Beryllocene is an organoberyllium compound with the chemical formula Be(C5H5)2, first prepared in 1959.[1] The colorless substance can be crystallized from petroleum ether in the form of white needles at −60°C and decomposes quickly upon contact with atmospheric oxygen and water.[2]
In contrast to the uncharged metallocenes of the transition metals V, Cr, Fe, Co, Ni, Ru and Os, which have a strictly symmetrical and therefore dipoleless structure, beryllocene has an electric dipole moment of 2.46 Debye (in benzene), or 2.24 Debye (in cyclohexane), indicating asymmetry of the molecule. In the IR spectrum there are signals at 1524, 1610, 1669, 1715 and 1733cm−1, which also indicate that the structure does correspond to that of ferrocene.[2] In contrast, the nuclear magnetic resonance spectrum shows only one signal down to −135°C, indicating either a symmetrical structure or a rapid fluctuation of the rings.[3]
Structure
Beryllocene shows different molecular geometries depending on the physical state. The low-temperature X-ray structure analysis shows a slipped sandwich structure, i.e. the rings are offset from each other - one ring is η5 coordinated with a Be-Cp distance of 152 pm, the second only η1 coordinated (Be-Cp distance: 181 pm).[4][5][6] The reason for the η5, η1 structure is that the orbitals of beryllocene can only be occupied with a maximum of 8 valence electrons. In the gas phase both rings η5 appear to be coordinated. In fact, one ring is significantly further from the central atom than the other (190 and 147pm) and the apparent η5 coordination is due to a rapid fluctuation of the bond.[7] Based on gas-phase electron diffraction studies at 120°C, Arne Haaland concluded in 1979 that the two rings are only about 80pm shifted from each other and are not coordinated η5,η1, but rather η5,η3.[3]
Like beryllocene, the octamethyl derivative Be(C5Me4H)2 has a slipped sandwich structure with η5,η1 coordination. In contrast Be(C5Me5)2 shows the classic η5,η5 coordination. In the crystal, however, the Be-C distances vary between 196.9(1) and 211.4(1) pm.[8]
↑Huheey, James E.; Keiter, Ellen A.; Keiter, Richard L.; Steudel, Ralf; Huheey, James E. (2003). Anorganische Chemie: Prinzipien von Struktur und Reaktivität (3., durchges. Aufled.). Berlin: de Gruyter. ISBN978-3-11-017903-3.
↑Riedel, Ralf; Alsfasser, Erwin, eds. (2007). Moderne anorganische Chemie: mit CD-ROM (3. Aufled.). Berlin: de Gruyter. ISBN978-3-11-019060-1.
↑Xie, Yaoming; Schaefer, Henry F.; Jemmis, Eluvathingal D. (2005). "Characteristics of novel sandwiched beryllium, magnesium, and calcium dimers: C5H5BeBeC5H5, C5H5MgMgC5H5, and C5H5CaCaC5H5". Chemical Physics Letters. 402 (4–6): 414–421. doi:10.1016/j.cplett.2004.11.106. ISSN0009-2614.
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