Theoretical investigation of four-center two-electron bonding involving boron derivatives

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.

Geometric and electronic structures of hydrogenated transition metal (Sc, Ti, Zr) clusters

“…The spatial extents of the orbital and the bond lengths are correlated because the stabilization of the hydrogen atom in these metal clusters results from the in-phase interaction of the atomic orbitals comprised of the 1s orbital of the hydrogen atom and the hybridized s and d orbitals of the metal atoms. This is somewhat akin to the stabilization of hydrogen atoms bound through three-center four-electron ͑3c-4e͒ bonds in boranes, 85 wherein the hydrogen atom is located at the intersection of the three boron p orbitals.…”

Thermochemistry of the Boranes

“…[126][127][128][129] However, a later analysis by Mach et al 130 showed that a structure with C s symmetry locates a global minimum that is approximately 25.1 kJ•mol Ϫ1 lower than that of T d . The rotational constants were derived from the structure of Mach et al 130 The vibrational frequencies ͑scaled by 0.943͒ were computed by Carpenter, 131 who also concluded that C s has the lower energy.…”

Scandium(III)‐Zeolites as New Heterogeneous Catalysts for Imino‐Diels–Alder Reactions

“…As its existence is associated with the overlap of the π* orbitals of each fragment, the bonding interaction will be found whenever two [TCNE] − fragments are placed at such a short distance from each other, either because the interaction between these two fragments is energetically stabilizing, or because some external force drives them together. Multicentered two‐electron (including four‐centered) bonding18 has been reported for several boranes,19 and for the structurally constrained, but not structurally characterized 1,3‐dehydro‐5,7‐adamantanediyl20a and pagodane dications;20b however, this is the first example for a carbon‐based system as well as being the first example of a bonding interaction residing over four atoms.…”