Single but Stronger UO, Double but Weaker UNMe Bonds: The Tale Told by Cp2UO and Cp2UNR

The free energies of reaction and the activation energies are calculated, with DFT (B3PW91) and small RECP (relativistic core potential) for uranium, for the reaction of Cp 2 UNMe and Cp 2 UO with MeC⋮CMe and H 3 Si−Cl that yields the corresponding addition products. CAS(2,7) and DFT calculations on Cp 2 UO and Cp 2 UNMe give similar results, which validates the use of DFT calculations in these cases. The calculated results mirror the experimental reaction of [1,2,4-(CMe 3 ) 3 C 5 H 2 ] 2 UNMe with dimethylacetylene and [1,2,4-(CMe 3 ) 3 C 5 H 2 ] 2 UO with Me 3 SiCl. The net reactions are controlled by the change in free energy between the products and reactants, not by the activation energies, and therefore by the nature of the UO and UNMe bonds in the initial and final states. A NBO analysis indicates that the U−O interaction in Cp 2 UO is composed of a single U−O σ bond with three lone pairs of electrons localized on oxygen, leading to a polarized U−O fragment. In contrast, the U−NMe interaction in Cp 2 UNMe is composed of a σ and π component and a lone pair of electrons localized on the nitrogen, resulting in a less polarized UNMe fragment, in accord with the lower electronegativity of NMe relative to O. The strongly polarized U (+) −O (-) bond is calculated to be about 70 kcal mol -1 stronger than the less polarized U NMe bond.