Coordinate Bond: Symbol, Definition, Examples

A coordinate bond is a subtype of a covalent bond. It is an alternative covalent bond in which the electron pair is shared by only one atom. In other words, the shared pair’s electrons are both from the same atom. Coordinate bonds may also be referred to as Dative bonds or dipolar bonds. Since it is a special type of covalent bond, a coordinate bond is also known as a Coordinate Covalent bond. Coordinate covalent bonds can be seen in processes involving two nonmetals, such as hydrogen atoms. It can also be produced in processes involving metal ions and ligands.

We use a wide range of chemical compounds that are created by coordinate bonds in our daily lives. A coordinate bond is formed by the sharing of electrons from a single atom. The other atom just receives the shared electron pair. An arrow indicates the direction of sharing. If a coordinate bond is created between the atoms A and B, with A serving as the donor and B serving as the receptor, the chemical compound with its bond is represented as A → B. On this page let us learn everything about the coordinate bond in detail. Read further to find more.

Coordinate Bond Explanation

Atoms share or donate electrons to attain a noble gas configuration. These noble gas structures are thought of as being in some way a “desirable” thing for an atom to have.

The octet rule governs an atom’s reactivity. In most cases, to fill the outermost orbital, the electrons within share electrons with other atoms and form covalent bonds. While in some cases, electron-rich atoms donate electrons to the electron-deficient atoms. This results in the formation of a coordinate bond. Thus, in coordinate covalent bond atoms are held together by the attraction between the donated electron pair and nuclei of the combining atoms.

Covalent Bond

Before understanding the concept of the Coordinate bond, let’s recall everything about the covalent bond.

1. Covalent bonding, in simple words, is the mutual sharing of valence electrons between atoms to attain the noble gas configuration of the participating individual atoms.  
2. In a covalent bond, the individual atoms are held together by the electrostatic force of attraction. This electrostatic force of attraction exists between the positively charged nuclei of the bonded atoms and the negatively charged electrons they share. 
3. The shared pair of electrons in a covalent bond are called the bonding pair of electrons, which results in the formation of a discrete group of atoms called a molecule. 
4. A covalent bond can exist between two atoms of the same element or elements close to each other in the periodic table. This bonding occurs primarily between non-metals; however, it can also be observed between nonmetals and metals.

What is a Coordinate Bond?

1. A coordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.
2. In the formation of a simple or ordinary covalent bond, each atom supplies at least one electron to the formation of the bond – but that is not the case every time. In the case of a coordinate covalent bond, one atom supplies both of the electrons, and the other atom does not supply any of the electrons.
3. This kind of bond is typically observed in the bonding of metal ions to ligands. However, nonmetals can also participate in this bonding.
4. The reaction between Lewis acid and base is a coordinate covalent bond. Lewis bases or ligands are neutral molecules or ions that contain lone pair of electrons. These lone pairs of electrons are donated to the metal ions. The metal ions act as lewis acid (acceptor) or electron-deficient species.
5. Common ligands are ammonia \(\left({{\text{N}}{{\text{H}}_{\text{3}}}} \right)\), water \(\left( {{{\text{H}}_2}{\text{O}}} \right)\), and halide ions \(\left({{\text{C}}{{\text{l}}^ – }{\text{,}}\,{\text{B}}{{\text{r}}^ – }} \right)\). Ligands are considered Lewis bases because they are sharing their electron pairs with the metal ion. Metal ions are always positive as they have fewer electrons and are quite attractive to lone pairs of electrons present in the lewis bases(donors).

Formation of Coordinate Covalent Bond

The formation of a coordinate covalent bond can be easily demonstrated by the reaction between ammonia and hydrochloric acid, which is as follows.
The reaction between ammonia and hydrochloric acid

The overall reaction is – \({\text{N}}{{\text{H}}_3}(~{\text{g}}) + {\text{HCl}}({\text{g}}) \to {\text{N}}{{\text{H}}_4}{\text{Cl}}({\text{s}})\)

The electron and proton movement during the reaction is as shown below.

