Reductive coupling of carbon monoxide by U(III) complexes--a computational study.

The role of U((η-C(8)H(6){Si(i)Pr(3)-1,4}(2))(η-C(5)Me(5)) and U((η-C(8)H(6){Si(i)Pr(3)-1,4}(2))(η-C(5)Me(4)H) in the reductive di- tri- and tetramerization of CO has been modelled using density functional methods and U(C(8)H(8))(C(5)H(5)) as the metal fragment. The orbital structure of U(C(8)H(8))(C(5)H(5)) is described. CO binding to form a monocarbonyl U(C(8)H(8))(C(5)H(5))(CO) is found, by a variety of methods, to place spin density on the CO ligand via back-bonding from the U5f orbitals. A possible pathway for formation of the yne diolate complex [U(C(8)H(8))(C(5)H(5))](2)C(2)O(2) is proposed which involves dimerization of U(C(8)H(8))(C(5)H(5))CO via coordination of the CO O atoms to the opposing U atoms followed by C-C bond formation to form a zig-zag intermediate, stable at low temperatures. The intermediate then unfolds to form the yne diolate. The structures of [U(C(8)H(8))(C(5)H(5))]C(2)O(2), the deltate complex [U(C(8)H(8))(C(5)H(5))]C(3)O(3) and the squarate complex [U(C(8)H(8))(C(5)H(5))]C(4)O(4) are optimized and provide good models for the experimental compounds. The reaction of further CO with a zig-zag intermediate to form deltate and squarate complexes was explored using Th(C(8)H(8))(C(5)H(5)) as a model and low energy pathways are proposed.