What is a Coordinate Covalent Bond?

A coordinate covalent bond is also known as a dative bond, which is a type of covalent bond. It is formed between two atoms, where the two electrons required to form the bond come from the same atom resulting in a semi-polar bond. The study of coordinate covalent bond or dative bond is important to know about the special type of bonding that leads to different properties. Since covalent compounds are non-polar whereas coordinate bonds results always in polar compounds due to charge separation.

How is it Different from other Bonds?

An ionic bond forms when there is a complete transfer of valence shell electrons generating two oppositely charged ionic species. A Covalent bond is formed by the mutual sharing of two electrons by two atoms. Whereas, a coordinate covalent bond or a dative bond, is a two-electron bond in which one atom donates its lone pair of electrons and the other atom accepts it. 

Characteristics of a compound containing coordinate covalent bond:

• An electron pair donor should possess at least one pair of electrons in its valence shell to donate. 
• The atom accepting electrons should possess a vacant orbital in its valence shell and is called an “electron pair acceptor”. 
• Coordinate covalent bond or dative bond is represented by an arrow that points towards acceptor atom from the donor atom. 
• Coordinate covalent bond is rigid and directional like a covalent bond.

Properties of a compound with co-ordinate covalent bond:

• Compounds containing dative bonds due to their weak intermolecular forces either exist as gases or as liquids. 
• Some compounds because of their directional nature exhibit isomerism. 

• Even though these compounds are as stable as covalent compounds, they undergo molecular reactions in a slow pace. 
• Their melting and boiling points are lower than ionic but higher than covalent compounds. 
• They are soluble in non-polar solvents like n-hexane, DMSO (dimethyl sulfoxide).
• They are insoluble in polar solvents like water, ethanol. 

How to Identify a Coordinate Covalent or a Dative Bond in a Compound?

First, find the number of unpaired valence electrons of the central atom in a molecule. Lewis structures can be used to identify the valance electrons, lone pair electrons, and also bonding pair of electrons. The number of unpaired valence electrons will be equal to covalent bonds since covalent bonding is the result of one atom’s valence electrons shared with another atom. The number of bonds higher than this would belong to the coordinate covalent bond. 

The position of donor atom in the periodic table can affect the nature of the bond formed in a coordinate covalent bond. This includes polarity of the bond, bond length (single bond formation in some cases and double or triple bond formation in other cases).

Case A:

If the donor atom belongs to the second period the coordinate covalent bond leads to charge separation between the donor and acceptor atoms.

$$\text{A}---->\text{B } \to {\text{ A}}^{\delta +}----{\text{B}}^{\delta -}$$ (where A is the donor atom and B is the acceptor atom)

For example:

${\text{HNO}}_3$: The electronic configuration of nitrogen is ${\text{1s}}^{\text{2}}{\text{, 2s}}^{\text{2}}{\text{, 2p}}_{\text{x}}{}^{\text{1}}{\text{, 2p}}_{\text{y}}{}^{\text{1}}{\text{ and 2p}}_{\text{z}}{}^{\text{1}}$. Therefore, the number of unpaired valence electrons of nitrogen is three.

As you can see in the image given, there are four bonds with nitrogen, (i) a double bond with$\text{‘O’}$, (ii) a single bond with $\text{‘OH’}$and (iii) a bond with oxygen. The first and second bonds (i & ii) is a mutual sharing of electrons leading to the formation of a covalent bond. The third bond mentioned here should be the coordinate covalent bond since the nitrogen has one lone pair of electrons acts as a donor and the oxygen needs two electrons to complete its octet acts as an acceptor.”

Case B:

If the donor atom belongs to the third to seventh period the coordinate covalent bond leads to the formation of a double bond between the donor and acceptor atoms.

$\text{A}---->\text{B } \to \text{ A}=\text{B}$ (where A is the donor atom and B is the acceptor atom)

For example: 

${\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}$: The electronic configuration of Sulphur is$\left[\text{Ne}\right]{\text{ 3s}}^{\text{2}}{\text{, 3p}}_{\text{x}}{}^{\text{2}}{\text{, 3p}}_{\text{y}}{}^{\text{1}}{\text{ and 3p}}_{\text{z}}{}^{\text{1}}$. Therefore, the number of unpaired valence electrons of sulfur is two.

