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( Ligand)
In chemistry, a ligand is either an atom, ion, or molecule (see also functional group) that bonds to a central metal, generally involving formal donation of one or more of its electrons. The metal-ligand bonding ranges from covalent to more ionic. Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known involving Lewis acidic "ligands."[1] Ligands are classified in many ways their charge, size (bulk), the identity of the coordinating atom(s), and their denticity. The size of a ligand is indicated by its cone angle. In coordination chemistry, the ligands that are directly bonded to the metal (that is, share electrons), are sometimes called "inner sphere" ligands. "Outer-sphere" ligands are not directly attached to the metal, but are bonded, generally weakly, to the first coordination shell, affecting the inner sphere in subtle ways. The complex of the metal with the inner sphere ligands is then called a coordination complex, which can be neutral, cationic, or anionic). The complex, along with its counter ions (if required), is called a coordination compound. In general, ligands are viewed as donating electrons to the central atom. Bonding is often described using the formalisms of molecular orbital theory. In general, electron pairs occupy the HOMO of the ligands.
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