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How do we see colour?
Most transition metal compounds appear coloured. This is
because they absorb energy corresponding to certain parts
of the visible electromagnetic spectrum. The colour that is
seen is made up of the parts of the visible spectrum that
aren’t absorbed.
For example, a red
compound will
absorb all
frequencies of the
spectrum apart from
red light, which is
transmitted.
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What happens when light is absorbed?
In transition metal ions, the d sub-level is only partially filled.
This means that electrons can move between d orbitals.
In a transition metal
complex, the relative
energies of the d orbitals
change. Electrons can
be promoted to higher
energy orbitals.
For electrons to be promoted, they need to absorb light
energy of a particular frequency. This frequency depends
on the precise difference in energy between the d orbitals.
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Why do d orbitals change in energy?
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Factors affecting colours
The colour of a transition metal compound is determined by
the difference in energy between its d orbitals.
This can be affected by several factors:

size and type of ligands

coordination number

strength of metal–ligand bonds

oxidation state.

complex shape
[Cr(H2O)6]3+ [Ni(H2O)6]2+ [Fe(H2O)6]2+
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[Fe(H2O)6]3+
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Colours of complexes
A transition metal will appear different colours in complexes
with different ligands. For example:
[Cu(H2O)6]2+
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[CuCl4]2–
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Complex colours: true or false?
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Determining concentration
Ultraviolet–visible spectroscopy can be used to determine
the concentration of a transition metal complex solution.
A UV-Vis spectrometer
passes different frequencies
of light through a sample.
Some of the light is
absorbed while the rest
passes through. A detector
measures the absorbance
of the sample.
The amount of light absorbed is proportional to the
concentration of the absorbing species.
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UV–Vis spectroscopy
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Improving UV–Vis spectroscopy 1
Some transition metal complex ions have a very pale colour.
Accurate quantitative determination of the concentration of
these solutions is difficult, because the difference in
absorption between the different concentrations is too small.
Replacing the ligands in a complex
changes its colour. This can
increase the absorption of the
transition metal species, allowing
lower concentrations to be analysed
using UV–Vis spectroscopy.
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Improving UV–Vis spectroscopy 2
[Fe(H2O)6]2+ is a pale green colour. If the water ligands are
replaced by other types of ligands, a much stronger
coloured solution is produced. The absorption values
increase by 103, allowing analysis of lower concentrations.
[Fe(H2O)6]2+ + 3bipy
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[Fe(bipy)3]2+ + 6H2O
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Hydrolysis reactions
Transition metal salts dissolve in water to form aqua ions –
complexes with water molecule ligands. Some of these
aqueous complexes are acidic, for example [Fe(H2O)6]3+.
This is because the Fe3+ ion is strongly polarizing and
weakens the O–H bonds in the water ligands. The complex
ion releases an H+ ion, producing an acidic solution.
The reaction is called hydrolysis because the water molecule
is being split.
The general equation for this reaction is:
[M(H2O)6]3+ + H2O
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[M(H2O)5OH]2+ + H3O+
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Why are some aqua ions less acidic?
A solution of [Fe(H2O)6]3+ is highly acidic, whilst a solution
of [Fe(H2O)6]2+ is only very weakly acidic. This is because
the polarizing power of the metal ion depends on its size
and charge.
Smaller, more highlycharged metal ions exert a
greater polarizing effect on
the water ligands, so that
more O–H bonds break,
releasing H+ ions.
As a general rule, M3+ ions are significantly more acidic than
M2+ ions.
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Order the complex ions by acidity
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Hydrolysis of M2+ ions
A series of hydrolysis reactions can occur until the overall
charge on a complex is 0. For example, in transition metal
2+ ions, two H+ ions are removed.
[M(H2O)6]2+(aq) + H2O(l)
[M(H2O)5OH]+(s) + H3O+(aq)
[M(H2O)5OH]+(s) + H2O(l)
[M(H2O)4(OH)2](s) + H3O+(aq)
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Hydrolysis of M3+ ions
Three H+ ions are removed from transition metal 3+ ions.
[M(H2O)6]3+(aq) + H2O(l)
[M(H2O)5OH]2+(aq) + H3O+(aq)
[M(H2O)5OH]2+(aq) + H2O(l)
[M(H2O)4(OH)2]+(aq) + H3O+(aq)
[M(H2O)4(OH)2]+(aq) + H2O(l)
[M(H2O)3(OH)3](s) + H3O+(aq)
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Reactions with bases
If a base such as ammonia or hydroxide ions is added to a
solution of transition metal aqua ions, H+ ions are removed
from the water ligands until there is no overall charge on the
complex.
The final product is an uncharged, insoluble metal hydroxide
that forms a precipitate.
This reaction occurs in a series of steps, depending on
whether the metal is a 2+ or 3+ ion.
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Reactions with carbonate ions
Sodium carbonate acts as a base with M3+ aqua ions to
produce a hydrated metal hydroxide.
2[Cr(H2O)6
]3+
(aq) + 3CO3
2–
(aq)
2[Cr(H2O)3(OH)3](s) +
3CO2(g) + 3H2O(l)
However, adding sodium carbonate to M2+ ions produces an
insoluble metal carbonate. The M2+ ions are less acidic, and
carbonate ions are unable to remove protons from the water
ligands, so they displace the ligands instead.
[Cr(H2O)6]2+(aq) + CO32–(aq)
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CrCO3(s) + 6H2O(l)
