Lesson Explainer: Coordinate Covalent Bonds Chemistry • Second Year of Secondary School

In this explainer, we will learn how to describe and illustrate coordinate covalent bonds in simple molecules and metal compounds.

Covalent bonds are formed when two nonmetal atoms share a pair of electrons. Coordinate covalent bonds are a special class of covalent bonds that are formed when the bonding pair of electrons is donated by just one of the bonding atoms. They are also known as dative covalent bonds. The atom that donates the lone pair of electrons is called the donor atom and is considered a Lewis base. The atom that accepts the lone pair of electrons into a vacant orbital is called the acceptor atom and is a Lewis acid.

Ammonia molecules are neutrally charged polyatomic molecules that contain one nitrogen atom and three hydrogen atoms. The nitrogen atom has five valence electrons and three of these valence electrons are used to make covalent bonds with the three hydrogen atoms. The last two valence electrons are not bonded to any atoms whatsoever. The last two valence electrons exist as a lone pair of electrons and they can form a coordinate covalent bond with another acceptor atom.

Ammonia molecules can be converted into positively charged ammonium ions that have the chemical formula. The transformation happens when ammonia molecules react with positively charged hydrogen ions () or protons:

The following image shows how the lone pair of electrons on a single ammonia molecule can bond with a proton to form a single positively charged ammonium ion. It is important to note here that the coordinate covalent bond is represented with a single-sided arrow and the other covalent bonds are represented with ordinary straight lines. Single-sided arrows can be used to represent coordinate covalent bonds and differentiate them from other types of chemical bonds.

The ammonia molecule can be described as a Lewis base in this chemical reaction because it donates a lone pair of electrons to a positively charged hydrogen ion. The hydrogen ion can be described as a Lewis acid in this chemical reaction because it accepts a lone pair of electrons.

Ammonia molecules can also form coordinate covalent bonds when they react with boron trifluoride molecules (). The boron atom can get eight electrons in its valence shell and the same electron configuration as the nearest noble gas if it accepts a lone pair of electrons from an ammonia molecule. The ammonia molecule can be classed as a Lewis base in this chemical reaction because it donates a lone pair of electrons to the boron trifluoride molecule. The boron trifluoride molecule can be classed as a Lewis acid in this chemical reaction because it accepts a lone pair of electrons from the ammonia molecule. The following image shows the structure of the ammonia boron trifluoride molecule that is formed from the combination of one ammonia molecule and one boron trifluoride molecule.

Example 2: Understanding How Ammonia Molecules Bond Boron Trifluoride Molecules

The reaction between ammonia and boron trifluoride is shown below. What type of bond is formed between ammonia and boron trifluoride?

1. Coordinate covalent bond
2. Covalent bond
3. Metallic bond
4. Ionic bond
5. Hydrogen bond

Answer

Ammonia molecules have the chemical formula. They contain one nitrogen atom that has five valence electrons. Three of these valence electrons are used to make covalent bonds with hydrogen atoms. The other electrons form a single lone pair of electrons. This lone pair of electrons can be donated to the boron atom of a boron trifluoride molecule. The transfer of electrons from the ammonia molecule to the boron trifluoride molecule forms a coordinate covalent bond. We can use these statements to determine that option A is the correct answer to this question.

The boron trifluoride molecule can also act as a Lewis acid when it accepts a lone pair of electrons from a fluoride ion () and forms the boron tetrafluoride () ion. The fluoride ion acts as the Lewis base in this chemical reaction because it donates a lone pair of electrons to the neutrally charged boron trifluoride molecule. The reaction is summarized in the following figure.

Phosphine is a pnictogen hydride that has the chemical formula. The molecule contains one phosphorus atom that is covalently bonded to three other hydrogen atoms. Each phosphine molecule can form one coordinate covalent bond with a single hydrogen ion. The coordinate covalent bond is formed as the phosphorus atom donates its lone pair of electrons to a single hydrogen ion. The resulting phosphonium ion has the chemical formula and it has a tetrahedral shape that matches the tetrahedral shape of an ammonium ion.

Water molecules can form coordinate covalent bonds when they react with other hydrogen ions. The following image shows the Lewis structure diagram for a single water molecule. The figure uses black dots to represent the valence electrons of the oxygen atom and red dots to represent the valence electrons of the hydrogen atoms. The figure shows that the oxygen atom has a total of six valence electrons.