As you can see in the image given, there are six bonds with sulfur, (i) two double bonds with $\text{‘O’}$, (ii) two single bonds with $\text{‘OH’}$. The two single bonds with $\text{‘OH’}$ are a mutual sharing of electrons leading to the formation of a covalent bond. The two double bonds (i) mentioned here should be the coordinate covalent bond since the sulfur has two lone pairs of electrons acts as a donor and each oxygen needs two electrons to complete its octet acts as an acceptor.

Common Mistakes 

Do not confuse coordinate covalent bonds with ionic bonds, ionic bonds form between two atoms that have major difference in their electronegativity whereas coordinate covalent bond arises between similar electronegative atoms.

Practice Problems

1. What are the types of bond that ${\text{N}}_{\text{2}}\text{O}$contains?

Answer: Number of unpaired electrons of nitrogen is equal to three which is equal to the Number of covalent bonds.

The bonds between two nitrogen share mutual electrons and form 3 covalent bonds. The nitrogen bonded with oxygen donates its one lone pair of electrons to oxygen leading to charge separation resulting in a coordinate covalent bond.

2. The compound containing coordinate bond is

$\begin{array}{l}\text{A}{\text{.O}}_{\text{3}}\\ \text{B}{\text{.H}}_{\text{3}}{\text{O}}^{\text{+}}\\ \text{C}{\text{.NH}}_{\text{4}}^{\text{+}}\end{array}$

D. All of these

Answer: 

A. ${\text{O}}_{\text{3}}\text{: O=O}\to \text{O}$

$\text{‘O’}$ atom with its electronic configuration as ${\text{1s}}^{\text{2}}{\text{, 2s}}^{\text{2}}{\text{, 2p}}_{\text{x}}{}^{\text{2}}{\text{, 2p}}_{\text{y}}{}^{\text{1}}$and${\text{2p}}_{\text{z}}{}^{\text{1}}$has two unpaired electrons in its valence shell.

Two $\text{‘O’}$ atoms mutually share electrons to form two covalent bonds. The ${\text{2}}^{\text{nd}}\text{ ‘O’}$atom donates its lone pair of electrons to the ${\text{3}}^{\text{rd}}\text{ ‘O’}$atom forming a coordinate covalent bond.

B. ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{: H}}_{\text{2}}\text{O:}\to {\text{H}}^{\text{+}}$

In${\text{H}}_{\text{2}}\text{O}$molecule the oxygen has two lone pair of electrons.

$\text{‘O’}$ atom with its electronic configuration as ${\text{1s}}^{\text{2}}{\text{, 2s}}^{\text{2}}{\text{, 2p}}_{\text{x}}{}^{\text{2}}{\text{, 2p}}_{\text{y}}{}^{\text{1}}$and${\text{2p}}_{\text{z}}{}^{\text{1}}$has two lone pair of electrons and two unpaired electrons in its valence shell that form covalent bonds with two$\text{‘H’}$atoms. When a proton becomes available for the water molecule, the $\text{‘O’}$ atom donates its one lone pair of electrons to complete the valence of Hydrogen atom resulting in the formation of ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$.

C. ${\text{NH}}_{\text{4}}{}^{\text{+}}{\text{: H}}_{\text{3}}\text{N:}\to {\text{H}}^{\text{+}}$

The $\text{‘N’}$atom with its electronic configuration as${\text{1s}}^{\text{2}}{\text{, 2s}}^{\text{2}}{\text{, 2p}}_{\text{x}}{}^{\text{1}}{\text{, 2p}}_{\text{y}}{}^{\text{1}}$and${\text{2p}}_{\text{z}}{}^{\text{1}}$ has one lone pair of electrons and three unpaired electrons that form covalent bond with $\text{‘H’}$atoms. In ${\text{NH}}_{\text{3}}$molecule the nitrogen has one lone pair of electrons. When a proton becomes available for the water molecule, the $\text{‘N’}$ atom donates its lone pair of electrons to complete the valence of Hydrogen atom resulting in the formation of${\text{NH}}_{\text{4}}{}^{\text{+}}$. All the $\text{N-H}$bonds on ${\text{NH}}_{\text{4}}{}^{\text{+}}$ are single bonds.

Therefore, the correct option is (D) All of these contain coordinate covalent bond.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for      

• B.Sc. Chemistry, Biochemistry and Molecular biology
• M.Sc. Chemistry, Biochemistry and Molecular biology             
• M.Sc. Biological science
• M.Sc. in Biomolecular chemistry
• M.S. Biotechnology