Metal-Stabilized [B8H8]2- Derivatives with Dodecahedral Structure in the Solid and Solution States: [(Cp2MBH3)2B8H6] (Cp=η5-C5H5; M=Zr (1-Zr) and Hf (1-Hf)).

Despite the synthesis and structural characterization of closo-hydroborate dianions, [BnHn](2-) (n=6-12) more than 50 years ago, some ambiguity remains about the structure of [B8H8](2-). Although the solid-state structure of [B8H8](2-) was established by single-crystal X-ray studies in 1969, fast rearrangements in solution at accessible temperatures prevented its detailed characterization. We therefore stabilized a derivative of [B8H8](2-) by using Cp2MBH3 and structurally characterized two new octaborane analogues, [(Cp2MBH3)(2)B8H6] (Cp=eta(5)-C5H5; M=Zr (1-Zr) and Hf (1-Hf)), so that the dynamics of the B-8 skeleton were arrested. The solid-state structures of both 1-Zr and 1-Hf comprise a dodecahedron core protected by {Cp2MBH3} moieties on both sides of the cluster. Spectroscopic characterization (B-11 NMR) validates the intactness of the B-8 dodecahedron core in solution as well. Theoretical calculations establish that the two exo-{Cp2MBH3} fragments provide structural and electronic structural stability to the B-8 core and its intact dodecahedral dianionic nature. Furthermore, we propose isodesmic equations for the formation of higher analogues of the B-n core (n>8) guarded by different group 4 transition metals. Our analysis suggests that, as we move to higher polyhedra (n>10), the formation becomes unfavourable irrespective of metal.