In Section 2.1 we discuss the traditional methods of designing novel TMCs. These methods leverage the existing TMCs as templates and then deconstructs the complexes to extract available ligands. By combining different ligands together, new structures of TMCs are then designed. One obvious limitation of this complex-level design is that it highly relies on already synthesized TMCs, and it is simply a selection of all possible recombination of available ligands. As a result, ligands are not designed from scratch, which limits the discovery of novel TMCs because the chemical space of known TMCs is smaller than that of organic molecules and accordingly the variety of ligands is also limited. Section 2.2 discusses how ML can overcome this drawback and open new pathways for the de novo design of TMCs. The schematic timeline of representative open-source tools for TMCs design is shown in Figure .