When the ammonium ion, \({\text{N}}{{\text{H}}_4}^ + \), is formed –

1. The hydrogen atom shown in red is the fourth hydrogen atom, attached by a coordinate covalent bond to the ammonia molecule. As hydrogen has only one electron in its outermost shell, only the hydrogen’s nucleus is transferred from the chlorine to the nitrogen. 
2. The electron of the hydrogen atom is left behind on the chlorine to form a negative chloride ion. 
3. Once the ammonium ion has been formed, there lies no difference between the coordinate covalent and the ordinary covalent bonds. All of the hydrogens are equivalent in the molecule, and the extra positive charge carried by the hydrogen ion is distributed throughout the molecule.
4.  Although the electrons are shown differently in the diagram, there is no difference between them in reality. In simple diagrams, a coordinate bond is shown by a curved arrow \(\left( \to \right)\) the arrow points from the atom, donating the lone pair to the atom accepting it, i.e from lewis bases to lewis acids.

Characteristics of Coordinate Covalent Bond

1. Coordinate covalent bond mostly takes place between dissimilar atoms.
2. In this bonding, the atom or the lewis base that donates an electron pair from itself is termed as the donor and the atom or the lewis acid that accepts the donated pair of electrons is known as receptor or acceptor.
3. The other atom which accepts these shared pairs of electrons is known as a receptor or acceptor.
4. An arrow \( \to \) symbol represents this bond, pointing towards the acceptor from the donor atom.
5. The bond length, the strength of the coordinate covalent bond is almost similar to that of the covalent bond.
6. After sharing an electron pair each atom gets stability.

Examples of Coordinate Covalent Bond

Let’s now discuss about examples of coordinate covalent bond:

Formation of Ozone Molecule

Coordinate covalent bonds are also present in neutral molecules such as \({{\text{O}}_{\text{2}}}\).

Each oxygen atom in \({{\text{O}}_{\text{2}}}\) contributes \(1\) electron to each of the bonding pairs of electrons. This results in the formation of \(2\) ‘normal’ covalent bonds or a double covalent bond \({\text{O=O}}\). Besides the \(2\) bonding pairs of electrons, each oxygen atom also has two nonbonding pairs of electrons called the lone pairs of electrons.

In an ozone molecule, \({{\text{O}}_{\text{3}}}\), an oxygen molecule, \({{\text{O}}_{\text{2}}}\), reacts with an oxygen atom in the atmosphere.

Each oxygen atom in the \({{\text{O}}_{\text{2}}}\) molecule has the stable electronic configuration of the noble gas Neon. So, the only way the third oxygen atom could add to the oxygen atoms in \({{\text{O}}_{\text{2}}}\) is that one of the oxygen atoms present in \({{\text{O}}_{\text{2}}}\) to contribute both the non bonded pair of electrons to the new oxygen atom. This results in the formation of a coordinate covalent bond between the existing \({{\text{O}}_{\text{2}}}\) molecule and the new oxygen atom:

In the diagram given above, one of the oxygen atoms from the original \({{\text{O}}_{\text{2}}}\) molecule shown in black dots has contributed one pair of its non-bonded pair of electrons to the third oxygen atom(denoted by \({\text{X}}\)).

The \({\text{X}}\) oxygen atom has not contributed any of its existing electrons to this bonding, so this is a coordinate covalent bond (dative bond). However, once the coordinate covalent bond has been formed, it is in no way different from a ‘normal’ covalent bond. By looking at this Lewis structure for \({{\text{O}}_{\text{3}}}\) we could say that the single bond will always be the coordinate covalent bond

Formation of Ammonium Ion

In ammonia, the nitrogen atom acts as the lewis base and donates its lone pair of electrons to the empty orbital of \({{\text{H}}^ + }\) ion which acts as the lewis acid. Thus nitrogen is the donor, and \({{\text{H}}^ + }\) is the acceptor resulting in the formation of a co-ordinate bond.

Formation of Hydronium Ion

In the formation of hydronium ions, water acts as the Lewis base. An oxygen atom in water donates its lone pair of electrons to the vacant orbital of the \({{\text{H}}^ + }\) ion. The \({{\rm{H}}^ + }\) ion acts as the Lewis acid. Thus a dative bond is formed between water molecules and \({{\rm{H}}^ + }\) ions. The oxygen atom of water is the donor atom and \({{\rm{H}}^ + }\) is the acceptor atom.

