Chemistry by Inam Jazbi: Ligands or Co-ordinating Groups and its Types

Ligands or Co-ordinating Groups and its Types

The neutral molecules or ions surrounding the central metal atom or ion in complex by donating its lone pair of electrons are called ligands or co-coordinating groups. They are molecular or ionic species that get attached to the central metal atom or ion by donating its lone pair of electrons through co-ordinate bonds in a complex ion. In Lewis sense, they are referred as Lewis bases. They may be negative, neutral or positive.

Ambidentate Ligand

The ligands that can legate through two different atoms present in its molecule to central atom or ion are called Ambidentate Ligand. Some ambidentate ligands are NO₂^–, SCN^– or NCS^– etc.

1. The ligand NO₂^– can attach itself through N (as – NO₂^–) as well as through O (as ONO^–). The – NO₂^– is named as nitro and ONO^– is named as nitrito.

2. The ligand CNS^– may get attached through S (as – SCN^–) or through N (as – NCS^–). The ligand – SCN^– is named as thiocyanato or thiocyanato – S and the ligand –NCS^– is named as isothiocyanato or thiocyanatio –N.

Chelating Agents or Chelates and Chelation

The polydentate ligands are capable of occupying two or more coordination positions by donating two or more lone pairs of electrons to the same metal ion within the complex molecules i.e. they are coordinated to the central metal ion at more than two sites by two or more metal-ligand bonds are called chelating agents or chelating groups or chelating ligands because they form one or more ring structures called chelates (Greek word meaning crab’s claw).

The complexing process or coordination of a polydentate ligand or chelating agent to the same central atom or ion is called chelation or cylisation. The special type of complex ion or compound containing chelating groups or polydentate ligands with one or more ring structures around its central atom is called chelate complex or chelated complex or cyclic complex or simply chelates. The number of sigma electron pairs (or lone pairs) through which the polydentate ligands gets coordinated to the central atom or ion is called denticity.

Polydentate ligands have been classified as bidendate (e.g. glycinato;gly-, C₂O₄^3–, ethylenediamine; -en, sulphato; SO₄^2– etc), tridentate (e.g. –dien; diethylenetriamine, aspartate; asp^2– etc), tetradentate or quadridentate (e.g. nitrilotriacetato;nta^3–), triethylenetetraamine; -trien, etc), pentadentate and hexadentate depending upon whether the ligands have two, three, four, five and six donor atoms respectively.

Classification of Ligands According to Charge

According to charge, ligands are of three types namely anionic, neutral and cationic ligands. The name of anionic ligands end in –O while cationic ligand have suffix –ium.

1. Anionic Ligands or Negative Ligands

2. Neutral Ligands or Molecular Ligands

3. Cationic Ligands or Positive Ligands

1. Anionic Ligands or Negative Ligands

The ligands which carry negative charge on them are called anionic ligands. Anionic ligands generally form anionic complex ions. If the name of the ligands end –ide, –ite, or –ate , it is changed respectively to –ido, –ito, or –ato. For some simple ligands, the ending –ide is changed simply to –O. e.g.