The structures and energetics of the binuclear cyclooctatetraene uranium carbonyls (C8H8) 2U2 (CO) n (n = 2, 3, 4, 5) have been studied by density functional theory. The most interesting observation from this work is the prediction of low-energy structures in the tetracarbonyl system of the type (C8H8) 2U2 (η4-μ-C4O4), in which the four CO groups couple to form a bridging C4O4 squarate unit. Such a tetramerization of carbon monoxide to give a squarate unit by organouranium compounds has been observed experimentally by Cloke and co-workers in sandwich compounds of the type (η5-Me5C5) U (η8-C8H6SiR32) containing both five-membered and eight-membered rings. However, tetramerizations of CO groups to squarate were not predicted in theoretical studies of related (C8H8) 2Th2 (CO) 4 or (C5H5) 2M2 (CO) 4 systems (M = Th, U). These bridging squarate (C8H8) 2U2 (η4-μ-C4O4) structures found in this work are thermochemically favored to the extent that the lowest energy structure of the tricarbonyl (C8H8) 2U2 (CO) 3 is disfavored relative to disproportionation into such a bridging squarate tetracarbonyl structure and the lowest energy structure of the dicarbonyl (C8H8) 2U2 (CO) 2. In the remaining low-energy (C8H8) 2U2 (CO) n (n = 2, 3, 4, 5) structures, the carbonyl groups are all isolated, either as terminal CO groups similar to those bonding to d-block metals or as bridging η2-μ-CO groups bonded to uranium through both their carbon and oxygen atoms. The viability of formal uranium oxidation states from +3 to +6, as found experimentally in diverse stable molecules, leads to a variety of spin states and uranium-uranium bonding modes in the low-energy (C8H8) 2U2 (CO) n (n = 2, 3, 4, 5) structures. This contrasts with the previously studied thorium systems (C8H8) 2Th2 (CO) n (n = 2, 3, 4, 5) 6, where the maximum viable formal thorium oxidation state of +4 limits the range of accessible structure types, metal-metal bonding modes, and spin states.
Attia et al. (Fri,) studied this question.