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

Five electron ligands
(c) TITANOCENE AND DICHLOROBIS(CYCLOPENTADIENYL)TTTANIUM. Treatment
of titanium(IV) chloride with two equivalents of sodium cyclopentadienide
in dimethoxyethane affords the red crystalline complex Cp 2 TiCl 2 • Its structure is
typical of a large number of 16, 17 and 18 electron compounds of general formula
Cp 2 MX 2 which are formed by the elements of Group IV, V and VI. In these the
metal assumes distorted tetrahedral geometry. The chloride ligands in Cp 2 TiCl 2
are readily replaced by metathesis in non-aqueous solvents with alkali metal salts
MX to yield derivatives Cp 2 TiX 2 (X= e.g. F, Br, NCO, NCS, N 3 , OR, OCOR, SR).
Ammonium pentasulphide gives Cp 2 TiS 5 which has been used by Max Schmidt to
prepare cyclopolysulphur allotropes such as S7, s9 and slO' Partial hydrolysis of
the Ti-Cl bonds in Cp 2 TiCI 2 occurs in aqueous solutions.
Methyllithium reacts with Cp 2 TiCl 2 to yield the dimethyl complex Cp 2 TiMe 2 ,
which is an orange yellow solid, stable in air, and unreactive to cold water but not
very stable to heat. On photolysis in an atmosphere of hydrogen it affords a violet
complex which on heating is converted into o ne form of the 'titanocene' dimer.
vacuum
a 'titanocene' dimer
The monomer of titanocene has never been isolated. Decamethyltitanocene,
Ti(C 5 Me 5 ) 2 , however, has been prepared, as has bis(2,4-dimethylpentadienyl)­
titanium (p. 302).
(d) HYDROZIRCONATION. Zirconium forms a complex Cp 2 ZrCl 2 , analogous to
Cp 2 TiCl 2 , which is converted into the polymeric hydride 1/n[Cp 2 ZrHCl]" on
treatment with KBHBu~. This hydride readily adds across the double bond of an
alkene in a regiospecific fashion, that is, the zirconium adds to the less sterically
hindered end of the double bond. In the case of an interna! olefin the initial
addition is followed by a series of rapid isomerizations, so that the zirconium
finishes at the end of the carbon chain as in hydroboration (p. 66) or
hydroalumination (p. 80) (Fig. 9.8). lnsertion of carbon monoxide into the Zr-C
290