Two series of complexes MX(diimine)(CO)(3)] (M = Tc, Re) have been prepared, fully characterised and investigated for their ability to act as photocatalysts for the reduction of CO2 to CO. One series consists of complexes with different aromatic diimine ligands while keeping X = Br- constant. The second series describes complexes with diimine = 2,2-bipyridine and variations in the anionic ligand X-. Although numerous complexes of this type have been prepared and investigated before, a systematic study of their photocatalytic activity has not yet been carried out. Electrochemical and spectroscopic characterisation of these complexes has been performed with the objective of better understanding their respective activity in the photocatalytic CO2 reduction. Despite various modifications, catalytic activity is retained for all compounds exhibiting fluorescence, including (TcCl)-Tc-99(bipy)(CO)(3)], whereas nonfluorescing compounds did not convert CO2 to CO. The correlation of catalytic activity and spectroscopic or electrochemical properties such as absorption or emission wavelengths, redox potentials or Stern-Volmer constants for the reductive quenching of the excited complexes is difficult. Nevertheless, the study emphasises the possibility to obtain ReX(CO)(3)(diimine)] complexes with a wide range of physicochemical properties by ligand variations and the great potential of compounds of this class of complexes as inorganic photosensitisers.