The crystal and molecular structures of the compounds (π-C5H5)2ZrF2 and (π-C5H5)2Zrl2, have been elucidated by X-ray diffraction studies. The I–Zr–I and (π-C5H5)–Zr–(π-C5H5) angles in the di-iodide are 96·2 and 126·3°, and the F–Zr–F and (π-C5H5)–Zr–(π-C5H5) angles in the difluoride are 96·2 and 127·8°. The mean zirconium–carbon distance in the di-iodide, 2·48 Å, does not differ significantly from that in the difluoride, 2·50 Å. The zirconium–iodine and zirconium–fluorine distances, 2·85 and 1·98 Å, differ by 0·87 Å, whereas the covalent radii of iodine and fluorine atoms differ by 0·69 Å. Crystals of (π-C5H5)2ZrF2 are orthorhombic, of space group F2mm, with a= 6·076, b= 10·475, c= 14·848 Å, and Z= 4; the X-ray intensities were measured by means of a computer-controlled four-circle diffractometer employing Mo-Kα radiation, and least-squares refinement of the atomic co-ordinates and thermal parameters resulted in convergence at R 3·0% for 373 independent reflections. Crystals of (π-C5H5)2Zrl2 are monoclinic, of space group C2/c, with a= 14·20, b= 7·25, c= 13·53 Å, β= 117·1°, and Z= 4; photographic intensity data obtained with Mo-Kα radiation were estimated visually, and least-squares refinement of the atomic parameters was completed at R 5·7% for 749 independent reflections.