Monoalkyl boranes are relatively rare. When the alkyl group is small, such as methyl, the monoalkylboranes tend to redistribute to give mixtures of diborane and di- and trialkylboranes.  Monoalkylboranes typically exist as dimers of the form [RBH]. One example is thexylborane (ThxBH), produced by the hydroboration of tetramethylethylene:
:BH + 2 MeC=CMe →  [MeCHCMeBH]
A chiral example is monoisopinocampheylborane. Although often written as IpcBH, it is a dimer [IpcBH]. It is obtained by hydroboration of (−)‐α‐pinene with borane dimethyl sulfide.
Species of the form RBH are available for R = alkyl and halide. Monobromo- and monochloro-borane can be prepared from BMS and the corresponding boron trihalides. The stable complex of monochloroborane and 1,4-dioxane effects hydroboration of terminal alkenes.