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.