In this work the first examples of C-N bond activation by insertion into a geometrically constrained PIII-centre are shown. The mechanisms of these activation processes leading to new PV species were studied both experimentally and computationally. Interestingly, in the case of insertion of the PIII-centre into an N-C(O)H bond, an unstable phosphoranyl-formaldehyde intermediate is probably formed, which undergoes decarbonylation in the presence of a catalytic amount of HCl producing a hydrophosphorane.