Early on in the exploration of adsorption-triggered flexibility of MOFs, researchers in the field realized that small changes in their microscopic structure and intermolecular interactions could have important consequences on their flexibility. In the MIL-53 family of wine-rack type materials, for example, it became clear that the nature of the metal drastically influences the flexibility, with MIL-53(Fe) behaving very differently from its aluminium and chromium counterparts, and MIL-53(Ga) exhibiting intermediate phases not observed in MIL-53(Al), despite the two metals being close in nature. This observation was not limited to the metal ions, but was also true of the influence of linker nature: with NH 2 -substituted terephthalic acid linkers in amino-MIL-53 impacting its flexibility and favoring a narrow-pore phase, linker substitution was soon recognized as a valid strategy for tuning the flexibility of the material by controlling the amount of amino "tags" introduced -and therefore, the shape of adsorption isotherms. This line of research has borne very exciting results in the recent literature on the control and tuning of MOF flexibility, especially in order to meet specific performance targets for applications such as gas uptake, binary adsorption separation, etc. In a recent example of the control of MOF flexibility through choice of metal center, Chen et al. studied the structural dynamic response to different gases of a family of isonicotinatebased MOFs, M(4-PyC) 2 (M = Mg, Mn, or Cu; 4-PyC = 4pyridine carboxylic acid); these systems are shown in Figure . Through a comparison of adsorption-induced transitions with various guest molecules, the authors were able to establish that the structural flexibility of the MOFs is highly sensitive to the chemical hardness of the Lewis acidic metal ions (Mg 2+ > Mn 2+ > Cu 2+ ), with gate opening behavior observed at lower pressures for MOFs containing weaker M-N bonds (due to harder metal ions). On a very different family of materials, but following a similar approach, Garai et al. were able to tune the flexibility of DUT-49(Cu) family through postsynthetic metal exchange of Cu 2+ with a wide variety of metal cations: Mn 2+ , Fe 2+ , Ni 2+ , Zn 2+ , and Cd 2+ .