Extended Hückel molecular orbital study of the effects of edge-bridging hydrogen atoms on the lengths of boron–boron and metal–metal bonds in cluster compounds, and the crystal structure of benzyltrimethylammonium octahydrotriborate(1–)
- 1 January 1987
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 5,p. 1017-1025
- https://doi.org/10.1039/dt9870001017
Abstract
The origin of the lengthening of B–B and M–M (M = transition metal) connectivities in borane and transition-metal cluster compounds as a consequence of edge H-bridging is traced, via EHMO–FMO calculations on [B11H13]2–, [Os4H2(CO)12]2–, [Re3H(CO)12]2– and [Re3H2(CO)12]–, to asymmetry in the occupation of formerly degenerate orbitals of the cluster upon protonation. The unusual relative shortening of the bridged B–B connectivities in [B3H8]– is confirmed by an accurate, low-temperature crystallographic study of the ion as its [PhCH2NMe3]+ salt. Crystals are monoclinic, space group P21/c with four ion pairs in a cell of dimensions a= 11.225(4), b= 9.483(3), c= 13.218(4)Å, β= 111.70(3)°; R= 0.0569 for 2 903 data measured at 185 K. EHMO–FMO calculations show that the B–B edge shortening in [B3H8]– is strongly correlated with the asymmetric nature of the H-bridges, and that these two distortions are mutually self-regulating. A combined EHMO–FMO and MNDO study of the B–B edge protonation of 1,6-C2B4H6 suggests that the edge shortening which has previously been predicted may be incorrect.Keywords
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