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Coordinate Covalent Bonds (dative bonds)

Key Concepts

  • A coordinate covalent bond is also known as a co-ordinate covalent bond, a dative bond, or a dative covalent bond.

  • A covalent bond is formed when 2 electrons (a bonding pair) is shared between 2 atoms.

  • A 'normal' or 'conventional' covalent bond is formed when each of the 2 atoms to be bonded contribute 1 electron to the bonding pair of electrons.

  • A coordinate covalent bond is formed when both electrons forming the bonding pair of electrons are provided by the same atom.

  • An atom with a lone pair of electrons (non-bonding pair of electrons) is capable of forming a coordinate covalent bond.

  • Once the covalent bond is formed it is impossible to distinguish the origin of the electrons so a coordinate covalent bond is exactly the same as any other normal covalent bond.

Lewis Structures and the Coordinate Covalent Bond1

Consider the Lewis Structure (electron dot diagram) for a molecule of water:

  • •    
•  
•  
O   •
 
H
     
  H    
  An atom of oxygen has 6 valence electrons which are represented as black dots () in the Lewis structure (electron dot diagram) for water.
Each atom of hydrogen has 1 valence electron which is represented as a red dot () in the Lewis structure (electron dot diagram) for water.
The central oxygen atom shares 1 valence electron with each hydrogen atom so that it now has a share in 8 valence electrons (the stable Noble Gas electron configuration of neon).
The hydrogen atoms now have a share in 2 valence electrons which gives them the stable electron configuration of the Noble Gas helium.

In the water molecule above, both O-H covalent bonds are of the 'normal' or 'conventional' type because each atom contributes 1 electron to be shared between the two atoms.

Note that the central oxygen atom is surrounded by 2 bonding pairs of electrons, and, 2 lone pairs (non-bonding pairs) of electrons.

  • •    
•  
•  
O   •
 •
H
  • •    
  H    
  The bonding pairs of electrons are shown as black dots () in the Lewis structure (electron dot diagram) for water.
The lone pairs of electrons are shown as blue dots () in the Lewis structure (electron dot diagram) for water.

If a species (atom or ion) could react with a water molecule in such a way that the oxygen atom shared both the electrons in one of its lone pairs with the other species without this other species sharing any electrons with the oxygen atom, then a covalent bond would form and this covalent bond would be called a coordinate covalent bond (or a dative bond).

What kind of species could react with water in this way?
A hydrogen atom that has lost its valence electron to form a positively charged hydrogen ion could.

H atom proton
(hydron)
+ electron
H• H+ + e-

The proton (hydron2) has no electrons of its own to share with the central oxygen atom in a water molecule, but, if the oxygen atom shares a lone pair (non-bonding pair) of electrons with the H+ ion then the hydrogen gains a share in 2 electrons and achieves the stable electron configuration of the Noble Gas helium:

    • •    
H+
O   •
  •
H
    • •    
    H    
  O atom has provided both electrons to be shared with H+ ion, this is a coordinate covalent bond.
All the electrons are identical so all 3 of the O-H bonds are identical. All 3 of the O-H bonds are covalent as a result of electrons being shared NOT transferred between the atoms.

This is important because we need to know where to place the positive charge, does it stay with the hydrogen ion, does it move to the oxygen atom, or does it belong to the new molecule?
The bond between the oxygen and hydrogen ion is clearly covalent and NOT ionic, that is, there is sharing of 2 electrons between these two atoms so the positive charge does not belong on the hydrogen atom.
The oxygen atom has not lost any valence electrons so it will not carry a positive charge.
Therefore the positive charge must be shown as 'belonging' to the whole molecule.
We do this by surrounding the whole molecule in square brackets, and adding the charge as a superscript outside the brackets:

[           ] +
    • •      
H •  
•  
O   •
  •
H  
    • •      
    H      

This is the hydronium ion (or oxonium ion or hydroxonium ion), H3O+.

Coordinate covalent bonds can also be present in neutral molecules.

