The molecular geometries of π-cyclopentadienyl-trans-dicarbonyliodo(triphenylphosphine)molybdenum, (π-C₅H₅)-Mo(CO)₂(PPh₃)I, (I), and π-cyclopentadienyl-cis-carbonylchloro[1,2-bis(diphenylphosphino)ethane]molybdenum,(π-C₅H₅)Mo(CO)(PPh₂CH₂CH₂PPh₂)Cl, (II), have been determined by X-ray crystal-structure analysis. The compound (I) crystallizes in the triclinic space group P, with a= 9·475 ± 0·007, b= 10·667 ± 0·008, c= 12·370 ± 0·009 Å, α= 90° 52′± 4′, β= 93° 57′± 3′, γ= 113° 2′± 3′, and Z= 2. The compound (II) crystallizes in the orthorhombic space group Pbca, with a= 17·421 ± 0·009, b= 23·764 ± 0·014, c= 13·783 ± 0·010 Å, and Z= 8. The three-dimensional X-ray intensity data were collected on an automatic four-circle diffractometer with Mo-K_α radiation, and the atomic co-ordinates were deduced by Fourier and least-squares methods. The final values of the discrepancy index R are 7·5% over 3425 independent reflections for (I) and 6·7% over 1706 independent reflections for (II). Each molybdenum atom, formally seven-co-ordinate, is in a distorted square pyramid defined by the π-cyclopentadienyl ring at the apex and the other ligands at the corners of the base. The Mo–C(carbonyl) bond lengths are 1·984 ± 0·015 and 1·982 ± 0·018 Å in (I) and 1·938 ± 0·018 Å in (II), and the Mo–P bond lengths are 2·481 ± 0·005 Å in (I) and 2·496 ± 0·004 and 2·439 ± 0·005 Å in (II). The Mo–I and Mo–Cl distances are 2·858 ± 0·003 and 2·541 ± 0·005 Å, and the difference between these (0·32 Å) is close to the difference of 0·34 Å between the covalent radii of chlorine and iodine atoms. The Mo–C-(cyclopentadienyl) separations in (I) range from 2·297 to 2·367, mean 2·333 Å, while those in (II) range from 2·233 to 2·381, mean 2·310 Å.