Fig. 2. Plotted are the effective radii of protonated amines r _A,eff. vs. ionic radii of the divalent metals r _B of hypothetical compositions. The figure shows (a) iodide based and (b) formate based permutations with TFs between 0.8 and 1.0. The plots highlight the influence of the linking anion on the size-ranges of protonated amines and divalent metals where a hybrid perovskite structure is expected to form. The anisotropy of the anti–anti bridged formate and the associated smaller effective radius (r _X,eff.) leads to a larger ReO₃-like cavity, so that larger protonated amines can fit within the cavity for a fixed divalent metal. In the case of formates with a 4¹²·6³ topology and syn–anti connectivity, the size of the ReO₃ cavity is reduced and even smaller protonated amines lead to a perovskite-like architecture, as recently shown for [NH₄]Cd(HCOOH)₃.²¹ .