Binuclear Cyclopentadienylmolybdenum Carbonyl Derivatives: Where is the Missing MoMo Double-Bonded Species Cp2Mo2(CO)5?
journal contributionposted on 11.05.2009, 00:00 by Xiuhui Zhang, Qian-shu Li, Maofa Ge, Yaoming Xie, R. Bruce King, Henry F. Schaefer
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 MoMo 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 MoMo 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 MoMo triple bond. A higher energy singlet structure with one four-electron-donor bridging carbonyl group is also found for Cp2Mo2(CO)3. The MoMo 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.