Is Thorium a d Transition Metal or an Actinide? An Answer from a DFT Study of the Reaction between Pyridine N-Oxide and Cp2M(CH3)2 with M = Zr, Th, and U

The reaction of various actinide and transition metal bisalkyl complexes with pyridine N-oxide has been investigated at the DFT level for M = Zr, Th, and U. Rather than the expected oxygen transfer reaction, an ortho C-H activation is observed experimentally for Th and U and is explained theoretically. The oxygen transfer reaction implies the formation of ethane by coupling two methyl groups; however, the transition state lies very high in energy due to the electrostatic repulsion between the two negatively charged methyl groups. In contrast, C-H activation leads to a six-membered-ring transition state at low energy with a perfect alternation of charges. The latter is found to be kinetically accessible for Th and U but not for Zr, in agreement with the experimental observations. This is related to the ability of Th and U to hybridize their unoccupied 5f orbitals with the 6d in order to form a covalent M-Me bond, leading to strong M-Me ->sigma*(C-H) back-donation at the transition state. Thorium is thus behaving more like an actinide that can use the 5f orbitals for bonding than as a transition metal.