Transition metal compounds are formed from transition metals, which are elements found in the d-block of the periodic table. These metals, such as iron, copper, and nickel, can form various compounds by bonding with other elements. They often exhibit unique properties, including variable oxidation states and the ability to form colorful complexes. These compounds play essential roles in various applications, including catalysis, pigments, and biological systems. For example, hemoglobin, a protein in red blood cells, contains iron and is crucial for transporting oxygen in the body. Transition metal compounds are also widely used in industrial processes and materials science.