Condensed polyhedral boranes and analogous organometallic clusters: a molecular orbital and density functional theory study on the cap-cap interactions

Abstract

The interactions between the non-bonded atoms on adjacent units were assumed to be one of the major factors that hinder the exploration and advancement of macropolyhedral borane chemistry. In sandwich complexes involving boron as the bridging atom, the interaction between non-bonded atoms tends to be antibonding, but a closer analysis of various condensed systems shows that this cannot be generalized. The overlap populations (OPs) calculated for structures optimized at the B3LYP/6-31g* level [B₂₁H₁₈^1- (5), B₂₀H₁₆ (6), [Al(C₂B₄H₆)₂]^1- (7), B₁₂H₁₀^2- (8), B₁₀H₈^2- (9 and 14), B₁₁H₁₃ (10), B₁₀H₁₄ (11), C₂B₈H₁₂ (13) and B₂₀H₁₈^2- (15)] indicate bonding interactions between the caps, except for 7 and 13. This is substantiated by a detailed extended Huckel-based molecular orbital (MO) study using B₁₀H₁₄ as a model system to represent macropolyhedral boranes with higher fusions. An isolobal equivalent structure, [C₈H₆(Ru(CO)₂Me)₂] (17), studied at the B3LYP/LANL2DZ level has weak RuRu interactions. An analysis of the nature of the MOs in B₁₀H₁₄ (11) shows that there is no direct head on overlap of the cap orbitals that are antibonding; this is in contradiction to sandwiched molecules (7), where there are two occupied MOs with antibonding interactions. The m + n + o electron pair count (m is the number of cages involved in condensation, n is the number of vertices and o is the number of single vertex condensations) of sandwich complexes requires the filling of these two MOs. The negative OP between the carbon atoms in 13 is attributed to the greater electronegativity of carbon and is substantiated by a fragment MO analysis.