Beryllocene

Beryllocene is an organoberyllium compound with the chemical formula Be(C5H5)2. It was 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:


 * $$\mathrm{BeCl_2 +\ 2\ Na(C_5H_5)\ \xrightarrow[]{Et_2O} Be(C_5H_5)_2 + 2\ NaCl }$$

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 a clear 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 prove that the structure cannot correspond to that of ferrocene. In contrast, the nuclear magnetic resonance spectrum shows only one signal down to a temperature of −135 °C, indicating either a symmetrical structure or a rapid fluctuation of the rings.

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.

Ernesto Carmon et al. studied the structure of beryllocenes with sterically more demanding ligands: Be(C5Me4H)2 and Be(C5Me5)2 using X-ray crystallographic analysis. They found that in the solid state Be(C5Me4H)2 has a slipped sandwich structure with η5,η1 coordination, while Be(C5Me5)2 shows the classic η5,η5 coordination. In the crystal, however, the Be-C distances are not the same length, but vary between 196.9(1) and 211.4(1) pm.

Chemical
Beryllocene decomposes relatively quickly in tetrahydrofuran, forming a yellowish gel. Beryllocene crystals quickly form an oxide layer on the surface with atmospheric oxygen.

Beryllocene reacts violently in water to produce beryllium hydroxide and cyclopentadiene:


 * Be(C5H5)2 + 2 H2O -> Be(OH)2 + 2 C5H6

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


 * Be(C5H5)2 + FeCl2 -> BeCl2 + Fe(C5H5)2

There are reports in the literature that it can theoretically react exothermically with beryllium to generate C5H5BeBeC5H5.

Safety
Beryllocene is toxic and carcinogenic.