The central metal atom (or ion) refers to a metal atom that is the focal point of a coordination complex or a coordination compound. In coordination chemistry, a coordination complex is a molecular entity consisting of a central metal atom or ion surrounded by a set of ligands. These ligands are typically molecules or ions that can donate electron pairs to the central metal atom, forming coordination bonds.

The coordination complex can be thought of as a “core” formed by the central metal atom and its surrounding ligands. The central metal atom contributes to the overall properties and reactivity of the complex. Different central metal atoms can lead to coordination complexes with distinct characteristics, including color, stability, and chemical reactivity.

Some key points about the central metal atom:

1. Role: The central metal atom is the site of coordination in a coordination complex. It forms bonds with ligands, creating the overall structure of the complex.
2. Valence: The valence of the central metal atom determines the number of coordination bonds it can form. Transition metals are often used due to their variable oxidation states and availability of d orbitals for bonding.
3. Coordination Number: The coordination number of the central metal atom is the number of bonds it forms with ligands. It depends on both the valence and size of the metal atom.
4. Geometry: The arrangement of ligands around the central metal atom determines the complex’s geometry. Different coordination numbers result in different geometries (e.g., octahedral, tetrahedral, square planar).
5. Physical Properties: The central metal atom contributes to properties such as color, magnetic behavior, and stability of the coordination complex.
6. Chemical Reactivity: The reactivity of the central metal atom can influence the overall chemical behavior of the coordination complex.
7. Catalysis: Some coordination complexes with specific central metal atoms act as catalysts in various chemical reactions.
8. Examples: Common central metal atoms include transition metals like iron (Fe), copper (Cu), and platinum (Pt). Non-transition metals like magnesium (Mg) and zinc (Zn) can also act as central atoms in coordination complexes.

Coordination complexes with different central metal atoms and ligands exhibit a wide range of properties and applications. They are important in fields such as catalysis, materials science, bioinorganic chemistry, and more.