Hoofdligand

Hoofdligand is a term used in coordination chemistry to describe the primary ligand bound to a central metal atom or ion. It is the ligand that largely defines the coordination environment, geometry, and electronic structure of the complex. The hoofdligand is typically contrasted with bijliganden, secondary ligands, or solvent/ counter-ions that complete the coordination sphere but often bind more weakly or are more labile.

The hoofdligand can be monodentate or multidentate and often exerts a strong influence on the properties of the complex, such as geometry (for example, octahedral or square planar) and electronic characteristics. Its denticity, bite angle, steric profile, and electronic donation are key factors in determining reactivity, stability, and catalytic behavior. In multi-ligand complexes, the hoofdligand is generally considered the dominant binding partner, with bijliganden filling remaining coordination sites.

Common examples of hoofdliganden include multidentate ligands such as 2,2'-bipyridine, 1,10-phenanthroline, terpyridine, and porphyrin-like macrocycles, as well as Schiff base ligands. These are frequently paired with smaller or more labile bijliganden such as chloride, water, nitrate, or other neutral or anionic ligands to complete the metal’s coordination sphere. In dinuclear or bridged complexes, bridging ligands may also function as hoofdliganden for one of the metal centers.

In Dutch-language literature, hoofdligand is used to distinguish the primary binding partner from secondary ligands; in English, it is often rendered as “main” or “primary” ligand. The concept is descriptive and context-dependent, rather than a formal IUPAC designation.