Covalent Bonding and Dative Bonds

Learning Objectives (OCR Spec 2.2.2)

By the end of this lesson you should be able to:

• Describe a covalent bond as the shared pair of electrons between two atoms
• Draw dot-and-cross diagrams for simple molecules and polyatomic ions
• Distinguish between single, double and triple bonds
• Explain and recognise dative (coordinate) covalent bonds
• Explain exceptions to the octet rule (expanded octets, incomplete octets, odd electron species)

What Is a Covalent Bond?

A covalent bond is a shared pair of electrons between two atoms. The shared pair is attracted to both nuclei simultaneously, holding the atoms together.

Covalent bonding is the usual mode of bonding between two non-metals, where neither atom has a strong enough tendency to form an ion outright, so they share electrons instead to achieve a full outer shell.

Dot-and-Cross Diagrams for Covalent Molecules

Use dots for one element's electrons and crosses for the other's. Show overlapping electrons in the "bond" region between atoms.

H2

   H  x.  H        H - H

One shared pair (1 electron from each H). Each H effectively has 2 electrons, matching helium.

HCl

   H  x.  :Cl:  ..
              ..

One shared pair. Cl has 3 lone pairs plus the shared pair = 8 outer electrons.

H2O

Oxygen has 6 outer electrons. It shares 2 of them with 2 Hs, forming 2 bond pairs and retaining 2 lone pairs.

        ..
     H-O-H
        ..

NH3

Nitrogen has 5 outer electrons, shares 3, retains 1 lone pair. Three N-H bond pairs, one lone pair.

CH4

Carbon has 4 outer electrons, shares each with one H, forming 4 bond pairs, no lone pairs.

CO2

Two C=O double bonds. Each O provides 2 electrons to the double bond, each C provides 2 electrons. Both O atoms have 2 lone pairs each.

   O = C = O

N2

Nitrogen atoms share three pairs of electrons to achieve octets, giving a very strong triple bond (bond enthalpy 945 kJ mol-1).

   :N :::N:

Each N has one lone pair and three bond pairs.

Multiple Bonds

• Single bond: 1 shared pair (2 electrons)
• Double bond: 2 shared pairs (4 electrons)
• Triple bond: 3 shared pairs (6 electrons)

More bond pairs = shorter and stronger bond.

Bond	Bond length (pm)	Bond enthalpy (kJ mol-1)
C-C	154	347
C=C	134	612
C::C	120	838
N-N	145	163
N=N	125	418
N::N	110	945

Dative (Coordinate) Covalent Bonds

A dative covalent bond (also called a coordinate bond) is a covalent bond in which both of the shared electrons are donated by the same atom. The donor atom must have a lone pair; the acceptor must have an empty orbital.

Once formed, a dative bond is identical in length, strength and properties to a "normal" covalent bond - you cannot tell them apart experimentally. It is just the origin of the bonding pair that differs.

A dative bond is represented by an arrow pointing from donor to acceptor: D -> A.

Example 1: The Ammonium Ion, NH4+

NH3 has a lone pair on nitrogen. When NH3 reacts with H+ (which has an empty 1s orbital), the lone pair forms a dative bond:

NH3 + H+ -> NH4+

In NH4+ all four N-H bonds are equivalent, even though one originated as a dative bond.

       H
       |
   H - N+- H    (+ charge; four bond pairs, no lone pair)
       |
       H

Example 2: The Oxonium (Hydronium) Ion, H3O+

H2O has two lone pairs; one forms a dative bond with H+:

H2O + H+ -> H3O+

       H
       |
   H - O+- H    (pyramidal, like NH3)

Example 3: The Tetrahydroxoaluminate Ion, [Al(OH)4]-

Al^3+ has empty orbitals. Four OH- ions each donate a lone pair into an empty orbital on Al to form the complex ion:

Al^3+ + 4OH- -> [Al(OH)4]-

All four Al-O bonds are dative in origin.

Example 4: Carbon Monoxide, CO

CO contains a particularly striking dative bond. Consider the bonding:

• C and O form a double covalent bond (4 electrons shared)
• O also donates a lone pair into an empty orbital on C

The result is a triple bond :C::O:, with the C-O bond length (113 pm) and bond enthalpy (1077 kJ mol-1) closer to a triple bond than a double.

Example 5: The Aluminium Chloride Dimer, Al2Cl6

AlCl3 has only 6 electrons around Al (an incomplete octet). Two AlCl3 molecules join via two dative bonds (a Cl lone pair from each donates to the other Al):

    Cl      Cl      Cl
     \     / \     /
      Al      Al
     /     \ /     \
    Cl      Cl      Cl

Exceptions to the Octet Rule

Although most covalent species achieve an octet, there are notable exceptions:

Incomplete octets