New theoretical insight into high coordination number complexes in actinides-centered borane

The coordination number of a given element behaving to understand its chemistry shows a great interest, which greatly promotes the extension and development of new materials, but remains challenging. Herein we report a new record high coordination number (CN) for actinide established in the cage-like An(BH)₂₄ (An = Th to Cm) via using relativistic quantum chemistry methods. Analysis of U(BH)_n (n = 1 to 24) confirms these series of systems being as geometric minima, with the BH as a ligand located in the first shell around the uranium. Contrast, global searches reveal the low CN half-cage structure for UB₂₄, which is extended to the series of AnB₂₄ and prevails over the competing structural isomers such as cage. The intrinsic geometric difference for AnB₂₄ and An(BH)₂₄ mainly arise from the B sp³ hybridization in borane inducing strong interactions between An 5f6d7s hybrid orbitals and B 2p_z orbitals in An(BH)₂₄ comparing to that of AnB₂₄. The fundamental trend presents a valuable insight for future experimental endeavor searching for isolable complexes with high-coordination actinide and provides a new structural motif of boron clusters and nanostructures.