Bare group 10 metal atoms as vertices in polyhedral metallaboranes: spherical aromaticity in the icosahedral MB₁₁H₁₁...

The MB_n-1H_n-1 (M = Ni, Pd, Pt; n = 5 to 14) clusters with bare group 10 metal vertices have been examined by density functional theory. The lowest energy structures are found to be the corresponding closo deltahedra. This suggests that the group 10 metal atom can serve as four skeletal electron donors thereby providing the 2n + 2 skeletal electrons for these closo systems in accord with the Wade-Mingos rules. A molecular orbital analysis of the icosahedral NiB₁₁H₁₁ suggests interpretation as a spherical aromatic system with 32 core electrons in a filled 1S²1P⁶1D¹⁰1F¹⁴ shell, analogous to superatomic 3D aromatic species. The molecular orbital analysis indicates that the valence s and d orbitals (mainly d_z²) of the metal participate in delocalized bonding with the boron cage, while the remaining d-electrons reside in non-bonding orbitals localized on the metal. The result is a closed-shell, diamagnetic cluster with substantial aromatic stabilization. Comparisons between Ni, Pd, and Pt reveal similar structural trends, with the Ni-based clusters generally showing the largest stabilization of the closo form. The absence of any protective ligands in these clusters means that the metal centers are coordinatively unsaturated. However, the pronounced stability owing to delocalized skeletal bonding suggests that if isolated, such species would be resilient icosahedral clusters. This theoretical study provides a systematic baseline for bare metal metallaborane clusters, highlighting their viability and unique electronic structure (spherical aromaticity), while also noting the potential (yet untested) reactivity of an exposed transition-metal vertex.