Dimetallic Imido Complexes of Molybdenum and Tungsten with Bridged Bis(η<sup>5</sup>-cyclopentadienyl) Ligands. Molecular Structure of [(MoO)<sub>2</sub>(μ-N<i><sup>t</sup></i>Bu)<sub>2</sub>{μ-(η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>SiMe<sub>2</sub>}]

New imido and oxo dimetallic complexes of Mo and W with the monobridged or dibridged bis(cyclopentadienyl) ligands Cp<sup>1</sup>Cp or Cp<sup>2</sup>Cp [Cp<sup>1</sup>Cp = (η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>SiMe<sub>2</sub>, Cp<sup>2</sup>Cp = (η<sup>5</sup>-C<sub>5</sub>H<sub>3</sub>)<sub>2</sub>(SiMe<sub>2</sub>)<sub>2</sub>] have been synthesized. The Cp<sup>2</sup>Cp complexes have been obtained as both <i>cis</i> and <i>trans</i> isomers which differ in the stereodisposition of the two metal fragments with respect to the Cp<sup>2</sup>Cp system, the stereochemistry being unchanged in all the reactions described. Addition of H<sub>2</sub>N<i><sup>t</sup></i>Bu to [(MCl<sub>4</sub>)<sub>2</sub>(μ-Cp<i><sup>n</sup></i>Cp)] (M = Mo, <i>n</i> = 1, <b>2a</b>; <i>n</i> = 2, <b>2b</b>; M = W, <i>n</i> = 2, <b>2c</b>) affords the imido M(V) complexes [{M(N<i><sup>t</sup></i>Bu)Cl<sub>2</sub>}<sub>2</sub>(μ-Cp<i><sup>n</sup></i>Cp)] (<b>4a−c</b>). Complexes <b>4a,c</b> are oxidized by PCl<sub>5</sub> to give [{M(N<i><sup>t</sup></i>Bu)Cl<sub>3</sub>}<sub>2</sub>(μ-Cp<i><sup>n</sup></i>Cp)] (<b>5a,c</b>). Both <b>4a</b> and <b>5a</b> afford [{MoCl(μ-N<i><sup>t</sup></i>Bu)}<sub>2</sub>(μ-Cp<sup>1</sup>Cp)] (<b>6</b>) by reduction with sodium amalgam. The differences found between this reaction and the related reductions of the mononuclear Cp analogues are discussed. Reaction of <b>6</b> with HgO affords the oxo complex [{MoO(μ-N<i><sup>t</sup></i>Bu)}<sub>2</sub>(μ-Cp<sup>1</sup>Cp)] (<b>7</b>). [(MoOCl)<sub>2</sub>(μ-O)(μ-Cp<sup>2</sup>Cp)] (<b>3</b>) is obtained by reaction of [{Mo(CO)<sub>3</sub>Cl}<sub>2</sub>(μ-Cp<sup>2</sup>Cp)] with CH<sub>3</sub>NO<sub>2</sub> or bubbling air through a THF solution of <i>cis</i>-[(MoCl<sub>4</sub>)<sub>2</sub>(μ-Cp<sup>2</sup>Cp)]. The molecular structure of <b>7</b> has been determined by a single-crystal X-ray analysis.