Preferred Coordination Sites for Metal Fragments in σ,π-Bimetallic Complexes. Detailed Mechanistic Insight from Heteroatom, Bridging Ligand, Solvent, Temperature, and Pressure Effects on the Irreversible Exchange of Coordination Sites<sup>1</sup>

New heterobimetallic complexes with σ,π-bridging thiophene and selenophene ligands, (η<sup>1</sup>:η<sup>5</sup>-XCRCHCHCMn(CO)<sub>5</sub>)Cr(CO)<sub>3</sub> (X = S, R = H (<b>1</b>), R = Me (<b>2</b>); X = Se, R = H (<b>3</b>)), were synthesized from (η<sup>5</sup>-XCRCHCHCLi)Cr(CO)<sub>3</sub> and Mn(CO)<sub>5</sub>Hal (Hal = Cl<sup>-</sup>, Br<sup>-</sup>, CF<sub>3</sub>SO<sub>3</sub><sup>-</sup>). The complexes <b>1</b>−<b>3</b> irreversibly convert at 0 °C in acetone into the complexes (η<sup>1</sup>:η<sup>5</sup>-XCRCHCHCCr(CO)<sub>5</sub>)Mn(CO)<sub>3</sub> (X = S, R = H (<b>4</b>), R = Me (<b>5</b>); X = Se, R = H (<b>6</b>)) by exchanging coordination sites. The σ,π- exchange of coordination sites is a first-order process and rate constants for the reaction of <b>2 </b>are (3.8 ± 0.1) × 10<sup>-5</sup> and (1.1 ± 0.1) × 10<sup>-5</sup> s<sup>-1</sup> in acetone and cyclohexane at 15 °C, respectively. This reaction shows no significant pressure dependence. The activation entropies for the exchange process are −16 ± 6 and −30 ± 11 J/(mol K) for <b>1</b> and <b>2</b>, respectively. The kinetic data suggest an intramolecular exchange mechanism involving bridging carbonyls without any direct involvement of the solvent. It is suggested that in the activated complex the metal centers are η<sup>1</sup>-bonded to the C2 of the thienyl ligand and that the free coordination sites are occupied by two bridging carbonyls. The bimetallic complexes (η<sup>1</sup>:η<sup>5</sup>-XCRCHCHCC(O)Mn(CO)<sub>5</sub>)Cr(CO)<sub>3</sub> (X = S, R = H (<b>7)</b>, R = Me (<b>8</b>); X = Se, R = H (<b>9</b>)) were also isolated from the reaction mixtures and could be obtained in higher yields by working under a CO atmosphere. The inserted carbonyl in the bridging ligand inhibits the metal fragments from exchanging coordination positions. Excess BuLi leads to the formation of the trimetallic five-membered ring complexes (μ-Hal){μ-(η<sup>1</sup>:η<sup>1</sup>:η<sup>5</sup>-SCRCHCHCC(O)Mn(CO)<sub>4</sub>)Cr(CO)<sub>3</sub>}Mn(CO)<sub>4</sub> (R = H, Hal = Cl (<b>10</b>); R = Me, Hal = Br (<b>11</b>)). The lithiated thiophene precursor exclusively attacks a carbonyl of Re(CO)<sub>5</sub>Br to give a bimetallic acylate, which after subsequent alkylation with Et<sub>3</sub>OBF<sub>4</sub> affords the bimetallic rhenium carbene complex (η<sup>1</sup>:η<sup>5</sup>-SCHCHCHCC(OEt)Re(CO)<sub>4</sub>Cl)Cr(CO)<sub>3</sub> (<b>12</b>). The target complex (η<sup>1</sup>:η<sup>5</sup>-SCHCHCHCRe(CO)<sub>5</sub>)Cr(CO)<sub>3</sub> (<b>13</b>), which could not be converted and did not insert a carbonyl, was obtained from Re(CO)<sub>5</sub>CF<sub>3</sub>SO<sub>3</sub> and the lithiated precursor. The structures of <b>7</b> and <b>10</b> were confirmed by single-crystal X-ray diffraction studies.