Manganese complexes bearing the chelating phosphane–borane ligand H3B·dppm [dppm = bis(diphenylphosphanyl)methane] have been prepared. Addition of H3B·dppm to Mn(CO)5Br using Na[BArF4] as a halide-abstracting reagent affords [Mn(CO)3(η2-H3B·dppm)][BArF4] (1). This reacts with CO to open the bidentate borane to afford [Mn(CO)4(η1-H3B·dppm)][BArF4] (2) in which the borane is now bound in a monodentate manner. The CO addition is reversible, and placing 2 under vacuum (hours) regenerates 1 quantitatively, demonstrating that the chelating phosphane–boranes can act as hemilabile ligands. The complexes 1 and 2 have been fully characterised by NMR spectroscopy and X-ray crystallography.