Covalent immobilization tends to be more complicated than the non-covalent counterpart but usually provides a robust connection through covalent bonds between the surface and the molecular catalyst. Both chemical and electrochemical reactions, e.g., alkyne-azide cycloaddition, could be utilized to create covalent bonds between an electrode and an organic moiety that either serves as a ligand or originates from a pre-established organometallic complex (Figure ). It is also possible to functionalize the surface through other bonds such as forming ester or amide bonds with a carboxylic acid. Pretreating the electrode surface to increase the number of functional groups provides possible binding sites for metal complexes through surface conjugation.