Transition metal complexes are chemical structures formed when transition metals bond with molecules or ions, known as ligands. These complexes can vary in shape, size, and color, depending on the metal and the ligands involved. Transition metals, found in the d-block of the periodic table, have unique properties that allow them to form multiple bonds and oxidation states. The study of transition metal complexes is important in various fields, including catalysis, biochemistry, and materials science. They play crucial roles in processes like photosynthesis and are used in applications such as dyes, pigments, and catalysts for chemical reactions.