Coordination compounds, also known as complex compounds or coordination complexes, are molecules or ions consisting of a central metal atom or ion surrounded by a set of coordinated ligands. These ligands are molecules or ions that form coordinate bonds with the central metal through the donation of electron pairs. The resulting coordination complex has a distinct structure, properties, and often, vibrant colors.

Here are some key features and characteristics of coordination compounds:

1. Central Metal Atom/Ion: This is typically a transition metal or sometimes a metal from the main group of the periodic table. The metal has one or more available d orbitals that can be involved in bonding.
2. Ligands: Ligands are molecules or ions that bond to the central metal. They can be neutral molecules or anions with available lone pairs of electrons. Ligands are classified based on their binding sites: monodentate (binding through a single atom), bidentate (binding through two atoms), tridentate (binding through three atoms), etc.
3. Coordination Number: The coordination number is the number of ligands directly bonded to the central metal. It determines the geometry of the complex. Common coordination numbers include 4 (tetrahedral or square planar) and 6 (octahedral).
4. Chelation: Chelation occurs when a ligand forms multiple bonds with a metal center to create a more stable ring-like structure known as a chelate.
5. Isomerism: Coordination compounds can exhibit various types of isomerism, including geometric isomerism (cis-trans isomerism) and structural isomerism (different arrangements of ligands).
6. Color: Many coordination compounds are colored due to the absorption of specific wavelengths of light. The color is a result of electronic transitions within the complex.
7. Magnetic Properties: Coordination compounds can exhibit magnetic properties, such as paramagnetism and diamagnetism, depending on the arrangement of electrons in the metal’s d orbitals.
8. Crystal Field Theory and Ligand Field Theory: These theories explain the electronic structure and properties of coordination complexes by considering the interaction between the metal’s d orbitals and the ligands.
9. Applications: Coordination compounds have diverse applications, including catalysis, medicine (such as cisplatin, a cancer drug), materials science (designing new materials), and more.
10. Naming: Coordination compounds are named systematically, indicating the central metal, the ligands, and their arrangement. The ligands are named before the metal, and prefixes like “di-” and “tri-” indicate the number of ligands.

Coordination compounds are of great importance in fields ranging from chemistry and biology to medicine and materials science. They offer insights into the behavior of metal ions in different environments and have practical applications in various industries.