Ligandenaustauschprozesse

Ligandenaustausch, or ligand exchange in English, is the process in which one or more ligands bound to a central metal ion in a coordination complex are replaced by other ligands. It is a fundamental reaction in coordination chemistry, enabling synthesis, catalysis, and biological processes without changing the oxidation state of the metal core.

Two broad kinetic pathways describe substitution: dissociative mechanisms, where a ligand departs to create a coordinatively unsaturated intermediate before the entering ligand binds; and associative mechanisms, where the entering ligand assists displacement by temporarily forming a higher-coordinate intermediate. An interchange mechanism can combine features of both. The observed rate depends on the metal’s identity and oxidation state, ligand denticity and charge, solvent, temperature, and the presence of competing ligands.

Factors influencing ligand exchange include the lability of the metal center, with weaker-field ligands and higher coordination numbers generally exchanging more readily, while inert complexes (for example, many Cr(III) and Co(III) octahedral species) show slow exchange. Solvent effects, pH, and chelation also play important roles. Complex geometry and trans effects further modulate rates and pathways.

Examples and applications span aqua complexes such as [M(H2O)6]n+ undergoing substitution with incoming ligands (e.g., NH3, en), and the activation of metal-based drugs such as cisplatin, which involves aquation and subsequent DNA binding. Ligand exchange is central to homogeneous catalysis, solvation chemistry, and the interpretation of reaction mechanisms in inorganic and bioinorganic systems.

Methods to study ligand exchange include kinetic measurements by UV-Vis spectroscopy, NMR exchange experiments (EXSY), and stopped-flow techniques, often complemented by activation-parameter analyses to distinguish mechanisms.