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–)

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.