Consider the oxygen molecule, O2.

  • •   • •  

O
O
Each oxygen atom contributes 1 electron to each of the bonding pairs of electrons resulting in the formation of 2 'normal' covalent bonds.
The result is a double covalent bond between the two oxygen atoms, O=O

Besides the 2 bonding pairs of electrons, each oxygen atom also has 2 lone pairs of electrons (non-bonding pairs of electrons).

Consider now that an oxygen molecule, O2, could react with an oxygen atom in the atmosphere to form the ozone molecule, O3.
Each oxygen atom in the O2 molecule has achieved an octet of electrons, the stable electron configuration of the Noble Gas neon, so the only way to add a third oxygen atom to the molecule is for one of the oxygen atoms in O2 to contribute both electrons in a lone pair of electrons to the new oxygen atom, that is, to form a coordinate covalent bond:

  • •   • •   • •  

O • • • • O
O
          • •  
Notice that one of the oxygen atoms from the original O2 molecule shown in black has contributed both electrons to the blue oxygen atom.
The blue oxygen atom has not contributed electrons to this bonding pair of electrons, so this is a coordinate covalent bond (dative bond).

However, once the coordinate covalent bond has been formed it is in no way different to a 'normal' covalent bond.
By looking at this Lewis structure for O3 we could say that the single bond will always be the coordinate covalent bond.3

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Worked Examples of Coordinate Covalent Bonds

Example 1. Draw the Lewis Structure (electron dot diagram) for the ammonium ion, NH4+, and identify any coordinate covalent (dative) bond formed, if any.

  1. Identify the central atom : N
    Nitrogen belongs to Group 15 (VA) of the Periodic Table therefore a nitrogen atom has 5 valence electrons
    Nitrogen belongs to Period 2 of the Periodic Table therefore it obeys the octet rule (will form molecules in order to achieve 8 electrons in its valence shell), which means that it can provide 3 of its valence electrons to be shared with another atom which will leave 1 lone pair of electrons left on the nitrogen atom in a molecule:
      • •  
    N
       
    Note the lone pair of electrons which are available to form a coordinate covalent bond if required.

  2. Identify the surrounding atoms: H
    H
    A hydrogen atom has only 1 valence electron and it will share that electron with another atom in order to achieve the electron configuration of the Noble Gas helium

  3. Note that it is only possible for 1 nitrogen atom to form 'normal' covalent bonds with each of 3 hydrogen atoms, corresponding to the ammonia, NH3, molecule:
        • •    
    H
    N
    H
           
        H    
    The valence shell of each hydrogen atom is full.
    The valence shell of the nitrogen atom is full.
    Note the lone pair of electrons on the nitrogen atom which are available to form a coordinate covalent bond IF the next atom does NOT contribute any electrons to be shared with the nitrogen atom.

  4. Recognise that the only way to covalently bond a fourth hydrogen to this molecule is if the hydrogen atom shares the lone pair of electrons that the nitrogen atom provides.
    HH+ + e- Since a hydrogen atom can only have a maximum of 2 electrons in its valence shell, the hydrogen atom must first lose its valence electron to form a positively charged ion before it can covalently bond to the nitrogen atom.

  5. Draw the Lewis structure (electron dot diagram) for the molecule:
        H    
        • •    
    H
    N
    H
           
        H    
    Count the number of valence electrons in the whole molecule: 8
    Determine the number of valence electrons there should be if each atom contributed all its valence electrons:
    5 (from N) + 4 (1 from each 4) = 9
    This molecule has 1 less electron than it should have (9 - 8 = 1), that is, there is 1 less negative charge than there should be, so overall this "molecule" is a positive ion with a charge of +1 (or just +)

  6. Draw the Lewis Structure (electron dot diagram) for the ion NH4+:

    [           ] +
        • •      
    H •  
    •  
    N   •
      •
    H  
        • •      
        H      
    Note that the Lewis structure (electron dot diagram) for the molecule is positioned within square brackets, and the positive charge is shown as a superscript outside the brackets.
    Note that once the coordinate covalent bond has been formed, there is no way to distinguish it from the other 'normal' covalent bonds.