Formation Of Ammonia Boron Trifluoride \(\left( {{\text{N}}{{\text{H}}_3}{\text{.B}}{{\text{F}}_3}} \right)\)

Boron trifluoride \(\left({{\text{B}}{{\text{F}}_3}}\right)\) is a compound that does not have a noble gas structure around the boron \(\left({\text{B}}\right)\) atom. The boron only has three pairs of electrons in its valence shell and requires a pair to complete the orbital.  Hence, \({\text{B}}{{\text{F}}_3}\) is electron deficient.

The lone pair on the nitrogen \(\left({\text{N}}\right)\) of the ammonia \(\left({{\text{N}}{{\text{H}}_3}} \right)\) molecule is used to overcome that deficiency, and a complex compound forms through a coordinate covalent bond. Here, \({\rm{B}}{{\rm{F}}_3},\) being electron deficient acts as the Lewis acid whereas \({\rm{N}}{{\rm{H}}_3}\) being an electron-rich species acts as the Lewis base.

Formation of Aluminum Chloride \(\left({{\text{A}}{{\text{l}}_2}{\text{C}}{{\text{l}}_6}} \right)\)

In \({\rm{AlC}}{{\rm{l}}_3},\) each aluminium \({\text{(Al)}}\) atom has a deficit of two electrons in its valence shell, and chlorine (Cl) has a lone pair of electrons. \({\text{Al}}\) forms a coordinate covalent bond with the \({\text{Cl}}\) atom on an adjacent \({\text{AlC}}{{\text{l}}_3}\) group. The \({\text{Al}}\) atom acts as the acceptor and the neighbouring \({\text{Cl}}\) atom acts as the donor. As each of two \({\text{Al}}\) atoms does this, then aluminium chloride is a covalent dimer molecule with the formula \({\text{A}}{{\text{l}}_{\text{2}}}{\text{C}}{{\text{l}}_{\text{6}}}\).

Formation of Carbon Monoxide \({\text{(CO)}}\)

Carbon \({\text{(C)}}\) has four electrons in its valence shell, and oxygen \({\text{(O)}}\) has six. Both carbon and oxygen share two electrons, one from each atom. In carbon monoxide \({\text{(CO)}}\), the octet rule is satisfied for oxygen atoms, but the carbon atom is a deficit of two electrons. So, oxygen shares its two electrons with carbon to form a coordinate covalent bond, in addition to the two regular (double) covalent bonds. Hence, in carbon monoxide, the electron rich oxygen atom acts as the donor, whereas the carbon atom acts as the acceptor.

Properties of Coordinate Compounds

1. Generally, a metal and an electron-rich ligand form this type of bonding.
2. Coordinate compounds have much higher melting and boiling points than covalent compounds but lower than ionic compounds.
3. These compounds are sparingly soluble in water.
4. Coordinate compounds do not ionize in water and are poor conductors of electricity.
5. Some of these compounds exhibit isomerism.
6. The compounds are rigid, polar, and directional.
7. This bond is weaker than Ionic bonding.

Summary

The compounds having a coordinate bond plays an important role in our day-to-day life. From the major greenhouse gas carbon monoxide \({\text{(CO)}}\) to the protector of ultraviolet radiation, ozone. From dyes and pigments to chemotherapy, from bio-inorganic chemistry to synthetic detergents, all are composed of coordination compounds.

FAQs on Coordinate Bond

Q.1. Are coordinate covalent bonds stronger or weaker than regular covalent bonds?
Ans: A coordinate covalent bond is of similar strength compared to that of the covalent bond. This is because once the coordinate bond is formed there lies no difference between the normal covalent bond and the coordinate covalent bond.

Q.2. How do you know if a bond is a dative bond?
Ans: A dative bond can be identified by the arrow which points from the atom donating electrons to the atom accepting electrons. It is almost similar to the covalent bond.

Q.3. What is the difference between a coordinate covalent bond and a covalent bond?
Ans: Coordinate covalent bonds have one species donate both electrons to forming the bond while usually covalent bonds have one electron come from each atom.

Q.4. What do you understand by a lone pair and shared pair of electrons?
Ans: The non-bonded pair of electrons present on an atom of a molecule that can be shared with other atoms are called lone pairs of electrons whereas the bonded pair of electrons that are shared equally between two bonded atoms are called shared pairs of electrons.

Q.5. Is a co-ordinate bond a strong bond?
Ans: Coordinate covalent bonds are often quite strong. This is due to the fact that the bonds are similar to all other interatomic bonds.

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