Five-ligand compounds, valence-shell

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

While compoimds with three coordination are known, these species are strong Lewis acids and usually achieve four coordination by formation of three-center two-electron bonds, for example, AI2CI6 (1) (see Bridging Ligand). Aluminum is not restricted to an octet of electrons in its valence shell and compounds with coordination numbers of five and six may be prepared, for example, Cl3Al-2NMe3 (2), [AlFg] " (3). 

The scandium, yttrium, or lanthanide atoms in these compounds evidently form such bridges to make better use of their valence shell electrons and orbitals. (C5H5)2MR monomers would contain only 14 valence shell electrons (five from each cyclopentadienyl ligand, one from the alkyl group in addition to the three metal electrons), well short of the 16 or preferably 18 valence shell electrons normally present in stable organotransition metal compounds " dimerization to (C5H5)2M(p-R)2M(C5H5)2 raises the valence shell electron count to 16 and uses a metal orbital that would be vacant in the monomer. 

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