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.
|Number of pages||9|
|Journal||Journal of the Chemical Society, Dalton Transactions|
|Publication status||Published - 1987|
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