Thermal ellipsoids were drawn at the 50% probability level. The borane protecting group could be fully removed using the nucleophilic amine pyrrolidine. Treatment of 1•BH3 with an excess of pyrrolidine resulted in a small shift in the 31 P{ 1 H} NMR spectrum from 75.8 ppm to 77.0 ppm (Δδ = 1.2 ppm), but the change of the broad quartet signal of 1•BH3 to a sharp singlet was indicative of loss of the quadrupolar boron centre. Removal of excess pyrrolidine in vacuo followed by filtration through a silica plug to remove pyrrolidine•BH3 afforded 1 as a purple oil. The analogous deprotection of 2•BH3 and 3•BH3 afforded the free azophosphines 2 and 3, respectively, and were accompanied by more significant changes in the 31 P{ 1 H} NMR chemical shifts on deprotection (Δδ = 9.8 ppm (2), and 8.6 ppm ( )). Interestingly, the P-aryl azophosphine 1 is significantly less stable in toluene solution than the P-alkyl analogue 2 (see Fig S1 and). Compound 2 is stable in the absence of air in toluene for several weeks with minimal decomposition, whereas 1 starts degrading within 24 hours (in both the light and the dark), and ultimately rearranges with loss of N2 to afford MesPPh2 as the major product as confirmed by NMR spectroscopy and mass spectrometry (Fig S1 and).