Complex Ions
and stuff like that.
Complex Ions
A complex ion is the exception to a
rule...
It is an ionic compound that has an
overall charge.
Complex ions are identified using square
brackets. [ ]
Complex Ions
Are soluble.
Usually formed from solutions containing
precipitates.
There are 7 you need to memorise.
Complex Ions
• Formed if a precipitate disappears when excess
reagent is added.
• Metal cations with several ligands attached.
• Ligands have a pair of non-bonding electrons e.g.
H20, NH3, OH-, SCN• Usually* the number of ligands is twice the charge on
the cation. E.g. Cu2+ forms [Cu(NH3)4]2+
* Except Al3+
Complex ions
Iron
Thiocyanate
Silver diamine
Aluminium
tetrahydroxide
Lead
tetrahydroxide
zinc
tetrahydroxide
Zinc tetraamine
FeSCN2+
Copper
tetraamine
[Cu(NH3)4]2+
[Ag(NH3)2]+
[Al(OH)4]-
[Pb(OH)4]2[Zn(OH)4]2[Zn(NH3)4]2+
Making "OH" Complexes
[Al(OH)4][Zn(OH)4]2[Pb(OH)4]2-
These hydroxide complexes are
made by adding excess
hydroxide to a hydroxide
precipitate.
Making NH3 Complexes
[Cu(NH3)4]2+
[Ag(NH3)2]+
[Zn(NH3)4]2+
These ammonia complexes are
made by adding excess
(ammonia) ammonium hydroxide
to a hydroxide precipitate.
[Al(OH)4]Aluminium ions + Sodium hydroxide =
Aluminium hydroxide ppt - Al(OH)3
Adding excess Sodium hydroxide =
Al(OH)3 + OH- --> [Al(OH)4]-
[Cu(NH3)4]2+
Copper ions + Ammonia =
When used in small
amounts OH- react,
when used in large
amounts (excess)
NH3 reacts.
Copper hydroxide ppt - Cu(OH)2
Adding excess Ammonia =
Cu(OH)2 + 2OH- --> [Cu(NH3)4]2+
add 2 drops of dilute NaOH solution
orange precipitate forms
Fe3+
Cloudy (precipitate);
Fe(OH)3 is NOT
soluble
New sample
add 2 drops KSCN solution
dark red solution confirms
Fe3+
Clear solution:
Product is
soluble
Observation
What it means
Species formed
Precipitate is formed
(solution becomes
cloudy)
A compound has
been formed (from
ions present) that is
not soluble.
The compound is not
charged. Use swap
and drop rule to find
formula for compound.
Example: Iron3+ ions
form a compound with
OH- ions.
Example:
Example: Mixing
Iron(III)nitrate solution
with sodium hydroxide
solution
Solution changes
colour and/or
becomes clear.
Example:
Iron(III)hydroxide
dissolves with
potassium thiocyanate
solution (red)
A complex ion has
been formed (from
ions present) that is
soluble.
Example: Iron3+ ions
form complex ions with
SCN- ions
(thiocyanate).
Fe(OH)3
The complex ion is
charged.
Learn formulae for 7
complex ions (square
brackets).
Example:
[Fe(SCN)]2+
Observation
What it means
Species formed
Precipitate is formed
(solution becomes
cloudy)
A compound has
been formed (from
ions present) that is
not soluble.
The compound is not
charged. Use swap
and drop rule to find
formula for compound.
Example: Iron3+ ions
form a compound with
OH- ions.
Example:
Example: Mixing
Iron(III)nitrate solution
with sodium hydroxide
solution
Solution changes
colour and/or
becomes clear.
Example:
Iron(III)hydroxide
dissolves with
potassium thiocyanate
solution (red)
A complex ion has
been formed (from
ions present) that is
soluble.
Example: Iron3+ ions
form complex ions with
SCN- ions
(thiocyanate).
Fe(OH)3
The complex ion is
charged.
Learn formulae for 7
complex ions (square
brackets).
Example:
[Fe(SCN)]2+
add 2 drops of dilute
NaOH solution.
no precipitate
NH4+, Na+
white precipitate forms
Al3+, Zn2+, Pb2+,
Mg2+, Ba2+
add NaOH solution,
heat, test gas with
red litmus.
add excess
NaOH
solution
precipitate
remains
Mg2+, Ba2+
white
precipitate
Ba2+
litmus goes
blue NH4+
precipitate
disappears
Al3+, Zn2+, Pb2+
add 2 drops, then
excess NH3 solution
Find the steps in the
scheme where the 7
complex ions are formed.
Hint: Look out for “excess”
amounts added and the
disappearance of a
precipitate.
white precipitate forms and
disappears, Zn2+
add dilute H2SO4
solution
colourless
solution
Mg2+
litmus
stays red
Na+
white precipitate forms
Al3+, Pb2+
add dilute H2SO4 solution
green precipitate forms
Fe2+
orange precipitate forms
Fe3+
colourless solution
Al3+
white precipitate
Pb2+
add 2 drops
KSCN
solution
dark red solution confirms
Fe3+
blue precipitate forms
Cu2+
add 2 drops, then excess
NH3 solution
blue precipitate then deep
blue solution
Cu2+
brown precipitate forms
Ag+
add 2 drops, then excess
NH3 solution
brown precipitate then
colourless solution Ag+
add excess
NaOH solution
precipitate
disappears
Al3+, Zn2+, Pb2+
Equations:
Al3+ + 4 OH-
→
[ Al(OH)4 ] -
Zn2+ + 4 OH-
→
[ Zn(OH)4 ] 2-
Pb2+ + 4 OH-
→
[ Pb(OH)4 ] 2-
Complete the
equations
and work out
the charge!
add 2 drops, then
excess NH3 solution
White precipitate forms
and disappears
Zn2+
Complete the
equation and
work out the
charge!
Equation:
Zn2+ + 4 NH3
→
[ Zn(NH3)4 ] 2+
add 2 drops KSCN
solution
Dark red solution confirms
Fe3+
Complete the
equation and
work out the
charge!
Equation:
Fe3+ + SCN-
→
[ FeSCN ] 2+
add 2 drops, then
excess NH3 solution
Blue precipitate, then
deep blue solution
Cu2+
Complete the
equation and
work out the
charge!
Equation:
Cu2+ + 4 NH3 →
[ Cu(NH3)4] 2+
add 2 drops, then
excess NH3 solution
Brown precipitate, then
colourless solution
Ag+
Complete the
equation and
work out the
charge!
Equation:
Ag+ + 2 NH3
→
[ Ag(NH3)2 ] +
Written questions for end of assessment
• You will be given a list of observations
• You need to methodically follow the flow charts and
make conclusions and equations as if you were
actually carrying it out in real life.
• Eg: Question 1
• No ppt with silver nitrate means sulfate or nitrate ion
• White precipitate with barium nitrate means it must
be sulfate ion
• Ba2+(aq) + SO42-(aq)  BaSO4(s)