Metallocene, carbon-free

Perhaps the most interesting of all the phosphorus analogues of metallocenes is the first carbon-free metallocene, [( ri -P5)2Ti]. This complex, prepared by the reaction of [Ti(naphthalene)2l with P4, contains parallel, eclipsed P5 rings.The P5 ligand in this and other complexes functions as a weaker donor but substantially stronger acceptor than the cyclopentadienyl ligand. 

The structure of arene sandwich compounds (like that of cyclopentadienyl, cycloheptatrienyl, etc., derivatives) and the character of the M-arene bond may also be considered on the basis of ligand field theory assuming C ,v symmetry of the compound (cf. Chapter 9). The ligands connected to the metal via carbon atoms create a strong field. Therefore, the splitting parameters are large. The nephelauxetic coefficients P = BIBq, where B is the Racah parameter for the complex and Bq is the Racah parameter for the free metal ion (5o and B represent interelectron repulsion), generally assume values 0.5 0.1 for metallocenes and arene sandwich complexes. Low values of nephelauxetic coefficients indicate considerable covalent character of the metal-hydrocarbon bond. 

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