The 95MO n.m.r. spectra of a series of compounds containing the fac-[MoVIO3] and cis-[ MoVIO2] units are reported, together with those for the series [MoX2(R2NO)2] (X = O or S; R = Et, PhCH2). Trends in the data are discussed. The stereochemistry in solution of certain of the cis-dioxo species is assigned by means of 13C n.m.r. spectroscopy. The crystal and molecular structure of [MoS2(Et2NO)2],0.5C6H6 has been determined by X-ray crystallography. The steric and electronic requirements of the terminal sulfide ligands significantly affect the detailed coordination geometry relative to that observed for the dioxo analogue [MoO2(Et2NO)2]. Crystal data: a 10.162(2), b 11.836(2), c 14.437(2) �, β 106.12(2)� P21/a; Z 4.