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

The role of U((eta-C8H6{(SiPr3)-Pr-i-1,4}(2))(eta-C5Me5) and U((eta-C8H6{(SiPr3)-Pr-i-1,4}(2))(eta-C5Me4H) in the reductive di-tri- and tetramerization of CO has been modelled using density functional methods and U(C8H8)(C5H5) as the metal fragment. The orbital structure of U(C8H8)(C5H5) is described. CO binding to form a monocarbonyl U(C8H8)(C5H5)(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(C8H8)(C5H5)](2)C2O2 is proposed which involves dimerization of U(C8H8)(C5H5)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(C8H8)(C5H5)]C2O2, the deltate complex [U(C8H8)(C5H5)]C3O3 and the squarate complex [U(C8H8)(C5H5)]C4O4 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(C8H8)(C5H5) as a model and low energy pathways are proposed.