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 Sites¹

New heterobimetallic complexes with σ,π-bridging thiophene and selenophene ligands, (η¹:η⁵-XCRCHCHCMn(CO)₅)Cr(CO)₃ (X = S, R = H (1), R = Me (2); X = Se, R = H (3)), were synthesized from (η⁵-XCRCHCHCLi)Cr(CO)₃ and Mn(CO)₅Hal (Hal = Cl^-, Br^-, CF₃SO₃^-). The complexes 1−3 irreversibly convert at 0 °C in acetone into the complexes (η¹:η⁵-XCRCHCHCCr(CO)₅)Mn(CO)₃ (X = S, R = H (4), R = Me (5); X = Se, R = H (6)) by exchanging coordination sites. The σ,π- exchange of coordination sites is a first-order process and rate constants for the reaction of 2 are (3.8 ± 0.1) × 10^-5 and (1.1 ± 0.1) × 10^-5 s^-1 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 1 and 2, 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 η¹-bonded to the C2 of the thienyl ligand and that the free coordination sites are occupied by two bridging carbonyls. The bimetallic complexes (η¹:η⁵-XCRCHCHCC(O)Mn(CO)₅)Cr(CO)₃ (X = S, R = H (7), R = Me (8); X = Se, R = H (9)) 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-(η¹:η¹:η⁵-SCRCHCHCC(O)Mn(CO)₄)Cr(CO)₃}Mn(CO)₄ (R = H, Hal = Cl (10); R = Me, Hal = Br (11)). The lithiated thiophene precursor exclusively attacks a carbonyl of Re(CO)₅Br to give a bimetallic acylate, which after subsequent alkylation with Et₃OBF₄ affords the bimetallic rhenium carbene complex (η¹:η⁵-SCHCHCHCC(OEt)Re(CO)₄Cl)Cr(CO)₃ (12). The target complex (η¹:η⁵-SCHCHCHCRe(CO)₅)Cr(CO)₃ (13), which could not be converted and did not insert a carbonyl, was obtained from Re(CO)₅CF₃SO₃ and the lithiated precursor. The structures of 7 and 10 were confirmed by single-crystal X-ray diffraction studies.