complexation

Complexation is a chemical process in which a central metal atom or ion binds to one or more ligands to form a coordination complex. Ligands donate electron pairs to the metal via donor atoms such as nitrogen, oxygen, or sulfur, and the resulting complex can be neutral or charged. The properties of the complex depend on the metal, the ligands, and the overall geometry.

Ligands vary in denticity, being monodentate (one donor atom) or polydentate (multiple donor sites). Polydentate ligands that form rings with the metal are called chelating ligands, and such complexes are often more stable due to the chelate effect. The coordination number and geometry of a complex depend on the metal and ligands; common geometries include octahedral, tetrahedral, and square planar.

The stability of a complex is described by formation (stability) constants, typically denoted Kf. Larger Kf values indicate more stable complexes. Complexation can proceed quickly or slowly, depending on kinetics, and the ligands influence both thermodynamics and reaction rates.

Applications of complexation are widespread. In analytical chemistry, complexometric titrations using ligands like EDTA determine metal concentrations. In biochemistry and inorganic chemistry, metal complexes are central to catalysis, sensing, and the function of metalloenzymes and metalloproteins. Examples include [Cu(NH3)4]2+ and [Fe(CN)6]3-. In medicine and environmental science, chelating agents bind toxic metals for therapy or remediation, and coordination chemistry underpins many imaging and treatment techniques.