X-function ligand

An X-function ligand is one which interacts with a metal center via a normal 2-electron covalent bond, composed of 1 electron from the metal and 1 electron from the X ligand. As such, an X-function ligand donates one electron to a metal center. Since the metal uses one electron in forming the M-X bond, each X-function ligand raises the valence of the metal center by one unit. Simple examples of X-type ligands include H and CH3, i.e. radicals. 

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