This Ph.D. thesis explores experimental and theoretical studies on reactive intermediates like carbenes, nitrenes, and cyclometalated complexes. It highlights the application of carbene chemistry in C-H functionalization, O-H insertions, and cyclopropanation reactions, with detailed investigations from both experimental and theoretical perspectives. The research extends to diaryl diazoalkanes in proton transfer reactions and examines photochemical and photocatalytic nitrene transfer reactions using DTF calculations. A significant focus is on developing efficient directed C-H functionalization of 2-phenyl pyridine with TMS-diazomethane or phenyl diazoacetate. Theoretical analyses reveal insights into the reactivities of metals and diazoalkanes. The study also calculates the photocatalytic reaction of indole with ethyl diazoacetate in protic and aprotic solvents, uncovering the formation of radical intermediates in protic solvents and triplet carbenes in aprotic ones, leading to distinct C-H functionalization products. Additionally, diazoalkanes are investigated in Lewis acid-catalyzed O-H insertion reactions using tropylium tetrafluoroborate. The thesis further delves into light-mediated cyclopropanation reactions of (poly)unsaturated carbocycles and NHC boranes, as well as the photochemical and photocatalytic reactions of iminoiodinanes with allenes. Finally, it details the photochemical mechanism of C(sp2)-H functionalization
