Comparative ten-vertex metallaborane chemistry: some nido-6-metalladecaboranes of tungsten, rhenium, ruthenium, osmium, and iridium; including the crystal and molecular structures of [6,6,6,6,6-(PMe2Ph)3H2-nido-6-WB9H13] and [6,6,6-(PMe2Ph)3-nido-6-OsB9H13]

Abstract

Reaction of [WH₆(PMe₂Ph)₃] with [NEt₄][B₉H₁₄] yields the first wolfraborane, [6,6,6,6,6(PMe₂Ph)₃H₂-nido-6-WB₉H₁₃]; the halcyon-blue air-stable compound has been characterised by a single-crystal X-ray diffraction analysis and by n.m.r. spectroscopy. Crystals are monoclinic, space group P2₁/n, with a= 1 571.7(5), b= 1 049.1 (5), c= 1 961.5(8) pm, β= 94.30(3)°, and Z= 4. The molecular structure closely resembles that of nido-B₁₀H₁₄ with the BH unit at position 6 being subrogated by a WH₂(PMe₂Ph)₃ metal centre. In solution the compound exhibits fluxionality of the five exo-polyhedral ligands (PMe₂Ph)₃H₂ on the metal centre. Reaction of mer-[OsCl₃(PMe₂Ph)₃] with [NBu₄][B₉H₁₄] yields the new red air-stable osmaborane [6,6,6-(PMe₂Ph)₃-nido-6-OsB₉H₁₃] which has also been characterized by a single-crystal X-ray diffraction analysis and n.m.r. spectroscopy. Crystals are monoclinic, space group P2₁/c, with a= 957.8(1), b= 1 990.0(4), c= 1 671.4(3) pm, β= 94.01(1)° and Z= 4. The structure comprises nido-decaborane-like molecules in which the 6-BH unit has been subrogated by the isolobal Os(PMe₂Ph)₃ unit. The analogous reaction with mer-[RuCl₃(PMe₂Ph)₃] yields the new dark amber ruthenaborane [6,6,6-(PMe₂Ph)₃-nido-6-RuB₉H₁₃], and with [ReH₅(PMe₂Ph)₃] and [NEt₄][B₉H₁₄] the previously reported rhenaborane [6,6,6,6-(PMe₂Ph)₃H-nido-6-ReB₉H₁₃] is formed in an improved yield of 65%. The new tungsten and osmium species together with the known nido-6-metalladecaboranes [(PMe₂Ph)₃HReB₉H₁₃] and [(PR₃)₂HIrB₉H₁₃](R = Me or Ph) permit the structural and n.m.r. comparison of the metallaboranes across four adjacent groups of the Periodic Table; the systematic variation of the exo-polyhedral metal ligands across the sequence WH₂(PMe₂Ph)₃, ReH(PMe₂Ph)₃, Os(PMe₂Ph)₃, IrH (PR₃)₂ implies that each 18-electron metal centre is a three-orbital two-electron contributor to the cluster bonding, just like 6-BH in the structural parent nido-B₁₀H₁₄. For the compounds studied there is a progressive diminution of ca. 5 pm per elemental step in both the M–B and M–P interatomic distances across the series W, Re, Os, Ir, but the ¹¹B and ¹H n.m.r. properties indicate that the cluster bonding retains essentially nido-decaborane-like behaviour throughout.