STRUCTURE AND NONRIGIDITY OF B9H92- AND B9H10- - COMPARISONS OF BNHN2- AND BNHN- SYSTEMS(1)

Ab initio calculations at MP2/6-31G, B3LYP/6-3lG*, and B3LYP/6-311+G* levels indicate relatively high potential barriers for the intramole cular rearrangement of B9H92- via the single DSD (diamond-square-diamo nd) or the double DSD mechanism, 28.4 and 21.3 kcal/mol, respectively. However, its open face-protonated form, B9H10- (2b) is highly fluctio nal. Two other minima, 2a (C-2 nu) and 2c (C-3 nu) have energies 1.1 a nd 9.7 kcal/mol higher than 2b. Facile rotation of the BH2 group in 2a is coupled with reversible opening of the boron cage from the close t o a nido form. Structure 2c, with a face-bound proton, is a very shall ow minimum along the degenerate rearrangement pathway of 2b. None of t he optimized structures of B9H10- give calculated B-11 chemical shifts corresponding to experiment; the NMR assignments and interpretation n eed to be refined. Members of the BnHn+1- series (n = 6-8, 10, and 12) have both common as well as specific features. All BnHn+1- species ar e fluxional, due to rapid proton migration. The additional hydrogen H rotates over the whole boron octahedron in B6H7- and over one side of the bipyramid in B7H8-. In B10H11-, the ''extra'' hydrogen H migrate s around the faces of a ''polar region'' near the apical borons more r eadily than from pole to the other; as in B9H10-, H migration in B8H9 - is accompanied by a skeletal rearrangement. The proton affinities (P A) in the BnHn2- series decrease with increasing size, since the Coulo mb repulsion in the dianions becomes smaller and the coordination numb er at the protonation sites increases.