Triple-Decker [Cp*Fe(C8H6)Fe(C8H7)]

The title compound, [2(1,2,3,3a,6a-η)-1,4-dihydropentalenyl][μ-(l,2,3,3a,6a-η:3a,4,5,6,6a-η)pentalene]-[1(η 5 )-pentamethylcyclopentadienyl]diiron, [Fe 2 (C 8 H 7 )-(C 10 H 15 )(C 8 H 6 )], has a metallocenic structure with the two pentalene ligands adopting an eclipsed conformation. In the central pentalene ligand, all the peripheral bonds are of similar length whereas the bridge bond is longer; there are two significantly different Fe-C distances, i.e. a mean value of 2.121 (3) I for the Fe-C bridge distances and a mean value of 2.028(3)A for the distances from the Fe atoms to all other C atoms. In the terminal pentalene ligand, the C-C and C-Fe distances in the ring bonded to the Fe atom are all similar.

Data collection: CAD-4 Software (Enraf-Nonius, 1989). Cell refinement: CAD-4 Software. Program(s) used to solve structure: MULTAN11/84 (Main, Germain & Woolfson, 1984). Program(s) used to refine structure: SDP-PIus (Frenz, 1985). Molecular graphics: ORTEPII (Johnson, 1976). value quadrupole doublet (between Fe 2+ and Fe 3+) on a M6ssbauer time scale to as low as 1.5 K. The possibility that polymers derived from pentalene might exhibit interesting electrical and/or magnetic properties associated with electron delocalization and electronic cooperative interactions in odd-electron systems prompted us to prepare, characterize and model the monomers. As part of this general structural study, we present here the title compound, (I), which can be envisioned as the smallest subunit of the polymers.
Me Fe (I) Few crystal structures of pentalene complexes have been described. In addition to those mentioned above, one nickel complex (Kitano, Kashiwagi & Kinoshita, 1973), one lithium complex (Stezowski, Hoier, Wilhelm, Clark & Schleyer, 1985), two iron complexes (Churchill & Lin, 1973;Bunel et al., 1988) and five ruthenium complexes (Howard, Knox, Riera, Stone & Woodward, 1974;Howard, Knox, Stone, Szary & Woodward, 1974;Howard & Woodward, 1978;Howard, Stansfield & Woodward, 1979) have been reported. The five ruthenium complexes are polynuclear with the Ru atoms on the same side of the pentalene plane. The other metal complexes are dinuclear, similar to the title compound, with the two metal atoms on opposite sides of the pentalene plane.
The title compound, (I), has a metallocenic structure (Fig. 1). The two pentalene ligands adopt an eclipsed conformation. The dihedral angle between the two pentalene mean planes is 2.0(1) °. The Cp* ligand is not totally eclipsed with respect to the central pentalene ligand. The dihedral angle between the Cp* mean plane and the central pentalene mean plane is 5.5 (1) ° (Fig. 2). The Cp* and terminal pentalene ligands are planar but the central pentalene is slightly distorted showing a chair-like conformation; there is a local pseudo-twofold axis along the C21--C25 bond and a local pseudo-mirror plane along C23--.C27. The Cremer & Pople (1975) total puckering amplitude, QT, is 0.076 (3). The dihedral angle between the tips (C26-C28 and C22--C24) and the middle (C22, C24, C21, C25, C26 and C28) of the chair are 4.7 (2) and 5.1 (2) °, respectively. In the peripheral bonds of the central pentalene ligand, the bond alternation is small, with the C--C bridge bond displaying a longer length. In the related binuclear iron pentalene complex (Bunel et al., 1988), the same situation has also been observed, while in the homologous cobalt complex, the bond alternation is small in the whole pentalene moiety (Manrlquez et al., 1995). In the terminal pentalene, the ring bonded to the Fe atom shows all the bonds to be of similar length and consistent with the values described for the cyclopentadienide rings (like Cp*). The Fe-atom positions are nearly centered with respect to the ligand rings (Fig. 2). The slippage between the ring centroid and the projection of iron on the ring plane are 0.093 (6) for Fel and the central pentalene, 0.089 (6) for Fe2 and the central pentalene, 0.027 ~13) for Fe2 and the terminal pentalene, and 0.018(14)A for Fel and the Cp* ligand. The Fel--Cp* mean-plane distance is 1.643 (1), the Fe 1-pentalene mean plane distance is 1.665 (1) and the Fe2-pentalene mean-plane distances are 1.667 (1) for the central and . This situation has also been described for the previously mentioned binuclear iron complex (Bunel et al., 1988).
The molecules are packed in the crystal in such a way that the ligands adopt two almost perpendicular orientations (Fig. 3). This means that there are no 7r interactions between the two molecules. This packing is different to that found in the other pentalene complexes showing molecular stacks.

Experimental
The title compound was prepared according to the method of Oelckers, Ch~ivez, Manriquez & Rom~in (1993). Suitable crystals were grown by slow evaporation from a hexane solution in an inert atmosphere. For the diffraction measurement, a crystal was mounted in a nitrogen-filled capillary.    (3) C2--C 12 1.502 (5) (5)