Crystal structure of tri-µ-carbonyl-pentacarbonylbis-(π-cyclopentadienylrhodio)di-iron(Rh–Rh)(Fe–Fe)(4Rh–Fe), (π-C5H5)2Rh2Fe2(CO)8

The molecular structure of (π-C5H5)2Rh2Fe2(CO)8, has been determined from X-ray diffraction studies. The compound crystallizes in the centrosymmetric orthorhombic space group Pnma(D2h16, No. 62) with a= 17·979(15), b= 12·079(12), c= 9·386(8)A and Z= 4. A full three-dimensional analysis has led to the location of all non-hydrogen atoms. The structure has been refined by least-squares methods to R 7·90% for the 627 unique non-zero reflections. The molecule has crystallographically imposed Cs symmetry with the four metal atoms defining an irregular tetrahedron with Rh–Rh 2·648(3), Rh–Fe 2·570(5)–2·598(5), and Fe–Fe 2·539(7)A. The π-C5H5 ligands and a bridging carbonyl group are associated with the two rhodium atoms while there are three terminal CO ligands attached to Fe(1) and two to Fe(2).An unusual feature of the structure is the presence of two asymmetric carbonyl bridges, Fe(2)–CO–Rh and Fe(2)–CO–Rh′, with Fe–C 1·79(4) and Rh–C 2·18(3)A. The overall disposition of ligands around the tetrahedral metal-atom core is such that it proves impossible to assign 18 outer valence electrons to each metal atom.