A strategy based on the use of Lewis acids (LAs) is another potential option for reversibly modulating the electronic/spatial environment around metal centers that bear multifunctional ligands (Figure , right). The LA-mediated process can cause a substantial change in both the electronic state and spatial environment of the metal center due to the change in the number of ligands. This feature clearly distinguishes LA-mediated processes from LB-mediated ones, as the latter commonly proceeds without changing the number of ligands. However, examples of such LA-mediated reactions have been severely limited, because LAs often trigger the irreversible decomposition of the organometallic complex via the abstraction of ligands to form an LA-LB adduct. A judicious combination of multifunctional ligands and LAs is thus essential for constructing an LA-mediated system that can reversibly regulate the electronic/spatial environment around the metal centers. It should be noted that Fan et al. have reported an example that relies predominantly on an electrostatic interactions involving the reversible complexation of Na + and a crown-ether moiety included in a Rh complex that bears Aza-CrownPhos; the reactivity of the Rh complex can be regulated through the Na + -mediated interconversion. The authors evaluated the change in the spatial environment around the Rh center using multinuclear NMR spectroscopy and ESI mass spectrometry, even though a quantitative evaluation of how much the field expanded/diminished via the reaction was not discussed.