Video Transcript
In this video, we will learn how to describe and illustrate coordinate covalent bonds
in simple molecules and metal compounds. Molecules are formed when two or more atoms bond together. The atoms may be the same element or from different elements; it does not matter. When the atoms that are bonded together are nonmetals, they are held together by
strong covalent bonds.
Ammonia is an example of a molecule that contains strong covalent bonds. One nitrogen atom lies at the center of the molecule, and it is bonded to three
hydrogen atoms. Notice that both of these elements are nonmetals. In ammonia, three single covalent bonds are formed. Each bond is a normal single covalent bond. A covalent bond, remember, is a shared pair of electrons. In a normal single covalent bond, one electron comes from each atom in the bond.
The central nitrogen atom has five valence electrons. Remember, the nitrogen is found in group 15 or five of the periodic table. Nitrogen uses up three of its valence electrons in forming these bonds. Because a nitrogen atom has five valence electrons, there are two valence electrons
left over. These electrons form a lone pair. A lone pair is a pair of electrons found in the outer shell of an atom that are not
involved in forming any covalent bonds.
The structure we see here for ammonia is known as a dot cross structure or a
dot cross diagram. In this structure, dots represent valence shell electrons for the nitrogen atom and
crosses represent valence electrons for the hydrogen atoms. Dots and crosses are placed in overlapping circles to represent single covalent
bonds.
Lewis structures may also be used to indicate bonding pairs of electrons and lone
pairs of electrons in the molecule. A Lewis structure is a little different to a dot cross diagram, as each pair of
electrons is represented by two dots. Whether we use two dots or straight lines to represent the bonding pairs of electrons
in our Lewis structure for ammonia, the lone pair of electrons is always
visible.
An ammonium molecule can form an ammonium ion by using the lone pair of electrons
that are located on the central nitrogen atom to form a coordinate covalent
bond. When a coordinate covalent bond is formed, this lone pair of electrons on one atom is
used to form a covalent bond with another atom that does not have any electrons
available to share. The atom donating the electron pair into the coordinate covalent bond is called the
donor atom. The atom receiving the electron pair must have a vacant orbital. In this case, the proton, or H+ ion, can accept the lone pair of electrons into its
vacant orbital. The H+ ion does not have any electrons available to share.
In the ammonium ion, which contains a coordinate covalent bond, nitrogen is the donor
atom and hydrogen is the acceptor atom. A coordinate covalent bond can also be known as a dative covalent bond or simply as a
coordinate bond or a dative bond. It’s important to realize that a coordinate covalent bond is just like any other
single covalent bond once it’s been formed. It’s just the origin of the shared pair of electrons that is different.
When using a displayed formula, a coordinate covalent bond is shown using an
arrow. The direction of donation is indicated by the direction of the arrow. Notice that in the ammonium ion the arrow is pointing from the nitrogen donor atom to
the hydrogen atom, which is the acceptor atom. This arrow is not seen in a dot cross diagram or a Lewis structure.
Another feature of the displayed formula of the ammonium ion is that the positive
charge that exists on the nitrogen atom is also shown. It may be seen located on the nitrogen atom itself or placed outside square brackets
that surround the entire structure. Many other examples of coordinate covalent bonding exist.
Hydrogen ions do not exist in isolation in liquid water. If a hydrogen chloride molecule is dissolved into water, a hydrogen ion is
transferred from the hydrogen chloride molecule to the water molecule. The water molecule contains two lone pairs of electrons. Water can use one of these lone pairs to form a coordinate covalent bond to the
hydrogen ion. This reaction forms the hydronium ion, H3O+, and a chloride ion. The hydronium ion, H3O+, may also be referred to as the hydroxonium ion in some
texts.
The hydronium ion contains a coordinate covalent bond. Notice that the molecule has a net positive charge as a result of the oxygen donor
atom, which originated in the water molecule, donating one of its lone pairs to the
hydrogen ion, which originated in the hydrogen chloride molecule.
Carbon monoxide is an oxide of carbon that is formed when hydrocarbon fuels are
burned in a limited supply of oxygen. It is a poisonous gas, and its formation must be avoided in domestic homes when fuels
are ignited. Carbon monoxide consists of one carbon atom and one oxygen atom. The carbon atom uses two of its valence electrons to form two covalent bonds with the
oxygen atom. The oxygen atom uses a lone pair of electrons to form a third bond to carbon, which
is a coordinate covalent bond. There is no net charge on the molecule of carbon monoxide.
