No abstract

Factors contributing to the stability of 4c2e bonds have been investigated by means of ab initio calculations on boranes and carboranes. Electron density plots and "deformation energies", the energy required to deform the deprotonated species into a geometry optimal for interacting with the added proton, have been used to help evaluate the propensity of 4c2e vs 3c2e bonding. Two molecules exhibiting 4c2e bonding, CB5H7 (1) and B6H7-(2), have been optimized at MP2/6-31G* with the "extra" hydrogen capping a triangular face or bridging two boron atoms. Face-capping is predicted to be more stable than BHB bridging by 16.8 kcal/mol (CB5IU) and by 12.1 kcal/mol (B6H7-). Computed chemical shifts (IGLO) are in reasonable agreement with experiment. The "antipodal effect" is manifested by the boron atom opposite carbon in CBsH7; the upfield shift relative to B6H7-is calculated to be 20 ppm. one unbound electron) are the simplest possible 4c2e systems. Thus, in e.g. H42+,4 LL,2+,5 Na42+,5 and (CH3)42+,5 only one bonding MO, formed by the overlap of the four inward-oriented orbitals, is occupied. However, such unsupported Td complexes, at best, are only high-lying metastable local minima (Li42+, Na42+); H42+ and (CH3)42+ are not bound.Recently, a "purely organic" 4c2e Td representative has been realized,6 the 1,3-dehydro-5,7-adamantanediyl dication (1, Figure 1). In contrast to the X42+ tetramers, 4c2e bonding in 1 is facilitated by the rigid external framework. The four tetrahedrally-oriented atomic orbitals overlap in the center of the cage.The excellent agreement between the calculated and experimental chemical shifts for the tetrahedral geometry establishes the structure of 1 securely.6The c/ojo-carborane CB3H7 and the isoelectronic B6H7-anion are inorganic examples of 4c2e bonds involving a triply bridging hydrogen. The intent of this contribution is to explore the factors which lead to stable 4c2e bonds. MethodsAll calculations employed the GAUSSIAN 88 or GAUSSIAN 90 program systems.7'8 Geometries were fully optimized at the HF/6-31G* and MP2/6-31G* levels, and analytical frequencies were calculated at HF/6-31G*; 0.89 was used as the zero-point correction factor.

No abstract

No abstract