Preferred coordination sites for metal fragments in sigma,pi-bimetallic complexes. Detailed mechanistic insight from heteroatom, bridging ligand, solvent, temperature, and pressure effects on the irreversible exchange of coordination sites

New heterobimetallic complexes with sigma,pi-bridging thiophene and selenophene ligands, (eta(1):eta(5)-XCRCHCHCMn(CO)(5))Cr(CO)(3) (X = S, R = H (1), R = Me (2); X = Se, R = H (3)), were synthesized from (eta(5)-XCRCHCHCLi)Cr(CO)(3) and Mn(CO)(5)Hal (Hal = Cl-, Br-, CF3SO3-). The complexes 1-3 irreversibly convert at 0 degrees C in acetone into the complexes (eta(1):eta(5)-XCRCHCHCCr(CO)(5))Mn(Co)3 (X = S, R = H (4), R = Me (5); X = Se, R = H (6)) by exchanging coordination sites. The sigma,pi- exchange of coordination sites is a first-order process and rate constants for the reaction of 2 are (3.8 +/- 0.1) x 10(-5) and (1.1 +/- 0.1) x 10(-5) s(-1) in acetone and cyclohexane at 15 degrees 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 yl-bonded to the C2 of the thienyl ligand and that the free coordination sites are occupied by two bridging carbonyls. The bimetallic complexes (eta(1):eta(5)-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 (mu-Hal){mu-(eta(1):eta (1):eta(5)- SCRCHCHCC(O)Mn(CO)(4))Cr(CO)(3))Mn(CO)(4)Cr(CO3}Mn(CO)(4)(R = H, Hal = Cl (10); R = Me, Hal = Br(l1)). The lithiated thiophene precursor exclusively attacks a carbonyl of Re(CO)(5)Br to give a bimetallic acylate, which after subsequent alkylation with Et3OBF4 affords the bimetallic rhenium carbene complex (eta(1):eta(5)-SCHCHCHCC(OEt)Re(CO)(4)Cl)Cr(CO)(3) (12). The target complex (eta(1):eta(5)-SCHCHCHCRe(CO)(5))Cr(CO)(3) (13), which could not be converted and did not insert a carbonyl, was obtained from Re(CO)(5)CF3SO3 and the lithiated precursor. The structures of a and 10 were confirmed by single-crystal X-ray diffraction studies.