Rule breaker boron clusters: a new class of hypoelectronic osmaborane clusters [(Cp*Os)₂BH] (n = 6-10)

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 [BH]^2- (n = 6-12). Herein, we report the isolation of a series of osmaborane clusters [(Cp*Os)₂BH], 1-5, (n = 6-10) by the thermolysis of an in situ generated intermediate, obtained from the rapid condensation of [Cp*OsBr₂]₂ and [LiBH₄·THF], with [BH₃·THF] or [BH₃·SMe₂]. Interestingly, all these clusters show unusual less spherical deltahedral shapes that can be generated from canonical [BH]^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.