A pincer type ligand provides an additional coordination site or a proton relay depending on the metal oxidation state.

Coordination complex systems containing phosphine ligands are used in artificial photosynthesis utilizing their unique stereoelectronic properties. Mono-, di- and tetraphosphines act as optimized ligand systems for complexation.

Pd@(BiPy‐PEG‐OMe) is a catalyst comprised of palladium nanoparticles (Pd‐NPs) stabilized and made water soluble by 2,2′‐bipyridine‐end‐functionalized polyethylene glycol monomethyl ether (BiPy‐PEG‐OMe). The catalyst has been used in the past for nitrile hydrogenation. In this work, we prove that it is also very active for photocatalytic hydrogen generation, which might be true for more catalysts of its kind and therefore worthy of further investigation. Using the inexpensive photosensitizer Eosin Y, high turnover numbers (TONs) of over 4 500 were achieved for the evolution of molecular hydrogen from pure water under visible‐light irradiation. Replacing Eosin Y, which showed only a short lifetime under experimental conditions (i.e. a few hours), by a novel osmium‐based metal complex, which is also characterized by its crystal structure, the longevity of the system can be boosted to over one and a half months with a maximum TON of 1 500. Combining excellent yield and stability is a clear goal for further research.

Artificial photosynthesis with respect to water splitting is usually divided into water oxidation catalysis (WOC) and the hydrogen evolution reaction (HER). Though in the combined redox dissociation of water into oxygen and hydrogen no protons and electrons occur, both half reactions show photoinduced proton coupled electron transfer (PCET). Regarding a classical approach, photosensitizers (PS) deliver electrons and protons that are accepted by water reduction catalysts (WRC) containing suitable basic atoms like nitrogen working as proton relays. However, the mechanisms of PCET reactions differ, where concerted proton electron transfer (CPET) is an elementary step. In CPET simultaneous electron and proton transfer occurs in the femtoseconds range, being rapid when compared to the periods for coupled vibrations and solvent modes. This has to be distinguished from stepwise electron and proton transfer, leading to underlying thermodynamics of the intermediates. DFT calculations based on X‐ray diffraction (XRD) data help to specify the different reaction pathways. A plethora of experimental procedures are used in order to verify the theoretical predictions. Among them femtosecond pump‐probe spectroscopic measurements play an important role. Furthermore, cyclic voltammetry (CV) has proven to be also a powerful tool. In the case of electrochemical PCET rotating‐disk electrode voltammetry, electrochemical impedance spectroscopy and spectroelectrochemistry complete the experimental tools. In this minireview a selection of examples, where PCET occurs is discussed with respect to possible mechanisms and used methods.