Complexes Mo2O3B4(B = diethyl dithiocarbamate, O-ethyldithiocarbonate, or diethyl or diphenyl-phosphorothiolothionate), MoO2B2, and Mo2O3B2(CH3CO2)2(B = diethyl dithiocarbamate), and MoOB2(C5H5N)(B = ethyldithiocarbonate) have been prepared. Their electronic, i.r., and 1H n.m.r. spectra are reported. For these and other oxomolybdenum(V) complexes a correlation is found between Mo–O stretching frequencies and the frequency of the first d–d transition.