Beryllocene

Beryllocene is an organoberyllium compound with the chemical formula Be(C₅H₅)₂, first prepared in 1959. 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.

Preparation

Beryllocene can be prepared by reacting beryllium chloride and sodium cyclopentadienide in benzene or diethyl ether:

:$\mathrm$

Properties

Physical

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 1733 cm⁻¹, which also indicate that the structure does correspond to that of ferrocene. 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.

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). 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 147 pm) and the apparent η⁵ coordination is due to a rapid fluctuation of the bond. Based on gas-phase electron diffraction studies at 120 °C, Arne Haaland concluded in 1979 that the two rings are only about 80 pm shifted from each other and are not coordinated η⁵,η¹, but rather η⁵,η³.

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.

Chemical

Beryllocene decomposes relatively quickly in tetrahydrofuran, forming a yellowish gel. It reacts violently in water to produce beryllium hydroxide and cyclopentadiene:
:

Like magnesocene, beryllocene also forms ferrocene with iron(II) chloride. The driving force is the formation of the very stable ferrocene molecule.
:

It is predicted to react with beryllium to generate C₅H₅BeBeC₅H₅.

Safety

Beryllocene is toxic and carcinogenic.

References

{{Cyclopentadienide complexes

Metallocenes
Beryllium compounds
Substances discovered in the 1950s