The dissertation explores the synthesis and characterization of new transition metal complexes using 5-hydroxypyrazoline ligands. These ligands were successfully synthesized and reacted with various metal precursors (Ni, Fe, Zn, Mo, and Cu) to form complexes with distinct coordination modes. Characterization techniques revealed that the Ni complex exhibited O, N, O´-coordination, with the ligand in a planar configuration. The oxygen donors were trans to each other, while the nitrogen donor was positioned cis. The nickel center's remaining coordination sites were filled by base molecules (NH3, DMAP, or phosphanes), influencing the complex's geometry—resulting in either a diamagnetic square planar or an octahedral paramagnetic complex. Additionally, 5-hydroxypyrazoline ligands reacted with dimethyl zinc to create zinc complexes with a RO-Zn-Me motif, and the introduction of N,N,N’,N’-tetramethylethylenediamine led to the formation of a novel zinc-based seven-membered system. The coordination chemistry of Fe, Cu, and Mo complexes was also examined. The second part of the dissertation focused on catalytic experiments using Ni complexes as pre-catalysts in C(sp2)–C(sp3) cross-coupling reactions, achieving excellent yields and selectivities. DFT calculations and various experiments elucidated the reaction mechanism. The study also highlighted the tunable catalytic properties of Ni complexes through the addition of primary amines, le
