Many Metals Form Metallocene Complexes Where a Metal Ion Is

Multiple Choice

Many metals form metallocene complexes where a metal ion is sandwiched between two aromatic rings. The ion is considered to be equally coordinated to all carbons in the ring.
For example cyclopentadience (C₅H₆) reacts with sodium to form the ionic compound (C₅H₅) ^- Na⁺. The ring structure of the ligand is aromatic and the structure drawn as delocalised. This ionic salt is then used to produce a host of coordination complexes e.g with Fe, Mg, Be etc which consist of a divalent cation and two cycolpentadienyl rings. Most of these ions form a sandwich complex where the metal ion lies equidistantly between the two rings. However, in the gas phase, beryllium coordinates to one ring or the other but not both. By considering the properties of beryllium, suggest why this might be.

A) Beryllium is so small that it brings the two negatively charged rings too close to each other and they repel. Be can therefore only coordinate to one ring at a time.
B) The rings are too bulky and can't get close to the beryllium as it is so small.
C) Beryllium must be neutral in this compound.
D) The orbitals of the beryllium hybridize to form four covalent bonds rather than the sandwich structure.