Despite the synthesis and structural characterization of
closo-hydroborate dianions, [B
nH
n]
2− (
n=6–12) more than 50 years ago, some ambiguity remains about the structure of [B
8H
8]
2−. Although the solid-state structure of [B
8H
8]
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 [B
8H
8]
2− by using Cp
2MBH
3 and structurally characterized two new octaborane analogues, [(Cp
2MBH
3)
2B
8H
6] (Cp=η
5-C
5H
5; 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 {Cp
2MBH
3} moieties on both sides of the cluster. Spectroscopic characterization (
11B NMR) validates the intactness of the B
8 dodecahedron core in solution as well. Theoretical calculations establish that the two
exo-{Cp
2MBH
3} 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.
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