In the displayed formula of carbon monoxide, the coordinate covalent bond is again
shown as an arrow, indicating the direction of donation. If ammonia reacts with boron trifluoride, the lone pair on the nitrogen atom in
ammonia can be used to make a coordinate covalent bond with the boron atom in boron
trifluoride. In BF3, boron is electron-deficient, having only six electrons in its valence
shell. If boron becomes the acceptor atom, a nitrogen-to-boron covalent bond is made, and
boron fills its valence shell. Nitrogen is the donor atom in the formation of this coordinate covalent bond. Using a displayed formula for this molecule, an arrow from the nitrogen atom to the
boron atom is used to show the coordinate covalent bond.
Lewis acids are defined as species that can accept a pair of nonbonding electrons or
a lone pair of electrons. In all of the examples of coordinate covalent bonding that we have seen so far, the
acceptor atoms in the bond could be described as Lewis acids. In the molecule NH3BF3, the boron atom is the acceptor atom. It’s behaving as a Lewis acid. A Lewis base is a species that can donate a lone pair of electrons. In all of the examples of coordinate covalent bonding seen so far, the donor atoms
are behaving as Lewis bases.
Metal ions that are placed into liquid water attract water molecules very
strongly. This is because metal ions are relatively small and may have a high positive charge,
such as the aluminum three plus ion. The attractions are so great that coordinate covalent bonds are formed. The water molecules contain lone pairs on the oxygen atoms. And these are the donor atoms in this case. One aluminum three plus ion can form six coordinate covalent bonds to six separate
water molecules. Only one lone pair is used by each water molecule to form these coordinate covalent
bonds. The other lone pair is pointing away from the aluminum atom, so it isn’t involved in
the bonding.
The aluminum atom accepts these electrons into some vacant orbitals. The three plus charge on the aluminum ion is now evenly distributed across the entire
molecule. Since only six water molecules will fit around the central aluminum atom, this ion is
known as the hexaaquaaluminum ion.
Now it’s time for a question to test your understanding of coordinate covalent
bonding.
An ammonium ion, NH4+, contains one coordinate covalent bond. Which of the following displayed formulas correctly shows the structure of an
ammonium ion?
In the ammonium ion, which has the formula NH4+, nitrogen is the donor atom as far as
the formation of the coordinate covalent bond is concerned. We’re being asked to identify the displayed formula of the ammonium ion in this
question. The displayed formula shows all bonds and all atoms in the structure.
All of our possible structures here show the correct number and type of atom for the
ammonium ion. Each possible structure contains one nitrogen atom and four hydrogen atoms
present. All of our possible structures have the positive charge located in the right
place.
When writing displayed formulas, we use a straight line or a dash to represent normal
covalent bonds. And we should use an arrow to represent coordinate covalent bonds. In this case, the arrow must show the correct direction of donation of the lone pair,
that is, from the nitrogen atom to the hydrogen atom.
Remember that the nitrogen atom is the donor atom and the hydrogen atom is the
acceptor atom in this bond. The only structure here that is using the correct notation for a displayed formula is
structure (E). It’s the correct answer.
Let’s see why the other answers are not correct. In structure (A), we see a dotted line used to represent one of the bonds. A dotted line is often used to indicate that a bond is not a full bond; it’s only a
partial bond. An example of this would be a hydrogen bond formed between two molecules. Although ammonia molecules can form into molecular hydrogen bonds, this dotted line
does not represent the coordinate covalent bond in the ammonium ion. This structure is therefore an incorrect answer.
In structure (B), we see one of the covalent bonds represented as a wedge shape. A bold wedged line indicates that the bond is protruding out from the plane of the
drawing surface. It’s pointing out of the page. This structure does not represent the correct three-dimensional shape of this
molecule. And it does not show the coordinate covalent bond either. It’s not the correct answer.
Structure (C) includes a wavy line as one of the covalent bonds. A wavy line is used when the true stereochemistry or direction of the bond is
unknown. This is not a displayed formula, and it doesn’t contain our coordinate covalent
bond. So it’s not the correct answer.
Although structure (D) is very close to the correct answer, there is no coordinate
covalent bond depicted by an arrow shown at all. It shows four ordinary covalent bonds, and it’s therefore not the correct answer. The correct answer is therefore structure (E).
Now let us review the key points about coordinate covalent bonding. A normal single covalent bond contains one shared pair of electrons between
atoms. In a coordinate covalent bond, the shared pair of electrons originates from just one
of the atoms. The donor atom in a coordinate covalent bond must contain a lone pair in the valence
shell. Examples of molecules that contain coordinate covalent bonding include the ammonium
ion, the hydronium ion, and carbon monoxide.
