Complexes of main group elements with unsaturated hydrocarbons

11 Complexes of main group elements with unsaturated hydrocarbons 

Copper(I) and silver(I) possess filled shells of electrons. Nevertheless they form 

The bonding in cyclopentadienylsodium, CjHjNa, is generally regarded as ionic in character. The CgH ion has six 7r-electrons and according to Hiickers 4rH- 2 rule is aromatic. Considerable covalent character, however, probably exists in cyclopentadienyl derivatives of the elements of Groups II, III and IV. The cyclopentadienyl group exhibits a variety of bonding modes which may be compared with those found in transition metal complexes (p. 278). 

Bis-cyclopentadienylberyllium, obtained from BeCl and sodium cyclopen-tadienide in THF, is a very reactive compound, unstable in air and hydrolysing with violence. It sublimes under reduced pressure and decomposes at about 70°C. It has a large dipole moment (2.46 D) in benzene suggesting an unsymmetrical structure. Its proton n.m.r. spectrum, however, shows only one sharp peak from — 100° to 50°C. A single crystal X-ray diffraction study, carried out at — 120°C revealed a slip sandwich , in which one ring is symmetrically bound but the other is attached through only one carbon atom 

The molecules CpBeX (X = Cl, Br, Me) have C5 symmetry in which the ring is bonded in a symmetrical fashion. 

---