π-Cyclopentadienyl complexes of the type π-C5H5M(CO)nHgX (M = Mo or W, n= 3; M = Fe, n= 2; X = Cl, Br, l, or SCN) are immediately formed by exchange reactions between [π-C5H5M(CO)n]2Hg and HgX2 in acetone or methanol solution at room temperature. Polymetallic complexes of the type π-C5H5M(CO)nHgCo(CO)4 have also been prepared by similar reactions between [π-C5H5M(CO)n]2Hg and [Co(CO)4]2Hg. All these new complexes have been isolated as air-stable crystalline solids, and their infrared, ultraviolet, and nuclear magnetic resonance spectra are reported. In solution, reactants and products are in labile equilibrium although equilibrium constants usually strongly favour the products. The preparative scope of the reaction is discussed.