CO releasing manganese complexes reaction pathways and synthesis of multinuclear Mn compounds Abstract: Three different aspects of manganese complex chemistry are investigated in this work. The main part of this work is the investigation of the reaction pathways and products of manganese photoCORMs (CO releasing molecules for which the CO release can be triggered by UV or Vis irradiation). Secondly investigations of arginase model systems including the development of a HPLC method to determine and quantify arginase activity were carried out. Furthermore attempts to synthesize and characterize multinuclear manganese complexes with large ring shaped Schiff base ligands which might be used as model systems for the oxygen evolving complex (OEC) in photosystem II are made. The characterization of the multinuclear manganese complexes was problematic and therefore the focus of this work was the investigation of manganese photoCORMs and the possibility of their use as an alternative route to multinuclear manganese complexes. Very detailed information on the CO release pathways of [MnI(CO)3(κ3 tpm)]+ and [MnI(CO)3(κ3 bpza)] could be obtained by IR and UV Vis monitoring of the CO release. For the first time the remaining products of the CO release could be characterized as [MnIItpm2]2+ or [Mn2IIbpza4] and manganese oxides respectively. By showing in IR LSV spectroelectrochemical experiments that the CO release can also be triggered by oxidation an additional route of the CO release was investigated. The gathered information might be useful for further investigations of photoCORMs as drugs and also led to the right choice of tpa as ligand to obtain dinuclear manganese complexes ([Mn2III IV(μ O)2(κ4 tpa)2]3+ and [Mn2II II(μ OAc)2(κ4 tpa)2]2+) in a very simple way by the light or oxidation triggered CO release of [MnI(CO)3(κ3 tpa)]+. For [Mn2II II(μ OAc)2(κ4 tpa)2]2+ arginase activity could be proven by a newly developed HPLC method using a pre column derivatisation with OPA (ortho phthalaldehyde) to determine small amounts of ornithine in arginine rich solutions with a C18 column. Mass spectrometric investigations revealed the replacement of bridging acetates by hydroxides and therefore the possibility of this catalysis to be the same as the catalytic cycle of arginase. Overall this work shows that well defined multinuclear manganese complexes which can be used as model systems for metalloenzymes can be obtained by the use of manganese photoCORMs as precursors