Coliganden

Coliganden, or coligands, is a term used in coordination chemistry to describe ligands that bind to a metal center in conjunction with other ligands, forming the complete coordination sphere of a complex. The concept emphasizes the cooperative role of secondary ligands in defining the properties of a complex, rather than focusing solely on a single primary ligand. The usage of the term is not fully standardized; in some contexts coligands are simply the ligands present in the coordination sphere aside from the main ligands, while in others they are auxiliary ligands that modulate sterics, electronics, or bridging between metal centers.

Coligands can be monodentate, bidentate, or polydentate, and they may be terminal or bridging. Through sigma donation, pi-backbonding, and steric effects, coligands influence geometry (for example, octahedral, square-planar, or distorted structures), spin state, redox behavior, and catalytic activity. They can stabilize unusual oxidation states, assist in substrate activation, or help define the overall topology of multinuclear assemblies.

Common categories of coligands include halide and other anionic ligands, neutral donors such as amines and phosphines, and carbonyls, as well as multidentate chelating ligands that operate alongside a primary ligand set. In dinuclear and polynuclear complexes, bridging coligands like chloride, hydroxide, or carboxylate groups can link metal centers and shape the architecture of the complex.

In catalysis and materials science, careful selection of coligands allows tuning of activity, selectivity, stability, and electronic properties, enabling targeted applications in synthesis and functional materials.

See also: ligand, coordination chemistry, bridging ligand, dinuclear complex, ancillary ligand.