It was observed that coordination of the secondary phosphine to the parent borane BH3 could temper and control the reactivity. HPPh2•BH3 was deprotonated with sec-BuLi at -78 °C, and this was added dropwise to a stirring suspension of mesitylenediazonium tetrafluoroborate in THF at -78 °C to give an immediate purple colour and the formation of the target azophosphine-borane complex MesN2PPh2•BH3 (1•BH3) (Scheme 2). A broad quartet signal was observed at 75.8 ppm by 31 P{ 1 H} NMR spectroscopy, consistent with the P-B coupling expected for the product. The borane group renders 1•BH3 air-stable enough to be purified by column chromatography and the product was isolated as a purple solid in 42% yield.