(η4-s-cis-1,3-Butadiene)tetracarbonylchromium(0)

In the title complex, [Cr(C4H6)(CO)4], the Cr0 atom shows a distorted octahedral environment from four C atoms of the carbonyl ligands and the two π-bonds of the s-cis-1,3-butadiene ligand. The complex has an approximate non-crystallographic mirror symmetry m passing through the chromium atom, two carbonyl ligands and the mid-point of the central C—C bond of the s-cis-1,3-butadiene ligand. The C—C bond lengths in the s-cis-1,3-butadiene ligand alternate, the terminal distances being shorter than the central distance.

In the title complex, [Cr(C 4 H 6 )(CO) 4 ], the Cr 0 atom shows a distorted octahedral environment from four C atoms of the carbonyl ligands and the two -bonds of the s-cis-1,3butadiene ligand. The complex has an approximate noncrystallographic mirror symmetry m passing through the chromium atom, two carbonyl ligands and the mid-point of the central C-C bond of the s-cis-1,3-butadiene ligand. The C-C bond lengths in the s-cis-1,3-butadiene ligand alternate, the terminal distances being shorter than the central distance.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: SI2332).

Comment
Simple butadiene complexes of transition metals are of general interest because they are model systems that allow a deeper understanding of the bonding situation between transition metal centers and olefins that play an important role for example in catalysis. [Cr(C 4 H 6 )(CO) 4 ] that was first described in the 70s of the last century (Fischler et al. 1976) was subject to a number of spectroscopic (Kotzian et al. 1982) as well as theoretical studies (von Ragué Schleyer et al. 2000) and its chemistry was investigated (Kreiter & Özkar, 1978;Okamoto et al. 1991) with the focus on photochemical ligand exchange reactions (Fischler et al. 1976).
The coordination at Cr(0) in the title compound is best described as a distorted octahedron formed by four carbonyl ligands and one s-cis-1,3-butadiene ligand. The Cr-CO distances of the carbonyl ligands that are trans to the s-cis-1,3-butadiene ligand are slightly shorter than the two other Cr-CO distances (Table 1). This finding is in good agreement to Cr-CO distances in the structure of the related tetracarbonyl chromium(0) complex [Cr(C 19 H 23 NO 2 )(CO) 4 ]: d(Cr-CO trans ) = 1.884 (4), 1.887 (6) Å and d(Cr-CO) = 1.847 (5), 1.837 (4) Å (Pavkovic & Zaluzec, 1989). In the structure of the title complex the Cr-C distances to the terminal carbon atoms of the s-cis-1,3-butadiene ligand are longer compared to the respective distances to the central carbon atoms of the diene ligand. A similar trend to longer Cr-C distances for the terminal carbon atoms was found for example for the s-cis-1,3-butadiene chromium(1) complex [CrCp*(C 4 H 6 )(CO)] (Betz et al. 1993). As known from a few other chromium(0) complexes of s-cis-1,3-butadiene and related coordination compounds (Pavkovic & Zaluzec, 1989;Betz et al. 1993;Wang et al. 1990;Konietzny et al. 2010) in [Cr(C 4 H 6 )(CO) 4 ] the terminal C-C distances are significantly shorter than the central d(C-C) Δ(d(C-C)) = 0.057-0.065 Å. In contrast, for comparable iron(0) and manganese(0) complexes almost equilibrated C-C distances have been reported (Reiss, 2010;Reiss & Konietzny 2002), e. g. in the structure of the s-cis-1,3-butadiene iron (0)

Synthesis
[Cr(C 4 H 6 )(CO) 4 ] was synthesized according to a published procedure (Fischler, 1976). The crystal was obtained by slow evaporation of a solution of pentane.

Refinement
All hydrogen atoms were located from difference Fourier synthesis. For the terminal H atom pairs of the CH 2 groups common U iso (H) = 0.031 (4)/0.027 (4) Å 2 and individual U iso (H) = 0.027 (6) Fig. 1. Hydrogen atoms are drawn with an arbitrary radius and the displacement ellipsoids are shown at the 50% probability level.