The single-metal-ion-based molecular building blocks (MBBs) approach for the construction of metal–organic assemblies, in which hetero-coordinated single metal ions are rendered rigid and directional via nitrogen–oxygen chelation with judiciously selected ligands, has been implemented. Single-metal-ion-based MBBs of the general formula MNx(CO2)y constitute the building units of metal–organic frameworks (MOFs) and metal–organic polyhedra (MOPs) presented herein. The octahedral MBB, MN2(CO2)4, can occur as two structural isomers depending on the positioning of nitrogen atoms. The MN2(CO2)4 MBBs contain two rings of heterochelation, and depending on the position of the oxygen atoms involved in heterochelation it is possible to generate three different building units (BUs) from the cis-MN2(CO2)4 MBB and two BUs from the trans-MN2(CO2)4 MBB. Assembly of the different BUs derived from the cis-MN2(CO2)4 MBB, through a bifunctional ligand such as 2,5-pyridinedicarboxylic acid, permits the construction of diverse assemblies, such as a metal–organic 2D Kagomé lattice, a discrete octahedron, and a 3D diamondoid-like network. The fac-MN3(CO2)3 MBB mediates a BU with the appropriate geometry to facilitate the formation of a metal–organic cube, and the BU resulting from the mer-MN3(CO2)3 MBB is T-shaped. Tetrahedral building units (TBUs) can be derived either from MN4(CO2)2 or MN4(CO2)4 MBBs, from which zeolite-like MOFs have been constructed. Foremost, rationalization and systemization of such findings offer great potential toward the pursuit of the logical synthesis of functional metal–organic assemblies.