“…One idea, pioneered mainly by Shimizu and co-workers, , is to use layers that can coordinate to transition metals as shown in Figure b. Thus, the guanidinium cation with its three amino groups can be “substituted” by various transition-metal coordinated ligands with protons available for hydrogen bonding such as ammonia, − amines, − or water molecules. ,, The second coordination sphere of an octahedral hexaaqua or hexaammine metal complex, for example, can mimic two staggered guanidinium cations from neighboring layers when the complex is positioned with its C 3 axis perpendicular to the layers (Figure a, b). This arrangement, however, has been observed only in two compounds so far. , The preferred orientation of the metal complex seems to be rather with its C 4 axis normal to the layers and with hydrogen-bonded sulfonate groups that formally cap the faces (Figure c).…”