_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The first three periodic groups
Silver perchlorate dissolves in benzene and a crystalline adduct C 6 H 6 AgCl0 4
(dec. 145° in a sealed tube) can be isolated from the solution. A rather old X-ray
diffraction experiment ( 19 5 8) showed a chain structure.
3.11 Complexes of main group elements with unsaturated
hydrocarbons
Copper(l) and silver(I) possess filled d 10 shells of electrons. Nevertheless they form
complexes, discussed above, with unsaturated organic ligands. Can main group
elements which possess no filled d-orbitals ( e.g. Al) or in which the (n-1) d-orbitals
are very low in energy (e.g. Ga) form any similar complexes? It is now clear that
they can although their extent and variety is unlikely to rival those ofthe d-block
metals. Some evidence for weak n-interactions ofthis type is mentioned on p. 62
for mercury and on p. 33 for aluminium.
3 .11.1 Cyclopentadienyls
The bonding in cyclopentadienylsodium, C 5 H 5 Na, is generally regarded as ionic
in character. The C 5 H;ion has six n-electrons and according to Hiickel's 4n + 2
rule is aromatic. Considerable covalent character, however, probably exists in
cyclopentadienyl derivatives of the elements of Groups Il, 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 2 and sodium cyclopentadienide
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. lts 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 - l20°C
revealed a 'slip sandwich', in which one ring is symmetrically bound (f/ 5 ), but the
other is attached through only one carbon atom (f/ 1 ).
The molecules CpBeX (X= CI, Br, Me) have C 5 v symmetry in which the ring is
bonded in a symmetrical fashion.
Bis(cyclopentadienyl)magnesium can be made in good yield by a reaction
which is remarkable for an organometallic compound. Solid magnesium reacts
with cyclopentadiene at 500-600°C, the metal then having an appreciable
vapour pressure. The product m.p. 176°C, sublimes readily at 100°C. It has a
symmetrical sandwich structure, similar to that offerrocene (p. 2 79). It dissolves
in hydrocarbons, suggesting that intermolecular interactions are low. The biscyclopentadienyls
of calcium, strontium and barium are made from the metal and
cyclopentadiene in THF or liquid ammonia. The bonding is considered to be
largely ionic. The environment ofthe metal in solid Cp 2 Ca shows that the organic
ligands form weak bridges between the units, in a way reminiscent of the
corresponding lanthanide compounds.
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