ISOMER PREFERENCES IN 5-VERTEX, 6-VERTEX, AND 7-VERTEX MONOHETEROBORANES - A THEORETICAL-STUDY

Ab initio calculations at HF/6-31G and MP2/6-31G* are performed on a series of 5-, 6-, and 7-vertex heterosubstituted closo-boranes with NH , S, or PH as the heterovertex. For a trigonal bipyramidal arrangement , the heteroatom prefers the apical position (I). The equatorial posit ion is preferred for the pentagonal bipyramid (III). The only isomer p ossible for the octahedron is found to be a minima (II). The rule of t opological charge stabilization and the ring-cap orbital overlap match is used to rationalize the observed trends.