Definition of a Coordination Compound
A coordination compound is a substance composed of a central metal atom or ion that is chemically bonded to a surrounding group of molecules or ions, known as ligands. These bonds are typically coordinate covalent bonds, where the ligands donate a pair of electrons to the central metal.
Key Components: Central Metal and Ligands
The central metal atom, often a transition metal, acts as a Lewis acid by accepting electron pairs. Ligands, which act as Lewis bases, can be neutral molecules (like water or ammonia) or ions (like chloride or cyanide) that possess at least one lone pair of electrons available for donation. The number of ligand atoms directly attached to the central metal is called the coordination number.
A Practical Example: Tetraamminecopper(II) ion
A common example is the tetraamminecopper(II) ion, [Cu(NH₃)₄]²⁺. Here, the central metal ion is copper(II) (Cu²⁺), and the ligands are four ammonia (NH₃) molecules. Each ammonia molecule donates a lone pair of electrons to the copper ion, forming a coordinate covalent bond. This complex is known for its distinctive deep blue color.
Importance and Applications in Science
Coordination compounds are crucial in many fields, playing vital roles in biological systems, such as hemoglobin (iron complex) in blood for oxygen transport and chlorophyll (magnesium complex) in photosynthesis. Industrially, they are used as catalysts in chemical reactions, pigments in paints, and in various medicinal applications, including chemotherapy drugs and MRI contrast agents.