The preparation and properties of the complexes π-C5H5M(CO)2X3, where M = Mo or W, and X = Cl, Br, or I, are described. Their infrared spectra, and those of the complexes (π-C5H5)2MX2 and [(π-C5H5)2MX2]+HX2–, where M = Mo or W, and X = Br or I, are reported down to 70 cm.–1 and assignments to metal–halogen stretching frequencies are given.