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Example 2. Draw a Lewis Structure (electron dot diagram) for the carbon monoxide, CO, molecule and identify any coordinate covalent bonds, if any.

  1. Identify (select)4 the central atom : O
    Oxygen belongs to Group 16 (VIA) of the Periodic Table so it has 6 valence electrons.
    Oxygen belongs to Period 2 of the Periodic Table so it obeys the octet rule (achieves a share in 8 valence electrons in order to acquire the electron configuration of the Noble Gas neon)
      • •  

    O
       
    Note that an oxygen atom only needs a share in 2 more electrons in order to complete its valence shell.
    Note that there are also 2 lone pairs of electrons which could be used to form coordinate covalent bonds if required.

  2. Identify the surrounding atoms: C
    Carbon belongs to Group 14 (IVA) of the Periodic Table, it has 4 valence electrons.
    Carbon belongs to Period 2 of the Periodic Table so it will obey the octet rule (will achieve a share in 8 valance electrons to acquire the electron configuration of the Noble Gas neon):
       
    C
       
    Note that a carbon atom needs a share in 4 more electrons in order to complete its valence shell.
    Note that the oxygen atom above has only 2 electrons available to share with a carbon atom in the formation of 'normal' covalent bonds.

  3. Draw a Lewis structure using 'normal' covalent bonds only:
      • •      

    O
    C
    Note that the oxygen atom and the carbon atom have each contributed 1 electron to a bonding pair of electrons, and we have drawn 2 bonding pairs of electrons between them (this a double covalent bond).

    But is this a satisfactory description of the CO molecule?
    The oxygen atom has a share in 8 valence electrons.
    The carbon atom, however, only has a share in 6 valence electrons. It needs to gain a share in 2 more electrons without contributing any electrons itself, that is, carbon needs to share one of oxygen's lone pair of electrons.
    We need to make a coordinate covalent bond between the oxygen and carbon atoms which will transform the double bond we have drawn into a triple bond.

  4. Draw the completed Lewis structure for the CO molecule:

    O • •

    C
    Note that the oxygen atom still has a share in 8 valence electrons.
    Now the carbon atom also has a share in 8 valence electrons.

    Remember that although we have drawn the Lewis structure to clearly show the origin of each electron in the molecule using black and blue dots so that we can see how the coordinate covalent bond is formed, once all the covalent bonds have formed there is no way to identify which of the three bonds between the carbon and oxygen atom is the coordinate covalent bond because all three bonds are identical.


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1The following discussion does not take into account the shape or geometry of the molecules.
Refer to the Shapes of Molecules tutorial for a discussion of molecular shape.

2The preferred IUPAC name for the positively charged hydrogen ion, H+, is the hydron, but you are probably more likely to see, and use, the term proton.
99.99% of naturally occurring hydrogen atoms on earth are made up of 1 proton in the nucleus (zero neutrons) and 1 valence electron (the protium atom), so losing the electron results in a "naked" proton.
Only 0.01% of naturally occurring hydrogen atoms are made up of a nucleus containing 1 proton and 1 neutron and 1 valence electron (the deuterium atom), so losing the electron results in a nucleus containing 1 proton and 1 neutron (the deuteron ion).
The term hydron therefore refers to positively charged ions of the naturally occuring isotopic mix of hydrogen atoms.

3This is not really a very satisfactory description of the O3 molecule.
If this were a good description then we would see a short double bond and a longer single bond in each molecule of ozone.
Experiments have determined that the bonds between the oxygen atoms are the same length, 0.128 nm, and this length is about half-way between that expected for a single and a double bond.
The O3 molecule is described as a resonance hybrid of two forms.

4When there are only 2 atoms in the molecule (a diatomic molecule) it really doesn't matter which atom you select to be the 'central' atom.

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