Two of the valence electrons are used to make ordinary covalent bonds with hydrogen atoms. The other valence electrons are not bonded with any atoms and they exist as electron lone pairs. Water molecules can form the hydronium ion () when one of these electron lone pairs forms a coordinate covalent bond with a hydrogen ion ().

Hydronium ions are formed when acids are dissolved in water because the acid molecules release hydrogen ions. This equation shows how hydronium ions can be formed when hydrochloric acid is dissolved in water:

The hydrogen chloride molecules break apart and they form positively charged hydrogen ions () and negatively charged chloride ions (). Coordinate covalent bonds are formed when the positively charged hydrogen ions accept a lone pair of electrons from water molecules. Two Lewis structure diagrams for the hydronium ion () are shown in the following figure.

The carbon monoxide molecule is a rather unusual diatomic molecule that is made up of one carbon atom and one oxygen atom. The carbon and oxygen atoms are linked together with three covalent bonds or what can be called one carbon–oxygen () triple bond. Two of the bonds are formed due to the overlap of two valence electrons from the carbon atom and two valence electrons from the oxygen atom. The two bonds can be considered to be normal covalent bonds because the bonding electrons come from both of the bonding atoms. The last bond is formed because the oxygen atom donates another pair of electrons to the carbon–oxygen chemical bond. This final bond can be considered a coordinate covalent bond because the bonding electrons come from one of the bonding atoms. The following figure shows the dot-and-cross diagram for a single carbon monoxide molecule.

Example 3: Identifying the Correct Representation of Carbon Monoxide Molecules

Carbon monoxide () contains a triple bond between the atoms of carbon and oxygen, one of which includes a coordinate covalent bond. How can the bonding in be shown using a dot-and-cross diagram? Only include the valence electrons.

Answer

Carbon monoxide molecules contain one carbon atom and one oxygen atom. The carbon atom has four valence electrons, and the oxygen atom has six valence electrons. Carbon monoxide molecules contain one triple bond. Two of these bonds are formed through the overlap of one pair of electrons from the carbon atom and one pair of electrons from the oxygen atom. The last covalent bond is formed as the oxygen atom donates a lone pair of electrons. This bonding can be summarized to say that carbon monoxide molecules have one coordinate covalent bond and two other ordinary covalent bonds. Option C is the only figure that correctly displays all this information, and we can conclude that option C must be the correct answer to this question.

Nitric acid () is a fascinating molecule because it has an unusual set of covalent bonds. The following dot-and-cross diagram shows the types of covalent bonds that are formed in a single nitric acid molecule. The nitrogen atom makes an ordinary single covalent bond with one oxygen atom and it makes a double covalent bond with a second oxygen atom. The nitrogen atom also makes one coordinate covalent bond as it donates a lone pair of electrons to a third oxygen atom. The bonding is quite complex, and it can be easier to understand if it is shown in terms of a highly simplified displayed formula. The displayed formula for the nitric acid molecule is shown on the right-hand side of the following figure.

Many metal ions have the ability to form coordinate covalent bonds with up to six different water molecules. Most metal ions can only form up to six covalent bonds with water molecules because of steric hindrance effects. It is impossible to pack more than six water molecules around most metal ion particles because water molecules are relatively large and the space around a metal ion is relatively small. The hydrated cobalt metal ion complex has the formula. The structure of the hydrated cobalt metal ion complex is shown in the following figure.

Beryllium chloride () molecules are relatively simple triatomic molecules when they are in the gas phase. Each beryllium atom is bonded to two chlorine atoms and there are only weak van der Waals forces between the adjacent beryllium chloride molecules. The following image shows the structure of a single gas phase beryllium chloride molecule. The figure shows that the molecule has two ordinary covalent bonds and that the beryllium atoms have two empty orbitals that can accept lone pairs of electrons.

The beryllium chloride molecules polymerize with each other when they transition from the gas phase into the solid phase. Polymers are created as coordinate bonds form between adjacent beryllium chloride molecules. The coordinate bonds are formed as electron pairs are donated from the chlorine atom of one beryllium chloride molecule to the electron-deficient domains of other beryllium chloride molecules. The following image shows the structure of the dimer that is formed as beryllium chloride molecules start to transition from the gas phase into the solid phase.

The polymerization process continues as the electron lone pairs of one chloride atom are donated to the electron-deficient domains of other beryllium chloride molecules. This polymerization process eventually ends up making a long polymer that has the formula. Each beryllium atom ends up having two coordinate covalent bonds and a total of four covalent bonds. The following image shows the structure of the beryllium chloride polymer that is formed when beryllium chloride molecules transition from the gas phase into the solid phase.