Co-ordinate Covalent Bonds and

Resonance Structures

Co-ordinate Covalent Bond

• A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei.

• In a simple covalent bond, each atom supplies one electron to the bond - but that doesn't have to be the case.

• A co-ordinate bond is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.

Example: NH4+

 

Rules for molecules with an overall charge:

1. When figuring out the number of electrons available (ie. counting the valence electrons), add or subtract electrons as indicated by the charge.

2. Create the Lewis structure the same as always

3. Put [square] brackets around the structure

4. Write the charge as a superscript

Do you remember polyatomic ions?

Formula Name Formula Name

NH4+/NH3

Ammonium/Ammonia CrO4-2 Chromate

C2H3O2- Acetate Cr2O7

-2 Dichromate

CH3COO- Acetate MnO4- Permanganate

CN- Cyanide MnO4-2 Manganate

CO3-2 Carbonate NO2

- Nitrite

HCO3- Bicarbonate NO3

- Nitrate

C2O4-2 Oxalate OH- Hydroxide

ClO- Hypochlorite PO4-3 Phosphate

ClO2- Chlorite SCN- Thiocyanate

ClO3- Chlorate SO3

-2 Sulfite

ClO4- Perchlorate SO4

-2 Sulfate

S2O3-2 Thiosulfate HSO4

- Hydrogen sulfate

BrO- Hypobromite IO3- Iodate

AsO2-3 Arsenite SeO4

-2 Selenate

BrO3- Bromate HSO3

- Hydrogen sulfite

Try it: CO3-2

C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-

O O

C

O

Try it: CO3-2

C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-

O O

C

O

Try it: CO3-2

C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-

O O

C

O

Try it: CO3-2

O O

C

O

All 24 e- have been used, buy C does NOT have a complete octet….. Need to make a double bond….Does it matter which O it makes the double bond with?

Try it: CO3-2

O O

C

O

The bond can go here but it could also go between the C and either of the other Os

Finished Product

C CC

-2

Resonance Structures

• The Lewis structure for CO3 shows two different types of bonds, single and double.

• Double bonds also have a shorter bond length, the distance between the nuclei of the two atoms in the bond, than single bonds do. Thus, if the above Lewis structure for nitrate were correct, the carbonate ion would have one bond that is shorter and stronger than the other two.

• This is not the case. All three of the bonds in the carbonate ion are the same strength and the same length.

• Interestingly, the behaviour of the bonds suggests they are longer than double bonds and shorter than single bonds.

• They are also stronger than single bonds but not as strong as double bonds.

Resonance Structures (2)

• So the carbonate ion would actually 3 bonds, with the bond lengths between the characteristic single and double bond lengths

• The important points to remember about resonance forms are:

The molecule is not rapidly oscillating between different discrete forms

There is only one form of the each molecule, we need to draw two (or more) Lewis structures because a single structure is insufficient to describe the real structure

Drawing them:• Expressing resonance when drawing Lewis

structures is done by drawing each of the possible resonance forms and placing double-headed arrows between them

Carbonate ion:

C CC

-2

Try these:

1. O3

2. NO3-1

3. HCO2-1

4. CN-1

5. SO32-

6. BrO-1

7. PO43-

8. IO3-1

9. CHO2 -1

10. NO2+1