PROPERTIES OF ATOMS IN MOLECULES - STRUCTURES AND REACTIVITIES OF BORANES AND CARBORANES

The energies, geometries, and charge distributions of the closo-, nido-, and arachno-boranes and closo-carboranes are determined in SCF calculations using a contracted (9s, 5p) basis set of Huzinaga, supplemented with polarization functions. Good agreement is obtained between the experimental and calculated geometries. Molecular structures are assigned on the basis of the bond paths defined by the topology of the charge density using the theory of atoms in molecules. This theory is used to characterize the atomic interactions and to account for the stabilities of these electron-deficient systems. It is shown that their stability is a consequence of the delocalization of charge over the surfaces of the three- and four-membered rings of atoms that result from the formation of bonds of reduced order, a delocalization that is itself essential to the formation of the ring bonds. The Laplacian of the charge density is used to predict the sites of nucleophilic and electrophilic attack in these molecules, predictions that are in agreement with known experimental findings.