Studies on tetraborane(8) carbonyl, B4H8·CO: its isomeric composition in the gas phase and in solution, and its reactions with alkenes

¹¹B NMR spectra of tetraborane(8) carbonyl, B₄H₈·CO 1, reveal a changeover in the distribution of isomers in going from toluene solution to the gas phase. Fortuitously the distribution is 60:40 in each case, but comparison with published electron diffraction data and ab initio/IGLO computations indicates that the CO group is disposed endo with respect to the B₄ `butterfly' framework in the predominant isomer in the gas phase, and exo in solution. The results also allow conclusions to be drawn about the geometries of other B₄H₈·L isomers on the basis of their reported proton NMR chemical shifts. Reactions of B₄H₈·CO with ethene and propene at ca. 30 bar yield products, R₄B₄H₄·CO (R=Et 2 or Pr³ 3), in which all four wingtip hydrogen atoms of the tetraborane carbonyl have been replaced by alkyl groups. Variable-temperature ¹¹B and ¹H NMR spectra of 2 and 3 reveal interesting fluxional behaviour.