Redetermination of (η4-s-cis-1,3-butadiene)tricarbonyliron(0)

The crystal structure of the title compound, [Fe(C4H6)(CO)3], was previously reported by Mills & Robinson [Acta Cryst. (1963) ▶, 16, 758–761]. The compound crystallizes in the centrosymmetric space goup Pnma with the complex located on a mirror plane. The redetermination of this structure at 100 K yielded almost equilibrated C—C bond lengths within the butadiene ligand according to a metal-to-ligand bonding–back-bonding mechanism. The C—C bond lengths presented herein are significantly shorter than those reported earlier. The H-atom positions that have not been reported so far were located by difference Fourier maps. The positional parameters of all H atoms and individual U iso values were refined freely.

The crystal structure of the title compound, [Fe(C 4 H 6 )(CO) 3 ], was previously reported by Mills & Robinson [Acta Cryst. (1963), 16, 758-761]. The compound crystallizes in the centrosymmetric space goup Pnma with the complex located on a mirror plane. The redetermination of this structure at 100 K yielded almost equilibrated C-C bond lengths within the butadiene ligand according to a metal-to-ligand bondingback-bonding mechanism. The C-C bond lengths presented herein are significantly shorter than those reported earlier.
The H-atom positions that have not been reported so far were located by difference Fourier maps. The positional parameters of all H atoms and individual U iso values were refined freely.

Comment
Olefine complexes containing the {Fe(CO) 3 } fragment are intensively studied and are used as catalysts in a wide range of applications (Knöker, 2000;Sawyer et al., 2008). Diene and dienyl complexes of iron are common intermediates in iron catalyzed organic reactions (Pearson, 1981). Butadiene complexes have been structurally characterized since the 60 s of the last century (Porri, Lionelli, Allegra & Immirzi, 1965;Immirzi & Allegra, 1969). The first synthesis of the title compound dates back to the 30 s of the last century (Reihlen et al. 1930). Vibrational spectroscopic (Davidson, 1969) and 13 C-NMR spectroscopic studies (Kruczynski & Takats, 1976) were undertaken on the title compound. The structures of the title compound were derived from microwave spectroscopy (Kukolich et al., 1993) and quantum chemical calculations (Bühl und Thiel, 1997). The first crystal structure verified the constitution of the title complex consisting of three coordinated CO ligands and one η 4 -coordinated butadiene ligand (Mills & Robinson, 1963). This early structure determination, based on equi-inclination photographic data, did not report any information on the hydrogen atom positions. The standard uncertainties of the reported C-C bond lengths that range from 0.05 to 0.06 do not allow a detailed discussion of the bonding properties of the butadiene ligand.
The redetermination of η 4 -s-cis-1,3-butadienetricarbonyliron(0) at 100 K yielded significantly improved geometric parameters that enables a detailed discussion of the structure. The Fe(0) center of the title complex is coordinated by three carbonyl ligands and one s-cis-1,3-butadiene ligand. The different C-Fe(carbonyl) and C≡O bond lengths are equal within their standard uncertainties. The angles between the C≡O ligands are 93.11 (6) and 101.50 (4)°. The coordinated 1,3butadiene ligand shows the well known s-cis-conformation with the C-C bond lengths equilibrated according to the s.u.'s derived from the X-ray diffraction experiment. The C-C bond lengths are significant shorter (C1-C2 1.423 (1); C1-C1 i 1.418 (2) (librational corrected values; Schomaker and Trueblood, 1968)) than those reported for the crystal structure of this compound but they are in very good agreement to values derived from microwave spectra (Kukolich et al., 1993) and theoretical calculations (Bühl & Thiel, 1997). The values are in good agreement to values derived for the analogous bis(1,3-cyclohexadiene)monocarbonyliron(0) complex (Reiß, 2002). This equilibration is a consequence of a bonding-back bonding mechanism between the diene ligand and the metal center (Reiß & Konietzny, 2002).
The coordination figure at Fe(0) is best described as a square pyramid with the sterically more demanding s-cis-1,3butadiene ligand occupying two coordination sites of the basis. The hydrogen atom of the coordinated s-cis-1,3-butadiene ligand are diplaced of the plane defined by its four carbon atoms. H1 and H2a are only slightly displaced by 11.8 (5)° and 11.1 (6)°, respectively, whereas H2b shows a dihedral angle of 43.4 (7)°. The refined hydrogen atom positions are in very good agreement to results from microwave spectroscopy (Kukolich et al., 1993). This redetermination at low temperature yielded improved and significant shorter C-C bond lengths and locates the not yet reported hydrogen atom positions for the s-cis-1,3-butadiene ligand by crystallographic methods. All geometric parameters derived are in very good agreement with other experimental and theoretical results reported in the last decades.

supplementary materials sup-2 Experimental
The title compound was synthesizes according to procedures reported in the literature (Reihlen et al., 1930). The low melting Fe(0) complex crystallizes readily as block shaped crystals on slow cooling.

Refinement
All hydrogen atoms were located from difference Fourier synthesis. In the final refinement the positional parameters of all atoms, the anisotropic displacement parameters of all non-hydrogen atoms and the U iso values of all hydrogen atoms were refined freely. Fig. 1. Hydrogen atoms are drawn with an arbitrary radius and the displacement ellipsoids are shown at the 50% probability level. Symmetrie codes: i = x, -y+1/2, z.   (17)