However, depending on the absorbing atom, ligand types, and property of interest, previously known heuristics may not provide sufficient information, and close study of a small group of compounds is required to discover and understand new trends. Farges pointed out that conventional heuristics fail for MnO and MnCO 3 , which share the same oxidation state and coordination number, yet exhibit radically different Mn K-edge shapes and a large 4.5 eV shift between their peaks . Known coordination number heuristics are also not as straightforward to apply to heavier transition metals like Fe . Furthermore, oxidation number heuristics differ based on the type(s) of ligand anions due to differences in charge transfer processes.