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Adding bases to complexes
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Balance the equations
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Amphoteric character
Some metal hydroxides can react as both an acid and as a
base and are known as amphoteric.
For example, chromium hydroxide will react in the
following ways:
With strong acids:
Cr(H2O)3(OH)3 + 3H3O+
[Cr(H2O)6]3+ + 3H2O
With strong, excess alkali:
Cr(H2O)3(OH)3 + 3OH–
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[Cr(OH)6]3– + 3H2O
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Reaction definitions
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Ligand substitution
A ligand substitution reaction occurs when a ligand in a
complex ion is replaced by another type of ligand molecule.
When concentrated hydrochloric acid is added
to a solution of hexaaquacopper(II), chloride
ions replace the water molecules as ligands.
[Cu(H2O)6]2+ + 4Cl–
[CuCl4]2– + 6H2O
Chloride ions are much larger than water
molecules so only four can fit around the
copper ion. This means that the complex
changes shape from octahedral to
tetrahedral. The colour of the complex
changes from blue to green.
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Ligand substitution reactions
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Reactants and conditions
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Stability constants
Hexaaquacopper(II) undergoes ligand substitution with
ammonia in stages.
At each stage, one water molecule is replaced by one
ammonia molecule, until four molecules of water have been
replaced by four molecules of ammonia.
Each of these stages is an equilibrium reaction.
[Cu(H2O)6]2+ + NH3
[Cu(NH3)(H2O)5]2+ + H2O
[Cu(NH3)(H2O)5]2+ + NH3
[Cu(NH3)2(H2O)4]2+ + H2O
[Cu(NH3)2(H2O)4]2+ + NH3
[Cu(NH3)3(H2O)3]2+ + H2O
[Cu(NH3)3(H2O)3]2+ + NH3
[Cu(NH3)4(H2O)2]2+ + H2O
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Stability constants
For each stage of a reaction, an expression for the equilibrium
constant can be written. This equilibrium constant for the
overall reaction is called the stability constant: Kstab.
[Cu(H2O)6]2+(aq) + 4NH3(aq)
Kstab =
[Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)
[Cu(NH3)4(H2O)22+]
[Cu(H2O)62+] + [NH3]4
Note that the square brackets now mean concentration in
mol dm-3.
The concentration of water is left out because it is in great
excess and its concentration is almost constant.
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Comparing stability constants
The stability constant, Kstab, is the equilibrium
constant for the formation of a complex ion from its
constituent ions in solution.
Kstab values show how stable a complex ion is. Complex
ions with large Kstab values are easily formed.
Whether a ligand substitution will occur can be predicted
from Kstab values, by comparing that of the current complex
ion with the value for the substituted complex ion. The
most stable ion is most likely to occur.
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The chelate effect
Complex ions containing multidentate ligands such as
EDTA are called chelates. They have much larger Kstab
values and are far more stable than complex ions
containing unidentate ligands.
This is because of the effect of entropy. When a
multidentate ligand replaces unidentate ligands in a
complex, it releases many molecules, increasing the
entropy. The reverse reaction involves a large decrease in
entropy, which is why it is so unfavourable.
[M(H2O)6]2+ + EDTA4–
2 species
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[M(EDTA)]2– + 6H2O
7 species
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Predicting ligand substitution
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Haemoglobin
Haemoglobin is the molecule that causes blood to appear
red. It carries oxygen from the lungs to cells in the body.
Haemoglobin contains an Fe2+ ion
which forms a haem complex with a
tetradentate ligand called porphyrin.
It also binds to a unidentate globin
molecule. One coordination site is
left that can bind loosely to an
oxygen molecule.
Oxygen is a poor ligand that is easily released to cells, where
its concentration is low. Ligands that can form stronger
bonds with the Fe2+ ion, such as carbon monoxide, bind
irreversibly and destroy haemoglobin’s ability to carry
oxygen. These substances are toxic.
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Redox reactions
Transition metals are able to exist in many different oxidation
states, which is why they often undergo redox reactions.
Oxidation of transition metals occurs most
easily in alkaline solution. This is because
negative ions tend to form in alkaline
solution and it is easier to lose electrons
from a negatively-charged species.
Reduction of transition metals occurs most
easily in acidic solution.
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Oxidation of cobalt(II)
In ammoniacal solution, Co2+ is oxidized to Co3+ by oxygen
in the air. Several reactions occur because ammonia acts
as both a base and a ligand:
[Co(H2O)6]2+ + 2OH–
[Co(H2O)4(OH)2] + 6NH3
4[Co(NH3)6]2+ + O2 + 2H2O
[Co(H2O)4(OH)2] + 2H2O
[Co(NH3)6]2+ + 2OH– + 4H2O
4[Co(NH3)6]3+ + 4OH–
Co2+ can also be oxidized by hydrogen peroxide (H2O2) after
adding an alkali such as sodium hydroxide.
2[Co(OH)6]4– + H2O2
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2[Co(OH)6]3– + 2OH–
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Reduction of chromium(VI)
In aqueous solution, chromium has an oxidation state of 6+.
It exists in alkaline solution as CrO42– and as Cr2O72– in
acidic solution.
2CrO42– + 2H+
Cr2O72– + H2O
Chromium(VI) can be reduced to Cr3+ and Cr2+ by zinc in
acid solution .
Cr2+ is easily oxidized to Cr3+ in the presence of oxygen,
but hydrogen is produced during the reduction, which
excludes air.
Cr2O72– + 14H+ + 3Zn
Zn + 2Cr3+
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2Cr3+ + 7H2O + 3Zn2+
Zn2+ + 2Cr2+
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Identify the reaction
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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