Ligands and Its Types for IIT JEE

JEE Chemistry Ligands and Its Types and Co-ordination

For understanding the meaning and characteristics of a ligand is, we first need to understand the meaning of co-ordination chemistry and co-ordination compounds.

Co-ordination Chemistry

Co-ordination chemistry is a branch of chemistry which deals with the study of co-ordination compounds. The presence of co-ordination groups significantly changes the chemistry of a molecule. This branch of chemistry studies these changes and how they reflect upon during the chemical reactions and what can be utilized out of these different properties of these unique compounds.

Co-ordination Compounds

These are the compounds which contain platinum, cobalt, and other transition metals and are made up of two parts – a central atom and ligands. The ligands are bound to the central atom via means of co-ordination bonds. In these compounds, an atom or a group of atoms (called ligands) is/are bound to the central atom by utilizing a shared pair of electrons supplied by the coordinated group and not by the central atom. These compounds have strange empirical formulas and unique properties. 

They are often very brightly coloured compounds. Their major distinguishing feature is the presence of two, four, six, and sometimes even more chemical groups positioned geometrically around the metal ion (also known as the central atom). These groups (also known as the ligands) can be neutral molecules, cations or anions. Each co-co-ordinating group can be a separate entity, or all groups can be connected in one long, flexible molecule that wraps itself around the metal. Co-ordinating groups significantly change the chemical behavior of a metal. 

The colours of the compounds provide clues about their electronic energy levels, e.g., every plant depends on the green magnesium co-ordinating complex known as chlorophyll for carrying out the process of photosynthesis in order to synthesis their own food using sunlight, carbon dioxide and water. The combination of magnesium and its co-ordinating groups in chlorophyll has electronic properties that the free metal or ion does not have, and can absorb visible light and use the energy for chemical synthesis which either the free metal or ions cannot do. Another example of this is cytochromes (the co-ordination compounds of iron) that are essential for every oxygen inhaling organism for the breakdown and combustion of food and the storage of the energy released upon the breakdown and metabolism of that food. Most of the larger organisms need hemoglobin, another iron co-ordination complex in which the co-ordinating groups enable the iron to bind oxygen molecules without being oxidized. In fact, large areas of biochemistry are really the application of these transition metal based co-ordination compounds.

Ligand

The neutral molecules or ions (or atoms or group of atoms) which are directly attached to the central metal ion or atom through co-ordinate bonds in the complex ion are called ligand or ligands. In other words, any species capable of donating a pair of electrons to a metal is called a ligand. A ligand may be an ion, negatively or positively charged, or a neutral molecule.

There are a few requirements for an atom or a group of atoms or ions to behave as a ligand. These are:

  • • Ligands should have at least one lone pair of electrons

  • • Ligands should have the capability to donate their lone pair of electrons to the central metal atom or ion and form co-ordinate covalent bond(s) with it

  • • The Ligand behaves as a Lewis base while the metal atom or ion behaves as a Lewis acid, and an Lewis acid-base reaction takes place between them to form a co-ordination compound

  • Examples: Cl- , Br- , SO4 2- , NH2NH3 +, NH3, H2O, NH2CH2CH2NH2 etc.

    Types of ligands

    Ligands can be classified on the basis of many things. The most common classification of ligands is on the basis of their binding sites with the central metal atom or ion.

    On the basis of the number of sites, ligands can be classified as monodentate, bidentate, polydentate etc. ligands. 

  • Monodentate ligand: These are the ligands which coordinate to only site of a metal ion. In other words, only one pair of electrons can be donated to the metal ion. For instance: Cl–,SO42–, Br–, NH3, NH2NH3 +, H2O

  • Bidentate ligand: These are the ligands which occupy two sites of a metal ion. That is, it can be attached to two metal ion positions, e.g. NH2CH2CH2NH2 etc.

  • Polydentate ligands: These are the ligands which occupy many sites of the same metal ion. Example: EDTA etc. This category includes all the higher levels of dentate ligands above bidentat e.g., tridentate, tetradentate, pentadentate, hexadentate etc.

