Binuclear Cyclopentadienylmolybdenum Carbonyl Derivatives: Where is the Missing Mo═Mo Double-Bonded Species Cp2Mo2(CO)5?

Abstract

The cyclopentadienylmolybdenum carbonyls Cp2Mo2(CO)n (Cp = η5-C5H5; n = 6−1) have been studied by density functional theory. The two lowest energy structures predicted for Cp2Mo2(CO)6 lie within 4 kcal/mol of each other. Both have Mo−Mo single bonds of lengths 3.2−3.3 Å with all terminal carbonyl groups and correspond to stable compounds structurally characterized by X-ray diffraction. Similarly, the lowest energy structure predicted for Cp2Mo2(CO)4 has a formal Mo≡Mo triple bond of length ∼2.5 Å with four weakly semibridging carbonyl groups also corresponding to a stable compound structurally characterized by X-ray diffraction. The pentacarbonyl Cp2Mo2(CO)5, which is not known experimentally as a stable compound but only as a transient intermediate, is shown to have a structure with one symmetrical bridging two-electron donor and four terminal carbonyl groups as well as a formal Mo═Mo double bond. Furthermore, Cp2Mo2(CO)5 is predicted to be thermodynamically unstable with respect to disproportionation into Cp2Mo2(CO)6 + Cp2Mo2(CO)4. The lowest energy structure for Cp2Mo2(CO)3 is a triplet with a formal Mo≡Mo triple bond. A higher energy singlet structure with one four-electron-donor bridging carbonyl group is also found for Cp2Mo2(CO)3. The Mo≡Mo bond distances in the lowest energy more highly unsaturated Cp2Mo2(CO)2 and Cp2Mo2(CO) structures suggest formal bond orders no higher than 3 in the lowest energy structures and thus metal atoms with less than the favored 18-electron configurations.