Zirconium and hafnium are particularly favourable for the design of new MOF structures: in contrast to copper, for example, (which often forms dinuclear "paddlewheel" units ), there is a rich variety of known zirconium and hafnium clusters, with a wide range of nuclearities, geometries and coordination denticities. There are in fact over 1300 structures containing between 3 and 21 Zr or Hf ions in their molecular formula -forming either molecular clusters or being part of an extended framework (including MOFs) -in the CCDC (Cambridge Crystallographic Database) alone. From these structures, we can identify a wide variety of Zr and Hf metal-oxide inorganic substructures (derived by removing peripheral ligands), or "core" clusters. The metal ions have high charge density, arising from their +4 oxidation state, and so favour hard donor atoms such as oxygen, to which they coordinate strongly. Therefore, hydroxyl bridges and oxo-and hydroxocapped clusters are predominant in the Zr and Hf "core" clusters, and clusters are often found with ligands coordinating through oxygen, such as carboxylate groups. Although the most common cluster motif in Zr and Hf MOFs, e.g. in the UiO family [Error! Reference source not found.], is the M6(μ 3 -O)4(μ 3 -OH)4 cluster based on Zr6O8 octahedra, the "core" clusters range from small, low-nuclearity molecular clusters through to large structures such as infinite polymeric chains. Just as in other extended metal oxide structures, these metal (hydr)oxide clusters can share vertices, 8 edges, faces and coordination-polyhedra. As these structures increase in nuclearity, they increasingly resemble the structure of high temperature, tetragonal ZrO2. The standard method for synthesising Zr/Hf MOFs is solvothermal synthesis; the choice of solvent affects the MOF metal clusters, the degree of Zr or Hf cluster aggregation being greatly * Cambridge Crystallographic Database, search performed July 2020 affected in particular by the presence of water in the synthesis. In aqueous, and especially aqueous acidic conditions, the distorted-square tetrameric species [M4(OH)8•16H2O] 8+ is dominant. This cluster is surrounded by a structured coordination sphere of water molecules similar to other nanoparticles in solution. This water coordination sphere is potentially the source of further hydroxide bridges between metal centres during condensation of the clusters. The degree of hydrolysis, and subsequent aggregation, can be modulated by the use of additional reagents, such as acid, which in some MOF syntheses is required to form the crystalline framework, and in others is used to control particle morphology. Monocarboxylic acids are often chosen for this purpose; since they can possess a wide range of metal-ligand binding strengths, a judicious choice of carboxylic acid can "block" coordination sites on the cluster to further nucleophilic attack preventing, in the case of water/hydroxides, cluster aggregation, or for linkers, the formation of multi-cluster framework species. However, while it is now clear that water and modulating ligands are critical to the identity and stability of the resultant metal clusters (from ligand-terminated molecular clusters through to MOFs ), the exact interplay of these factors, especially in multi-species reaction mixtures, is not well understood.