Metal-metal multiple bonds with “half-bond” components in paramagnetic organometallics of f-block metals: Cyclopentadienyluranium carbonyls as molecular relatives of diuranium

Abstract

The experimentally observed reductive CO coupling in organouranium systems observed by Cloke and co-workers makes of interest the exploration of the structures and thermochemistry of simple Cp2U2(CO)n (n = 5, 4, 3, 2) derivatives. In this connection, the low-energy Cp2U2(CO)n (n = 2, 3, 4) structures contain two to four bridging η2-μ-CO groups supplemented, in some cases, by one or two terminal CO groups. No examples of CO coupling to form C2O2 ligands are found in any of these Cp2U2(CO)n structures in contrast to the previously studied Cp2Th2(CO)4 system in which CO coupling to form C2O2 ligands is observed. The retention of uranium valence electrons in low-energy Cp2U2(CO)n (n = 3, 4) structures, even after partial donation of uranium electrons to reduce Cp to Cp– and CO to CO2-, leads to a rich and complicated variety of UU bonding modes as well as spin states from singlet to quintet. Of particular interest are uranium-uranium multiple bonds containing multiple single electron half-bond components in triplet and quintet spin state Cp2U2(CO)n structures. The formal uranium-uranium multiple bonds in such Cp2U2(CO)n structures exhibit some of the features of the quintuple bond of diuranium, U2, in which a typical σ + 2π triple bond similar to the CC triple bond of acetylene is supplemented by no less than four separate one-electron half-bonds. Thus the UU bonds in the lowest energy Cp2U2(CO)n (n = 2, 3) structures can be interpreted as formal triple bonds of various types with unusually short bond distances of ∼2.5 Å and Wiberg bond indices of ∼3.