Synthesis and electronic structure analysis of the actinide allenylidenes, [{(NR)}An(CCCPh)] (An = U, Th; R = SiMe).

The reaction of [AnCl(NR)] (An = U, Th, R = SiMe) with generated lithium-3,3-diphenylcyclopropene results in the formation of [{(NR)}An(CH[double bond, length as m-dash]C[double bond, length as m-dash]CPh)] (An = U, ; Th, ) in good yields after work-up. Deprotonation of or with LDA/2.2.2-cryptand results in formation of the anionic allenylidenes, [Li(2.2.2-cryptand)][{(NR)}An(CCCPh)] (An = U, ; Th, ). The calculated C NMR chemical shifts of the C, C, and C nuclei in and nicely reproduce the experimentally assigned order, and exhibit a characteristic spin-orbit induced downfield shift at C due to involvement of the 5f orbitals in the Th-C bonds. Additionally, the bonding analyses for and show a delocalized multi-center character of the ligand π orbitals involving the actinide. While a single-triple-single-bond resonance structure (, An-C[triple bond, length as m-dash]C-CPh) predominates, the An[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]CPh resonance form contributes, as well, more so for than for .