Dimetallocene Carbonyls of the Third-Row Transition Metals: The Quest for High-Order Metal−Metal Multiple Bonds

Theoretical studies of the third-row transition-metal derivatives Cp<sub>2</sub>M<sub>2</sub>(CO) (Cp = η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>; M = Os, Re, W, Ta) indicate that the lowest-energy structures have lower spin states and similar or higher metal−metal bond multiplicities than the corresponding first-row transtion-metal derivatives. Therefore, Cp<sub>2</sub>Os<sub>2</sub>(CO) is predicted to be a singlet with an Os−Os formal quadruple bond, whereas Cp<sub>2</sub>Fe<sub>2</sub>(CO) is a triplet. Similarly, Cp<sub>2</sub>Re<sub>2</sub>(CO) is predicted to be a singlet with a very short rhenium−rhenium distance, which is consistent with the formal quintuple bond required to give both rhenium−rhenium atoms the favored 18-electron configuration. This contrasts with the manganese analogue Cp<sub>2</sub>Mn<sub>2</sub>(CO) for which the lowest-energy structure is a septet with a formal Mn−Mn single bond. The tungsten derivative Cp<sub>2</sub>W<sub>2</sub>(CO) is predicted to be triplet with a four-electron donor bridging carbonyl group. This contrasts with Cp<sub>2</sub>Cr<sub>2</sub>(CO) predicted to be a septet (S = 3) with a two-electron donor carbonyl group. For Cp<sub>2</sub>Ta<sub>2</sub>(CO), the lowest-energy structure is predicted to be a triplet with a formal TaTa triple bond and a four-electron donor carbonyl group. However, Cp<sub>2</sub>V<sub>2</sub>(CO) is predicted to be a quintet with a formal VV double bond. In addition to these Cp<sub>2</sub>M<sub>2</sub>(CO) structures with one Cp ring bonded to each metal atom, higher-energy Cp<sub>2</sub>M−MCO structures are found with both Cp rings bonded to the same metal atom. The lowest-energy Cp<sub>2</sub>M−MCO structures are triplets (M = Os, W) or quintets (M = Re) with agostic hydrogen atoms for M = Os and Re. In these structures, the spin density is concentrated on the metal atom of the MCO group. These results suggest that lower spin states clearly become more viable for highly unsaturated metal complexes upon descending the periodic table.