The irreversible formal exchange of the metal atoms in dinuclear complexes has been observed for the first time. In polar solvents complex 1 rearranges to give the thermodynamically more stable complex 2.

New heterobimetallic complexes with σ,π-bridging thiophene and selenophene ligands, (η 1 :η 5 -XCRCHCHCMn(CO) 5 )Cr(CO) 3 (X ) S, R ) H (1), R ) Me (2); X ) Se, R ) H (3)), were synthesized from (η 5 -XCRCHCHCLi)Cr(CO) 3 and Mn(CO) 5 Hal (Hal ) Cl -, Br -, CF 3 SO 3 -). The complexes 1-3 irreversibly convert at 0 °C in acetone into the complexes (η 1 :η 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 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 η 1 -bonded to the C2 of the thienyl ligand and that the free coordination sites are occupied by two bridging carbonyls. The bimetallic complexes (η 1 :η 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 (µ-Hal){µ-(η 1 :η 1 :η 5 -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) 5 Br to give a bimetallic acylate, which after subsequent alkylation with Et 3 OBF 4 affords the bimetallic rhenium carbene complex (η 1 :η 5 -SCHCHCHCC(OEt)Re(CO) 4 Cl)Cr(CO) 3 (12). The target complex (η 1 :η 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 CF 3 SO 3 and the lithiated precursor. The structures of 7 and 10 were confirmed by single-crystal X-ray diffraction studies.

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