron spin resonance spectra of the Au atom complexes of ethylene and oxygen molecules, Au(C2H4) and Au(O2), show anomalies due to 197Au nuclear quadrupole interactions. In the case of Au(C2H4), owing to an extremely large nuclear hyperfine interaction, the quadrupole interaction is manifested as a subtle intensity variation in the powder pattern of the normal transitions. In the case of Au(O2), the hyperfine interaction is small, and the quadrupole term is manifested in the form of forbidden transitions. Both complexes have a side-on structure where the metal atom is situated equidistant from the ligand termini. The following quadrupole coupling tensors were determined through analyses of the powder patterns via simulation: for Au(C2H4) Px = -2Py = -2Pz = 100 ± 15 MHz, and for Au(O2) P2 = -2Px = -2Py = 45 ± 5 MHz, where the z axis is parallel to the C-C or O-O bond of the ligand and the y axis is perpendicular to the plane of the complex. The electron distributions indicated by the quadrupole tensors are in accord with the structures and the bonding schemes envisaged for the respective complexes. © 1990 American Chemical Society.