Electron Counting in Carbaalane Clusters with Cubic Aluminum Core

Extended Hückel Theory (EHT) and Density Functional Theory (DFT) calculations on hexacapped cubic aluminum clusters of the type [(AlH)₈(μ₄-CH)_6−n (μ₄-H) _n ]^+/−q (n ≤ 6) indicate that their favored number of skeletal electron pairs (SEP), 12 SEPs for n ≥ 3, 14 SEPs for n = 2, 16 SEPs for n = 1 and 18 SEPs for n = 0, depends on the relative number of the two types of capping ligands: one-orbital ligands such as H and conical three-orbital CH units. Although only 16- and 18-SEP species have been isolated so far, DFT calculations indicate that 14-SEP clusters of the type [(AlR′)₈(μ₄-CR)₄(μ₄-X)₂]²⁺ (X = one-orbital ligand) should be synthesized. Computed 12-SEP models were not found to be energy minima, but this should not preclude the possibility for stabilizing such species, for example with bulky substituents. Changing the CR capping ligands into AlR units in the above series is expected to limit their electron counts to 12 SEPs. The incorporation of a main-group atom in the middle of the cube does not change the favored cluster electron count. However, calculations indicate that only 12-SEP species are likely to be synthesized. Dedicated to Professor Günter Schmid on the occasion of his 70th birthday