The preparation and some chemical properties are described for the complexes (π-C5H5)2MX2, where M = Mo or W and X = Cl, Br, or I; [(π-C5H5)2MX2]+Y–, where M = Mo or W, X = Cl or Br, and Y = HX2, X, or PF6; and [(π-C5H5)2ReX2]+X–, where X = Cl, Br, or I. The physical properties described and discussed include infrared and proton magnetic resonance spectra and, in particular, the electron spin resonance spectra of the molybdenum(V) and tungsten(V) complexes.