Metal-ligand interactions arise when a ligand donates a lone electron pair to empty orbitals of a metal ion, forming a coordinate bond that is dynamic as opposed to a covalent bonds (Figure ). Typical ligands are small organic molecules that contain at least one free pair of electrons and therefore act as Lewis bases. Such coordinate bonds can be formed by a variety of metal ions and neutral or anionic ligands covering a broad range of binding constants (K a = 10 3 -10 40 ) characteristic of strong covalent down to weaker non-covalent interactions. The complex stability generally increases with higher oxidation state and smaller size of central metal cation, for instance, following the Irvin-Williams series: Mn 2+ < Fe 2+ < Co 2+ < Ni 2+ < Cu 2+ > Zn 2+ , where copper ions form complexes with the highest equilibrium constants. On the ligand side, complex stability increases with (i) the ligand's basicity facilitating the donation of lone electron pairs and (ii) its denticity describing the number of ligand binding sites involved in the metal coordination (chelation). For example, a monodentate imidazole ligand forms metalligand complexes with substantially lower stability constants than its bidentate equivalent, histidine. Tough polymer networks based on metal-ligand coordination are typically formed by ligand-containing polymers that undergo chelation once a suitable metal cation is introduced. Beside the use of natural metal-chelating polysaccharides, such as alginate or carrageenan, prominent ligands include carboxylates, imidazole, among others. Consequently, metal-ligand interactions have successfully produced a variety of tough hydrogels with high capacity for energy dissipation.