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Since the pioneering electron counting rule for borane clusters was proposed by Wade, the structures and bonding of boron clusters and their derivatives have been elegantly rationalised. However, this rule and its modified versions faced problems explaining the electronic structures of less spherical deltahedra, unlike the core geometries of borate dianions [B n H n ] 2- ( n = 6-12). Herein, we report the isolation of a series of osmaborane clusters [(Cp*Os) 2 B n H n ], 1-5, ( n = 6-10) by the thermolysis of an in situ generated intermediate, obtained from the rapid condensation of [Cp*OsBr 2 ] 2 and [LiBH 4 ·THF], with [BH 3 ·THF] or [BH 3 ·SMe 2 ]. Interestingly, all these clusters show unusual less spherical deltahedral shapes that can be generated from canonical [B n H n ] 2- ( n = 8-12) shapes by doing diamond-square-diamond (DSD) rearrangements. The DSD rearrangements led to the generation of higher degree vertices, which are occupied by Os atoms and also generated Os-Os bonds in these clusters. Theoretical calculations revealed that these Cp*Os⋯OsCp* interactions in clusters 1-5 played a crucial role in their structural shape and electron count. These less spherical deltahedral clusters are rare, and most significantly, clusters 1-5 with ( n -1) skeleton electron pairs (SEPs) do not follow Wade-Mingos electron counting rules and can be classified as hypoelectronic closo clusters., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Among the series of stable closo -borate dianions, [B n H n ] 2- , the X-ray crystallographic structure of [B 7 H 7 ] 2- was determined only in 2011. To explore its chemistry and stability, we have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp 2 M) 2 B 9 H 11 ] (Cp = η 5 -C 5 H 5 ; M = Zr or Hf). The structures of [(Cp 2 M) 2 B 9 H 11 ] contain a pentagonal bipyramidal B 7 core, coordinated by two {Cp 2 M} and two {BH 2 } units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(Cp 2 M) 2 B 9 H 11 ] complexes are substantially more stable than the parent dianion, in either [B 7 H 7 ] 2- or ( n Bu 4 N) 2 [B 7 H 7 ]. Our theoretical study and chemical bonding analyses reveal that the surprising stability of the two new heptaborane metal complexes is due to multi-center covalent bonds related to the two exo -{Cp 2 M} units, as well as electrostatic interactions between the {Cp 2 M} units and the B 7 core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.