The unique geometry of tetrahedral, tetravalent molecules makes them valuable scaffolds in synthetic chemistry. Their ability to connect four functional groups in a rigid and well-separated tetrahedral arrangement has allowed development of new optoelectronic materials, thermally-stable energetic compounds, 2 catalysts with enhanced robustness or multi-catalytic sites, bioactive polypeptide frameworks, and pharmaceuticals. In crystalline reticular chemistry, tetrahedral cages are privileged secondary bonding units as their high symmetry facilitates packing, and in amorphous reticular chemistry, tetrahedral connectors have been used to construct hyper-crosslinked polymers or porous organic polymers. The two most-studied families of tetrahedral, tetravalent linkers are the tetraarylmethanes (A) and adamantanes (B) -both featuring a carbon skeleton. Our interest in the reactivity of geometrically constrained p-block amides led us to the family of phosphorus-nitrogen cages (C) reported by Holmes nearly 6 decades ago. Their quantitative, one-step, multi-gram synthesis from commodity reagents (PCl3, RNH2) is appealing from a practical perspective, and their high molecular symmetry makes them inherently suited for evolving a four-directional functional platform. Quadruple oxidation of some derivatives of C with azides, sulfur, and oxygen has also been reported, but no subsequent reactivity was possible since the resulting compounds do not feature sufficiently labile bonds. Salts of the binary polyanion P4N6 10-have also been reported, but their high temperature solid-state synthesis (>600 o C elemental melt) and insolubility have precluded further use in synthetic chemistry. We envisioned that conversion of C to a masked form of tetra-anion D would allow solution-phase tetravalent chemistry with a new inorganic synthon. Specifically, if the P III atoms in compound 1 could be oxidized to P V silylphosphinimines, the exo-cage N-Si bonds may be polar enough to engage in subsequent covalent metathesis with element halides. Here we validate this hypothesis and report the synthesis, structure, and reactivity of 2 as a new, remarkably robust, electron-rich, tetrahedral scaffold for molecular and macromolecular chemistry. Scheme 1. Synthesis of 1 and its sequential oxidation by Me3SiN3 to give 2.