Molecular structure of di-π-cyclopentadienylcobalt, (C5H5)2Co, by gaseous electron diffraction

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Abstract
Gaseous cobaltocene (CoCp2) has been investigated by electron diffraction at a nozzle‐tip temperature of about 120 °C. As expected, the molecule has the sandwichlike structure previously found for ferrocene (FeCp2) and nickelocene (NiCp2). The data show that any distortion from D5h or D5d molecular symmetry arising from the degenerate electronic ground state (Jahn–Teller effect) is small. Accordingly, the structure was refined in terms of three types of symmetric models: one with an eclipsed conformation of the rings (D5h), one with a staggered conformation (D5d), and one with freely rotating rings. The best agreement was obtained with the freely rotating ring models. However, the differences are small and it is clear that if large‐amplitude torsional motion of the rings had been included in the D5h and D5d models, the agreement from them would have been about equally good. Based on the assumption that the equilibrium conformation of CoCp2 can be approximated by one of these very symmetric models, we conclude that the barrier to internal rotation is low enough to permit large‐amplitude torsional motion and that the minimum energy conformation of the rings cannot be distinguished. The conclusion concerning the hindering potential is made somewhat uncertain by the unknown effect of Jahn–Teller distortion. The values of the important distances (ra), angles, and rms amplitudes of frame vibrations (l) with 2σ error estimates are r (Co–C) =2.113 Å (0.003), r (C–C) =1.430 Å (0.003), r (C–H) =1.095 Å (0.016) &C5,H (the angle between the plane of the C5 ring and the C–H bonds) = 3.7° (3.3), l (Co–C) =0.082 Å (0.004), l (C–C) =0.056 Å (0.003), l (C–H) =0.088 Å (0.015), l (Co...H) =0.137 Å (0.029). The metal–carbon bond length in CoCp2 is about 0.05 Å longer than in FeCp2 and about 0.08 Å shorter than in NiCp2. The weakening of the bonding to the rings as the atomic number of the metal atom increases is a dramatic consequence of the antibonding effects of e1g orbitals involving the metal atoms and the rings: These orbitals are empty in FeCp2, and singly and doubly occupied in CoCp2 and NiCp2. The apparent potential hindering internal rotation of the rings is greatest in FeCp2 and least in NiCp2.