  • The following table summarizes various monodentate ligands normally studied and encountered inday-to-day chemistry:
    Monodentate LigandName
    F, Cl, Br, IFluoro, Chloro, Bromo and Iodo
    NO2– and ONONitro and Nitrito
    CNCyano
    SCN and NCSThiocyanato and Isothiocyanato
    OHHydroxo
    CH3COOAcetato
    H2OAquo
    NH3Amine
    COCarbonyl
    NO+Nitrosyl
    C6H6N; pyPyridine


    The following table summarizes the common multidentate ligands (which are also known as chelating agents):
    SymbolLigand NameFormulaBonds
    enEthylenediamineNH2(CH2)2NH22
    pnPropylenediamineNH2CH2CH(CH3)NH22
    dienDiethylenetriamineNH2(CH2)2NH(CH2)2NH23
    trienTriethylenetetramineNH2(CH2)2NH(CH2)2NH(CH2)2NH24
    EDTAEthylene diamine tetra acetic acid(CH2COO)2N(CH2)2N(CH2COO)26
    oxOxalate(COO)22


    Another classification of ligands is on the basis of the molecular complex structure they form after forming co-ordination compounds. According to this classification, ligands are divided into two types – chelating agents and ambident ligands:

  • Chelating Agents: These are the ligands which are bonded with the same central metal atom or ion and form a ring type structure. Usually bidentate or polydentate ligands fall under this category. Chelating ligands generally form a ring structure around the central metal atom or ion. The most common example of these types of ligands is EDTA (ethylene di–amine tetra acetic acid).

  • Ambident Ligand: An ambident ligand is that ligand which binds with the central metal atom or ion through more than one site. Usually monodentate ligands fall under this category of ligands. The most common examples of these types of ligands are cyanides (M–CN) and isocyanides (M–NC).

  • Another classification of ligands is on the basis of their chemical nature. According to this classification, ligands are divided into the following types – inorganic ligands, neutral organic ligands, anionic organic ligands and cationic organic ligands:

  • Inorganic Ligands: These are the ligands which are of either ionic nature or other inorganic forms of chemical compounds. The most common examples of these types of ligands are the halide ions (such as the fluoride, chloride, bromide and iodide) and cyanometallates such as CN and SCN

  • Neutral Organic Ligands: These are the ligands which are of organic nature by origin and do not possess any type of charge on them. These are hence molecules and not ions. The most common example of this category of ligands includes pyrazine.

  • Anionic Organic Ligands: These are the ligands which are organic in nature and possess negative charge on them due to the presence of highly electronegative atoms in them, mainly oxygen or nitrogen. The most common example of this category of ligands includes oxalate.

  • Cationic Organic Ligands: These are the ligands which are organic in nature and possess positive charge on them due to the presence of pentavalent nitrogen atoms in them. The most common example of this category of ligands includes pyridine based ligands.

  • Another classification of ligands is based on the type of their covalent bonds. This classification is also sometimes referred to be based upon the LXZ Approach or the CBC Method (which stands for Covalent Bond Classification). This type of classification is mostly used in the organometallic chemistry. According to this classification, the ligands are divided into three types – L ligand, X ligands and Z ligands:

  • L Ligands: The L ligands are derived from charge-neutral precursors: NH3, amines,N-heterocycles such as pyridine, PR3, CO, alkenes etc.

  • X Ligands: The X ligands are derived from anionic precursors: halides, hydroxide, alkoxide alkyls—species that are one-electron neutral ligands, but two electron donors as anionic ligands. EDTA4- is classified as an L2X4 ligand, features four anions and two neutral donor sites. C5H5 is classified anL2X ligand.

  • Z Ligands: Z ligands are very rare in nature. They accept two electrons from the metal center. They donate none. The "ligand" acts as a Lewis acid that accepts electrons instead of the X and L ligands ' Lewis bases that donate electrons.

  • Another classification of ligands is based on the type of donor orbital involved for the donation of electrons to the central metal atom or ion. According to this type of classification, the ligands are of the following type – σ ligands, σ+π ligands, σ+π*/σ* ligands and π + π* ligands:

  • σ ligands: These involve only sigma bonding between the ligand and the central metal atom or ion.

  • σ + π ligands: These involve donation of lone pair through the ligand to the metal tom or ion.

  • σ + π*/ σ* ligands: These involve bonding to vacant π orbitals from σ.

  • π + π* ligands: These involve bonding to vacant π orbitals from π.