EXTENDED HUCKEL MO STUDIES OF BORANES .4. RATIONALIZATION AND PREDICTION OF ENDO-HYDROGEN AND BRIDGING-HYDROGEN POSITIONS IN NIDO AND ARACHNO SYSTEMS

An idealized form of extended Hückel calculations is presented that permits prediction of the sites of bridging- and endo- terminal-hydrogen atoms and the number of each type on boranes and their anions having stoichiometry equivalent to BnHn+4 or BnHn+6. Calculations are performed on the BnHn4−/6− core framework, endo hydrogens are placed on atoms having net charge greater than about −0.9, and bridging-hydrogen atoms are placed over B–B bonds having total HOMO overlap populations that are positive. Because the presence of endo-hydrogen atoms can change the identity of the core HOMO, it is necessary to run a sequence of calculations for each core yielding a predicted endo-hydrogen atom in which protons are added progressively to the BnHn+6 core until all sites are exhausted. Site exhaustion is indicated by (1) complete protonation of all edges of the cluster’s open face, (2) negative total HOMO overlap population for each remaining unprotonated edge of the open face, or (3) net positive charge for the sum of the HOMO overlap population and the net charge of the two atoms forming the B–B bond. Twenty-four boranes (all those for which experimental hydrogen locations have been proposed) are considered, all but two of which yield correct predictions of the number and location of endo- and bridging-hydrogen atoms; two species (B4H10 and B9H14−) yield predominantly correct predictions. Some generalizations are offered on the basis of the calculations to indicate patterns of endo- and bridging-hydrogen location for B., cores of given symmetry and boron atom connectivity. © 1990, American Chemical Society. All rights reserved.