The topology of the thermodynamic product is determined by a balance of entropic and enthalpic factors, with the components tending to fulfil Lehn's concept of maximal site occupancy, i.e. to form the greatest number of coordination bonds according to the denticity of the ligand and coordination number of the metal ion/node. For example, square-planar Pd(II) ions in combination with ditopic, or bis-monodentate, ligands will preferentially form PdnL2n-type assemblies. As demonstrated by extensive ligand engineering studies from the Fujita group, the value of n, in the absence of other factors, is largely controlled by the ligand structure, with the smallest possible assembly forming based on accessible conformations of the ligand (Fig. ). Although structural factors are some of the most important considerations in designing MOCs, guestinduced/-templated transformations, including by counteranions, can lead to the formation of unexpected species or structural rearrangements. 16 Whilst MOCs are most often formed as the thermodynamic product of self-assembly, there are notable examples of kinetically metastable species. The Mirkin group has pioneered the concept of the weak-link approach (WLA), in which a thermodynamic architecture is used to pre-organise components; subsequent modification of the structure leads to formation of a kinetically trapped assembly that cannot be formed through direct reaction of the building blocks. The range of building blocks, and therefore accessible structures, that can be used to assemble MOCs is essentially infinite. Transition metal ions with coordination numbers typically between 3 and 6, as well as lanthanide ions able to coordinate to up to 13 donors, and a range of coordination geometries have been used. Additionally, capping ligands can be used to form kinetically stable metal nodes with limited coordination numbers and geometries; the most extensively studied of these is the cis-protected Pd(II) ion, with two coordination sites blocked via a chelating bidentate ligand such as ethylenediamine (en). Through selection of ligand denticity and metal ion/node coordination number, topologically related assemblies can be obtained from different building blocks (Fig. ). 19 With regards to the ligand structure, donor units can be neutral or anionic, with a range of denticities, and arranged in any number of geometries from acyclic or cavitand-based ligand scaffolds. Through the balance of charges between the metal ions and ligands, MOCs can be prepared that are cationic, neutral, or anionic (Fig. ). Myriad studies have also been reported on the appendage of functional groups to both the external 21 and internal 22 surfaces of cages, endowing them with particular properties and functions.