Three-dimensional covalent connectors are valuable synthons for accessing crystalline or amorphous networks. Currently, fused polycyclic alkanes are employed as connectors in this context. We debut phosphorus-nitrogen (PN) cages as new 3-dimensional (3-D) inorganic connectors that yield crystalline and amorphous networks, including examples with gas porosity. We show that the high tunability of PN cages accelerates network diversification and the presence of a responsive 31P NMR spectroscopic handle provides structural insight. Collectively, this work unlocks a new and convenient 3-D synthon for reticular chemistry.