that interact by agostic interactions. X ligands are halides and pseudohalides. X ligands typically are derived from anionic precursors such as chloride but includes ligands where salts of anion do not really exist such as hydride and alkyl.

Especially in the area of organometallic chemistry, ligands are classified according to the "CBC Method" for Covalent Bond Classification, as popularized by M. L. H. Green