Bonding Analysis of the Ge-Ge Bonds in the Octagermacubane Ge8(Sit-butyl2methyl)6

Abstract

Quantum chemical calculations have been carried out at the BP86/def2-SVP level on Ge₈(Sit-butyl₂methyl)₆ (1) and the bonding situation has been analyzed with a variety of methods. The calculated equilibrium geometry of 1 is in good agreement with the reported x-ray structure analysis. The D3 correction for dispersion interactions as a sum of pairwise attractions leads to an overestimate of the effect of dispersion forces. Calculations at BP86-D3(BJ)/def2-SVP give shorter bonds for Ge(I)−Ge(I) than for Ge(0)−Ge(I), which is in contrast to the experimental values and the BP86/def2-SVP results. The NBO analysis suggests that the best Lewis structure of 1 has lone-pair orbitals at the Ge(0) atoms with occupation numbers of 1.70 e. A lone-pair character at Ge(0) albeit with less weight is also suggested by the shape of the HOMO, which is an antibonding orbital between the Ge(0) atoms with small contributions from the Ge(I) atoms. The LUMO of 1 is the corresponding bonding combination of the Ge(0) AOs, which can be explained with the reluctance of the heavier main-group atoms to s/p hybridization of the valence orbitals. The calculated bond order values suggest significant direct Ge(0)−Ge(0) interactions. This is supported by the shape of the HOMO and by the results of EDA-NOCV calculations. The deformation densities and the orbitals associated with the pairwise orbital interaction show that there is a direct charge flow between the Ge(0) atoms of the two fragments, but it is not completely separated from the Ge(0)−Ge(I) and Ge(I)−Ge(I) bond formation. The QTAIM calculations suggest that 1 has a cubic structure with a cage critical point but not a bond critical point for the Ge(0)−Ge(0) interactions. The dispersion interactions of the large substituents in 1 have a significant influence on the stability of the compound.