This lecture covers the concept of chelating ligands, the chelate effect, and the stability of metal complexes. It discusses the role of ligands like ethylenediamine and EDTA in metal speciation, as well as the formation of chelate complexes. The lecture also explores the hydrolysis of metal ions, the distribution of iron species at different pH levels, and the complexation in natural waters. Additionally, it touches on the importance of humic substances in metal complexation and the factors influencing the distribution of free and complexed metals in natural waters.