Regio- and Stereochemical Aspects of the Substitution Reaction between the Molybdenocene and Tungstenocene Dichlorides (η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>-R)<sub>2</sub>MCl<sub>2</sub> (R = CMe<sub>3</sub>, SiMe<sub>3</sub>; M = Mo, W) and Metallophosphide Anions [(CO)<sub>5</sub>M‘PPh<sub>2</sub>]Li (M‘ = Mo, W)

The reaction of metallophosphide anions [(CO)<sub>5</sub>M‘PPh<sub>2</sub>]Li (M‘ = Mo, W) with the metallocene dichlorides (η<sup>5</sup>-Cp‘)<sub>2</sub>MCl<sub>2</sub> (M = Mo, W; Cp‘ = C<sub>5</sub>H<sub>4</sub>−CMe<sub>3</sub> (noted <b>-CM</b>) or C<sub>5</sub>H<sub>4</sub>−SiMe<sub>3</sub> (noted <b>-SiM</b>)) leads to complexes (η<sup>5</sup>-Cp‘ ‘)(η<sup>5</sup>-Cp‘)MHCl (Cp‘ ‘ = C<sub>5</sub>H<sub>3</sub>-1-R-3-PPh<sub>2</sub>M‘(CO)<sub>5</sub>), which arise from a nucleophilic substitution on the cyclopentadienyl ligand. The reaction is partially (molybdenocene series) or completely (tungstenocene series) diastereogenic. In solution, a slow transformation of kinetic to thermodynamic products is detected in the molybdenum derivatives, while their tungsten homologues show a total stereostability. On the basis of X-ray structures and of NMR data the relative configurations of the stereoisomers are established.