First, we note that the lowest occupied molecular orbital (LUMO) of 2 is a combination of four P-N σ* antibonding orbitals, and is confined to a region inside the cage, where it is inaccessible for covalent intermolecular interactions (Figure , left). As phosphinimine degradation in protic media involves initial coordination of the solvent to the phosphorus atom, giving a five-coordinate intermediate, followed by generation of an acidic proton that catalyzes solvolysis, we propose that the cage-like nature of 2 affords a measure of geometric protection against such reactions. Consistently, degradation of 2 was indeed observed when solutions were spiked with added proton sources (macroscopic amounts of water or acetic acid). Second, we propose that it is an intrinsic geometric feature of small cages that all connected vertices undergo some distortion to accommodate a perturbation at any vertex. The molecular rigidity engendered by this cumulative distortion energy cost may enhance kinetic protection against reaction coordinates involving changes to geometry or coordination number at cage vertices. The robustness of 2 under a wide range of thermal, solvent, and redox conditions suggests it may be a practically useful scaffold, but only if exo-cage substitution chemistry were possible. The significant localization of the HOMO on the imine N atoms (Figure , middle) and the negative electrostatic potential at these sites (Figure , right) both underscore the possibility of nucleophilic behaviour, as envisioned in the limiting tetra-anionic representation D. Silylphosphinimines are known to react with a variety of main group element halides, and we selected phosphorus halides for ease of reaction monitoring. Scheme 3. Reactions of 2 with phosphorus halides.