CRYSTAL AND MOLECULAR STRUCTURE OF TRANS-1,4-BIS-(PI-CYCLOPENTADIENYLDICARBONYLIRON)BUTA-1,3-DIENE

The compound trans-1,4-bis-(π-cyclopentadienyldicarbonyliron)buta-1,3-diene, [π-C5H5Fe(CO)2]2C4H4, crystallizes in the centrosymmetric monoclinic space group P21/c (C2h5) with a = 6.794 ± 0.005 Å, b = 11.471 ± 0.010 Å, c = 11.787 ± 0.007 Å, β = 113.14 ± 0.02°, and Z = 2. Observed and calculated densities are 1.60 ± 0.01 and 1.596 g cm-3, respectively. A single-crystal X-ray diffraction study based on data out to sin θ = 0.38 (Mo Kα radiation) has led to the unambiguous location of all atoms, including the hydrogen atoms. The final discrepancy index for the 989 independent nonzero reflections is RF = 5.48%. The crystal consists of discrete molecular units of [π-C5H5Fe(CO)22]2C4H4, which have crystallographically imposed Ci symmetry. The FeCH=CHCH=CHFe system has an all-trans configuration and is closely planar, as are the π-cyclopentadienyl ligands. The iron-butadiene σ bonds are 1.987 ± 0.005 Å in length-i.e., some 0.12 Å shorter than the Fe-C(sp3) bond length of 2.11 ± 0.02 Å found in π-C5H5Fe(CO)2(σ-C5H5) and 0.07 Å shorter than the Fe-C(sp3) bond length of 2.06 ± 0.02 Å reported for π-C5H5Fe(CO)2(CH2CO2). Carbon-carbon distances within the butadiene system are 1.340 ± 0.007, 1.450 ± 0.011, and 1.340 ± 0.007 Å. © 1969, American Chemical Society. All rights reserved.