This Ph.D. thesis focusses on experimental and theoretical studies on reactive intermediates such as carbenes, nitrenes, and cyclometalated complexes. We demonstrated the application of carbene chemistry in C-H functionalization reactions, O-H insertions and cyclopropanation reactions and investigated reactions in detail from an experimental or theoretical perspective. Furthermore, we expanded the application of diaryl diazoalkanes towards proton transfer reactions. Next, we investigated photochemical and photocatalytic nitrene transfer reactions from a theoretical point of view using DTF calculations. In a last step, we applied our knowledge on DTF calculations towards the identification of photocatalytioc C-H functionalization reaction. In a first part, reactive carbene intermediates are at center stage. We were able to develop an efficient directed C-H functionalization of 2-phenyl pyridine with TMS-diazomethane or phenyl diazoacetate. Detailed theoretical investigations on the reaction mechanism uncovered insights into different reactivities of metals and diazoalkanes. In a next project, we calculated the photocatalytic reatcion of indole with ethyl diazoacete in protiv and aprotiv solvent., We were able to uncover the formation of a radical intermediates in protic solvent, while a triplet carbene is formed in aprotic solvent. In the course of the reaction the radical leads to a C2-H functionalization, while the triplet carbene formes the C3-H functionalization product in an unexpected 1,2-alkyl radical migration. Furthermore, we investigated diazoalkanes in Lewis acid catalyzed O-H insertion reactions using tropylium tetrafluoroborate as catalyst. [...] In a last step, we investigated carbene intermediates in light mediated cyclopropanation reactions of (poly)unsaturated carbocycles and NHC boranes. [...] In further studies on O-H insertion reactions, we investigated diaryl diazoalkanes as unbiased systems in proton transfer reactions. [...] Moreover, we investigated the photochemical and photocatalytic reactions of iminoiodinanes with allenes. [...] In a last past, we studies the photochemical reaction mechanism of the C(sp2)-H functionalization of 2-phenyl pyridine with maleimides in detail using (TD)-DFT calculations.weiterlesen