Title: Lesson 6 Complex Ions
Learning Objectives:
• Explain and use the terms ligand/complex/complex ion and ligand substitutions.
• Describe the formation of complexes in terms of dative (coordinate) bonding and shapes
of complexes.
Transition metal ions in solution
• When a transition metal compound such as cobalt chloride is dissolved
in water it becomes Co3+ and Cl- ions.
• Transition metal ions have a high charge density.
• The Co3+ ions attract water molecules around them to form something
we call a complex.
The bonds between
the water molecules
and the positive ion is
called a COORDINATE
BOND (Dative Bond).
Charged or not?
• If the complex is charged then it is called a
For example
How do we write complexes down?
• The symbol formula for this complex is
• Remember square brackets
• Remember overall charge
• Its name is
cobalt III
AfL – using whiteboards
Write the symbol formula for:
Ligands and bonding
• The molecules which bond to the metal ions are called
• Complexes form because the
of the ligand can bond
with the metal ion.
• Transition metals have more
so they can
bond with a large number of ligands.
• The number of ligands is called the
AfL – test your understanding
• What is the ligand and coordination number in each case:
Coordinate (dative) bonding
• When the lone pairs of the ligand bond to the metal we call
this
.
• It works with d-metals because of the metal’s electronic
structure. Empty orbitals mean that the lone pair electrons
have somewhere to go.
Shapes of complexes
• The shapes of complexes are determined by the number and
type of ligands.
• Complexes with 6 ligands are usually octahedral
Shapes of complexes
• The shapes of complexes are determined by the number and
type of ligands.
• Complexes with 4 ligands are usually tetrahedral
Square planar
• Some complexes are square planar.
Linear
• Some complexes are linear.
Polydentate ligands
• Some ligands can
bond to the metal
more than once.
• Each molecule of
ethandioate bonds to
the metal twice.
• Each molecule of
EDTA4- bonds to the
metal six times.
Polydentate ligands
• Why can they bond to a metal more than once?
• More than one
lone pairs
• EDTA4- has 6
atoms with lone
pairs (2 nitrogen
and 4 oxygen)
Chelating Agents
• Because EDTA4- can bond 6 times it is described as a
hexadentate (six toothed) ligand.
• It can grip the central ion in a six-pronged claw called a
CHELATE.
• Chelates are important in foods and biological systems. EDTA4can remove transition metals from solutions.
Ligand Exchange
• In aqueous solution, water molecules generally act as ligands but these can be replaced
in a process known as ligand exchange.
• Complexes often have distinctive colours so can be used in qualitative analysis.
Example: Test for iron (III) ion
• Left – [Fe(H2O)6]3+ - Orange colour
• Centre – Potassium thiocyanate solution (SCN- ion)
• Right – Complex formed when reactants on the left
and centre react together. One of the water ligands has
been replaced with the thiocyanate ion. (SCN-) 
FORMING BLOOD RED [Fe(SCN)(H20)5]2+
Lability
• Ligands can be exchanged in substitution
reactions because they are labile (can attach
on and off).
Other substitution reactions
[Cu(Cl)4]22+
Ligands

A ligand is a species with a
lone pair


Often negative ions
Common ligands include:






Water, H2O
Ammonia, NH3
Chloride, ClHydroxide, OHCyanide, CNThiocyanate, SCNMain Menu
Practical - Making transition metal complexes

Add potassium thiocyanate solution to a solution of iron (III)

Add conc. HCl (fume hood!) to 1 cm3 of a strong solution of cobalt (II). Repeat but
use conc. NH3 instead (fume hood!).

Add dilute NH3 to a copper (II) solution until no further change occurs

Record all observations

Suggest possible structures for the complexes you have formed and possible reaction
equations
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Transition metals and their ions – Important Catalysts

Transition elements and their compounds/complexes can act as catalysts.
(They allow chemical processes to proceed at an economic rate)
Two forms:
Heterogenous (different state to the reactants)
Homogenous (same state to the reactants)

 Transitional
metals have varying oxidation numbers and are able to coordinate to other molecules/ions to form complex ions
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The magnetic properties of Transition Metals and their
compounds

Every spinning electron in an atom or molecule can behave as a tiny magnet.

Electrons with opposite spins have no net magnetic effect.

Unpaired electrons found in the d block of transitions metals can be aligned to
lead to magnetic effects.
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Responses to externally applied magnetic fields

Diamagnetism – a property of all materials and produce a very week
opposition to an applied magnet

Paramagnetism – only occurs with substances with unpaired electrons.
Stronger than diamagnetism. Magnetisation is proportional to the applied field
and in the same direction.

Ferromagnetism – largest effect. Can produce magnetisation greater than the
applied field
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Iron, Cobalt and Nickel are ferromagnetic

The unpaired d electrons in large numbers of atoms line up with parallel spins
in regions called domains.

Domains are generally randomly orientated but will be ordered if exposed to
an external magnetic effect.

After external magnet is removed, domains remain aligned due to the long
range interaction between unpaired electrons in different atoms.
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Transition metals and their complexes show
paramagnetic properties

Transition metals with unpaired
electrons will be pulled into the
magnetic field.

Non Transition metals will moved out of
the magnetic field in the opposite
direction.

Paramagnetism increases with the
number of unpaired electrons (max at
Chromium)
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A Challenge

Working in small groups, complete the following activity on the structure of
some cobalt complexes.
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Key Points

Transition metals form ions with partially filled d-orbitals

Ligands are species with lone pairs

Ligands will form dative covalent bonds to transition metals forming ‘complex
ions’
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