Kinetics and Mechanism of Nucleophilic Substitutions on Coordinated Polyenes and Polyenyls. 3. Activation of η<sup>5</sup>-Cyclopentadienyl Ligands toward Nucleophilic Attack through η<sup>5</sup> → η<sup>3</sup> Ring Slippage and a Comparison with Reaction at C<sub>5</sub>H<sub>4</sub>O in [Ru(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(η<sup>4</sup>-C<sub>5</sub>H<sub>4</sub>O)(L)]<sup>+</sup> (L = CH<sub>3</sub>CN, Pyridine, Thiourea)

Complexes of the types [Ru(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(η<sup>4</sup>-C<sub>5</sub>H<sub>4</sub>O)]<sub>2</sub>(CF<sub>3</sub>SO<sub>3</sub>)<sub>2</sub> (<b>1</b>) and [Ru(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(η<sup>4</sup>-C<sub>5</sub>H<sub>4</sub>O)(L)]CF<sub>3</sub>SO<sub>3</sub> (L = CH<sub>3</sub>CN (<b>2</b>), pyridine (<b>3</b>), thiourea (<b>4</b>)) react with tertiary phosphines to give (i) 1,1‘- (<b>12</b>) or (ii) 1,2-disubstituted ruthenocenes (<b>13</b>) depending primarily on the basicity of the entering phosphine and the nature of L. Path i proceeds via the intermediacy of [Ru(η<sup>3</sup>-C<sub>5</sub>H<sub>5</sub>)(η<sup>4</sup>-C<sub>5</sub>H<sub>4</sub>O)(PR<sub>3</sub>)]<sub>2</sub><sup>2+</sup> (<b>5</b>) and [Ru(η<sup>3</sup>-C<sub>5</sub>H<sub>5</sub>)(η<sup>4</sup>-C<sub>5</sub>H<sub>4</sub>O)(PR<sub>3</sub>)(L)]<sup>+</sup> (<b>6</b>−<b>8</b>); i.e., the hapticity of the C<sub>5</sub>H<sub>5</sub> ligand is changed from η<sup>5</sup> to η<sup>3</sup> while forming an additional Ru−P bond. The η<sup>3</sup> bonding mode was established by <sup>1</sup>H and <sup>13</sup>C{<sup>1</sup>H} NMR spectroscopies. The kinetics of these reactions were studied in detail, providing enthalpies and entropies of both activation and reaction. The conversions to <b>6</b>−<b>8</b> are exothermic (Δ<i>H</i>° = −5.5 to −16.9 kcal mol<sup>-1</sup>) but entropically unfavorable (Δ<i>S</i>° = −44.8 to −19.0 cal K<sup>-1</sup> mol<sup>-1</sup>). The activation parameters and rate constants vary little with the phosphine, suggesting a preequilibrium between η<sup>5</sup> and η<sup>3</sup> species of the starting complexes where the latter reacts with the entering phosphine in the rate-determining step. The new η<sup>3</sup>-C<sub>5</sub>H<sub>5</sub> complexes are, with the exception of <b>5</b>, fluxional in solution due to an intramolecular enantiomeric equilibrium likely proceeding through a five-coordinate η<sup>1</sup>-C<sub>5</sub>H<sub>5</sub> intermediate. Path ii proceeds via η<sup>3</sup>-cyclopentenoyl complexes of the type [Ru(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(η<sup>3</sup>-C<sub>5</sub>H<sub>4</sub>O-2-PR<sub>3</sub>)(L)]<sup>+</sup> (<b>9</b>−<b>11</b>). Furthermore, <b>3</b> and <b>4</b> react with small and basic phosphines PR<sub>3</sub> = PMe<sub>3</sub> and Me<sub>2</sub>PCH<sub>2</sub>PMe<sub>2</sub> to give the half-sandwich complexes [Ru(η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>OH)(PR<sub>3</sub>)<sub>2</sub>L]<sup>+</sup> (<b>14</b>, <b>15</b>) together with free C<sub>5</sub>H<sub>4</sub>PR<sub>3</sub> (<b>16</b>).