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What is a Ligand?

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Do You Know the Ligand Definition?

A ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex in coordination chemistry. Formal donation of one or more of the ligand's electron pairs, often via Lewis Bases, is necessary for bonding with the metal. Metal ligand bonding can be either covalent or ionic in nature. The metal-ligand bond order can also vary from one to three. Lewis bases are considered ligands, while Lewis acidic "ligands" have been found in rare cases.

As we already defined ligands, now let’s take a look at examples of neutral ligands (neutral molecule), cationic ligands, and neutral ligands.

What is Ligand in Chemistry?

Ligand Examples

Occasionally ligands can be cations (NO+, N2H5+) and electron-pair acceptors. F, Cl, Br, I, S2, CN, NCS, OH, NH2are examples of anionic ligands, while NH3, H2O, NO, and CO are examples of neutral ligands. 

Metals and metalloids are almost always bound to ligands, while gaseous "naked" metal ions can be formed in a high vacuum. Ligand substitution rates, ligand reactivity, and redox are all factors that affect the reactivity of the central atom in a complex. In several practical fields, such as bioorganic and medicinal chemistry, homogeneous catalysis, and environmental chemistry, ligand selection is important.

Types of Ligands 

1. Monodentate Ligands

Monodentate ligands are also known as "one-toothed" ligands because they only bite the metal atom in one place.

2. Bidentate Ligands

Bidentate ligands are Lewis bases that donate two lone pairs of electrons to the central metal atom. Chelating ligand is a term used to describe them. Chelates refers to a complex containing chelating ligands.

3. Tridentate Ligands and Polydentate Ligands

To the central metal atom or ion, tridentate ligands have three lone pairs of electrons. Tetradentate molecules have four donor atoms, pentadentate molecules have five donor atoms, and hexadentate molecules have six donor atoms. Polydentate ligands are a common term for them.

4. Trans- Spanning Ligands

Bidentate ligands that can span coordination positions on opposite sides of a coordination complex are known as trans-spanning ligands.

5. Ambidentate Ligand

Ambidentate ligands, unlike polydentate ligands, can bind to the central atom in two separate ways. Thiocyanate, SCN, is a clear example of this, since it may bind to either the sulfur or nitrogen atom. Linkage isomerism is caused by such compounds. Polyfunctional ligands, especially proteins, may form isomers by bonding to a metal centre via different ligand atoms.

6. Bridging ligand

A bridging ligand is a molecule that binds two or more metal centres. Coordination polymers, which are made up of metal ion centres connected by bridging ligands, make up virtually all inorganic solids with simple formulas. Both anhydrous binary metal ion halides and pseudohalides fall into this category. In solution, bridging ligands are also present. Since polyatomic ligands like carbonate are ambidentate, they often bind to two or three metals at the same time. The prefix "" is often used to denote atoms that bridge metals. The presence of many bridging ligands makes most inorganic solids polymers. Because of their possible use as building blocks for the fabrication of practical multimetallic assemblies, bridging ligands, which are capable of coordinating multiple metal ions, have attracted a lot of attention.

7. Binucleating Ligand

Binucleating ligands are molecules that bind two metal ions together. Binucleating ligands usually involve bridging ligands like phenoxide, or pyrazine, as well as other donor groups that only bind to one of the two metal ions.

8. Metal–Ligand Multiple Bonds

Some ligands can bind to a metal centre by using the same atom but a different number of lone pairs. The metal-ligand bond angle (MXR) can be used to determine the bond order of the metal-ligand bond. This bond angle is often referred to as either linear or bent, depending on the degree to which the angle is bent. In its ionic form, an imido ligand, for example, has three lone pairs. One lone pair serves as a sigma X donor, while the other two serve as L-type pi donors. The MNR geometry is linear when both lone pairs are used in pi bonds. If either or both of these lone pairs are nonbonding, the MNR bond is bent, and the degree of the bend is determined.

9. Spectator Ligand

A spectator ligand is a closely coordinating polydentate ligand that eliminates active sites on metal but does not participate in chemical reactions. The reactivity of the metal centre to which they are bound is influenced by spectator ligands.

10. Chiral Ligands 

Chiral ligands can be used to create asymmetry in the coordination sphere. In several cases, the ligand is used as an optically pure group. Asymmetry results from coordination in some situations, such as secondary amines. For homogeneous catalyzes, such as asymmetric hydrogenation, chiral ligands are used.

Did You Know?

The number of times a ligand binds to a metal via noncontiguous donor sites is referred to as denticity (represented by). Many ligands may bind metal ions at multiple sites, which is typically due to the presence of lone pairs on multiple atoms. Chelating ligands are those that bind to more than one molecule. Bidentate ligands bind to two sites, while tridentate ligands bind to three sites. The angle formed by the two bonds of a bidentate chelate is known as the "bite angle." 

Chelating ligands are frequently made by joining donor groups together with organic linkers. Ethylenediamine, which is made by connecting two ammonia groups with ethylene (CH2CH2) linker, is a classic bidentate ligand. The hexadentate chelating agent EDTA, which can bind through six sites and fully surround certain metals, is an example of a polydentate ligand.

FAQ (Frequently Asked Questions)

1. What is a Protein's Ligand?

Ans: A ligand is a compound that forms a complex with a biomolecule to serve a biological function in biochemistry and pharmacology. The ligand in protein-ligand binding is usually a molecule that generates a signal by binding to a site on a target protein.

2. Is O2 Considered a Ligand?

Ans: Coordination compounds with O2 as a ligand are known as dioxygen complexes. Oxygen-carrying proteins including myoglobin, haemoglobin, hemerythrin, and hemocyanin are used to research these compounds. In 1938, the cobalt(II) complex reversibly bound O2 was demonstrated as the first synthetic oxygen complex.

3. Is Oxygen a Weak Ligand in the Field?

Ans: Since it cannot donate electrons by pi bonding, i.e. only sigma donors, it is not a weak ligand. This is because after donating one electron pair, oxygen gains a positive charge, making it impossible to donate another pair of electrons.