The use of fossil fuels is resulting in a significant rise in global temperature. To mitigate further detrimental effects to the environment and the climate, a transition from fossil fuels to renewable energy resources is urgently required. To enable this transition, efficient storage of renewable energy is required. Hydrogen and carbon-based fuels using CO2 as feedstock are interesting options, if they can be produced photocatalytically using sunlight as the energy source. Organometallic complexes are promising photocatalysts, with desirable properties such as strong visible light absorption, good photostability and good catalytic activity and selectivity. For these systems to succeed, a better understanding of the working of these systems is required. This research focussed on developing and studying new organometallic complexes using either ruthenium or rhenium metal centres and their application in photocatalytic systems for hydrogen evolution and CO2 reduction. New catalysts were synthesised and analysed using NMR-, UV-Vis-, emission and IR spectroscopy. Electrical properties of these complexes such as their reduction potentials were determined using cyclic voltammetry. The complexes were probed using various ultrafast spectroscopic techniques including transient absorption (TA) and time-resolved infrared spectroscopy (TRIR). These techniques can provide a better understanding of the interaction of these catalysts with light, which is the important first step in the photocatalytic cycle. In addition, the catalysts were studied with a variety of spectroelectrochemical techniques. In this approach, reduced intermediate species were electrochemically produced and studied using steady state spectroscopic. The study of these reduced intermediates is underappreciated in photocatalytic research, even though these species play an important role in the photocatalytic cycle. This thesis provides insights in how reduced intermediate species may influence the photocatalytic process. In addition, this thesis shows that immobilisation of these systems onto a semiconductor surface greatly influences the excited state properties and activity of these photocatalytic assemblies.