In 1973, Adams, Brice and Cotton demonstrated that in Cp2Mo2(CNMe)(CO)5 there is CNMe transfer between the molybdenum atoms, and by implication, carbonyl exchange.
Cp2Mo2(CO)6 exists in solution as a mixture of trans and gauche isomers. In most solvents, the predominant isomer is the trans. In 1989, Lindsell and Tomb showed that the ratio can be reversed by working in a mixture of DMSO and THF as the solvent. We have exploited this observation and used DMF as the solvent to study carbonyl exchange in 13CO enriched Cp2Mo2(CO)6.
The lowest energy dynamic process is carbonyl exchange in gauche-Cp2Mo2(CO)6. This results in carbonyls C1O and C3O exchanging via an intermediate gauche-Cp2Mo2(µ-CO)2(CO)4.
The next lowest dynamic process is the corresponding carbonyl exchange in in trans-Cp2Mo2(CO)6. This results in carbonyls C1O and C3O exchanging via an intermediate trans-Cp2Mo2(µ-CO)2(CO)4. This type of intermediate has previously been characterised for trans-Cp2Mo2(µ-CNCF3)2(CO)4 by Lentz and co-workers.
The highest energy dynamic process is rotation about the Mo-Mo bond. As was previously observed by Lindsell and Tomb, only interconversion between the trans and gauche isomers was observed. The barrier for gauche-gauche is too high to detect. This is presumably due to the difficulty in rotating one cyclopentadienyl group past the other.
Scheme. Rotation of the Mo-Mo bond in Cp2Mo2(CO)6. Viewed along the Mo-Mo bond with the carbonyls trans to the Mo-Mo bond omitted for clarity.