9-Vertex closo-boranes are new representatives of quasi-closo-polyhedra

AIM (Atoms in Molecules) analysis of electron density distribution based on the data of DFT B3LYP calculations was carried out for 9-vertex closo-borane and -monocarborane clusters ([closo-B9H9]2−, [closo-1-CB8H9]−). It was found that the calculated molecular graphs of the polyhedra studied differ notably from their traditional depictions as not all edges of the cages correspond to two-center bonds. Due to it, each species possesses two 6-member faces characterized by low values of electron density compared to other parts of the cages. The 6-member faces are characterized by negative Laplacian value which makes them prone to electrophilic attack. The calculated molecular graphs explain existence of low-frequency vibrational modes predicted by the calculations. Due to the features found 9-vertex closo-borane and -carboranes do not have the closo-structure traditionally ascribed to both by formal (2n + 2) -electron count. Similar to 11-vertex closo-species they should be named as quasi-closo-polyhedra.