Ligands, coordination number, denticity, chelation

Important terms

Before knowing the concepts in-depth, it’s essential to understand what these terms mean:

• Ligands: Ligand is a molecule or ion that is attached to the central metal atom for the formation of a coordination complex.
• Coordination number: The total number of ligands or the total no. of donor atoms attached to the central metal atom is the coordination number.
• Denticity: The total number of ligand groups or the total no. of donor groups attached to the central metal atom is termed denticity.
• Chelation: Chelation is the type of bonding between ions or molecules of metal ions. In Chelation formation or presence of two or more individual coordinate bonds between a polydentate ligand and the central metal atom.

Ligands

Ligands are the donor atoms donating lone pair of electrons to the central metal atom to form a coordination complex. They can be cations, anions, or neutral molecules. Ligand means to bind. Depending upon the no. of sites they bind, they are further classified into:

1. Monodentate ligand:

Ligands that bind on a single site are called Mono dentate ligands. Some of the examples include Br -, F-, Cl-, I-, OH-, CO, CN, 

  1. Bidentate ligand:

Ligands donating two lone pairs of electrons to the central metal atom are called bidentate ligands.

E.g., Ethylenediamine, Acetylacetonate ion, Phenanthroline, etc.

  2. Tridentate ligand/ Polydentate ligand:

Ions/atoms donating three lone pairs of electrons are called a tridentate ligand, four lone pairs of electrons are tetradentate electrons, five lone pairs of electrons are pentadentate, and the ligands donate six lone pairs of electrons are hexadentate ligands. All these ligands come under polydentate ligands. 

E.g., Diethyl triamine (diene), triethylenetetramine (triene), Ethylenediaminetetraacetate (EDTA). 

  3. Ambidentate ligand:

Ligands that have multiple attaching sites but bind simultaneously are called Ambidentate ligands.

E.g., CN-, NO2, etc

Depending on the type of ligand attached, the compounds are classified into:

• Homoleptic compounds: having the same ligand attached to the central metal atom. E.g., [Co (NH3)6] Cl3
• Heteroleptic compounds: having at least one different ligand attached to the central metal atom, E.g., [CoCl3(NH3)3]3+

Ligands can further be distinguished based on symmetry as well:

  4. Symmetrical ligand:

Symmetrical ligand are the ones where similar ligands are attached on either side of the central metal atom 

  5. Unsymmetrical ligand:

Unsymmetrical Ligands are those in which two donor atoms are different and have no mirror axis of symmetry.

Coordination number: 

The number of atoms an ion holds in a complex or Coordination complex is called coordination number. These numbers can range from 2-9. Complexes with higher Coordination numbers are rare. 

Example: [Mo(CN)8]4- has coordination number 8, [ZrF7]3- has  coordination number 7, [Zn(CN)4]2- and [Cu(CN)4]3- has coordination number 4, and [Ag(NH3)2]+, [AuCl2]- has coordination number 2. An ion or atom can have different Coordination numbers for different molecules, e.g., Al3+ has coordination number 4 in [AlCl4]- but 6 according to ligand field theory, theF6]3-. The bonds holding these complexes can be covalent, cationic, anionic, or electrostatic. The coordination number also depends on the ligands’ geometrical arrangement and the ligand field’s strength. 

In Coordination compounds, the central metal atom behaves as a Lewis acid, and hence it’s an electron pair acceptor.

Some properties of coordination complexes are:

1. The complexes formed due to transition elements are colorful because of the unpaired electrons present in the complex. These unpaired electrons absorb light in the electronic transition of the metals. E.g. Compounds containing Fe(III) impart yellowish-brown to brown color, whereas the complexes with Fe(II) impart green to pale green in the compound.
2. Coordination compounds exhibit various chemical activities, and they can be part of the inner sphere and outer spheres electron transfers.
3. If metal is present as the central atom, then the compounds inhibit magnetic properties due to the presence of unpaired electrons

Some of the applications of coordination compounds are:

1. In biology, Hemoglobin (Hb) consists of a Heme-complexion that has Fe (II) as the central ion.
2. Cisplatin complex is used in the treatment of cancer.
3. Extraction of gold and silver requires the Mac-Arthur process, where a complex of cyanide ions is formed.
4. Ni complexes with DMG are used to estimate the hardness of water using complexes like Ca2+, Mg2+ with EDTA.

Denticity

The total number of coordinating groups present in a ligand is called the denticity of a ligand. Denticity is usually used to determine the bonding of a ligand. Depending on the number of donor groups, ligands are classified into monodentate, bidentate, and polydentate ligands discussed above.

E.g., CN-, OH-, Cl-, Br-, NH3, H2O, CO, oxalate ion, ethylene diamine, ethylene diamine tetraacetic acid, etc.

It is observed that the denticity of the ligand present in the complex affects the stability of the complex. Complexes with high denticity form more stable complexes than those with lower denticity, and the phenomenon is called the chelate effect.

Chelation

The process where an atom or ion binds to the molecule is called chelation, and the ion or the atom bonded is called a chelate. The molecules formed due to chelation have a polydentate ligand and a central metal atom. Apart from chelates, these ligands can also be called chelators, chelation agents, chelants, etc. Most of the chelates are organic compounds, but some exceptions do exist.

The application of chelation provides nutrient supplements to remove toxic metals from water bodies by chelation. Chelation is also used in the medicinal application in Magnetic Resonance Technique (MRI) as the contrasting agent. Other applications include dental and oral, heavy metal detoxification, pharmaceuticals, etc.

The industrial application of chelation includes chelation in homogeneous catalysis, fertilizers, and water softener. While the chelation process has many applications, it has some drawbacks too. Some of them are

1. Chelation agents can lead to a respiratory disorder.
2. Chelation therapy leads to side effects like vomiting and fever in patients.

One of the mechanisms for chelation is mentioned below:

Cu2+ + en ⇌ [Cu(en)]2+

Cu2+ + 2 MeNH2 ⇌ [Cu (MeNH2)2]2+

In the first example, copper forms a chelating complex with ethylenediamine, whereas, in example 2, copper forms chelate with methylene diamine. Here the bidentate ligand is attached to two monodentate ligands indicating that both the reactions are similar. Examples of Chelates include:

1. Dimercaprol
2. Porphin
3. Cyanocobalamin (Vitamin B12)
4. Ethylenediamine etc.

Conclusion

Ligands, denticity, and chelation combine to form a Coordination complex. These Coordination complexes have Coordination numbers that can be determined by calculating the oxidation number of the compound.