The Transition Metals
23.1 The General Properties of Transition Metals:
- A transition metal forms at least one stable ion 23.1 The general properties of transition metals
with a part full d-shell of electrons. 23.2 complex formation and the shape of complex ions
- The d-block elements are Sc, Ti, V, Cr, Mn, Fe,
23.3 coloured ions
Co, Ni, Cu and Zn
- The transition elements are Ti, V, Cr, Mn, Fe, Co, 23.4 variable oxidation states of transition elements
Ni and Cu. 23.5 catalysis
- Sc is not a transition metal because it only
forms Sc3+ and that has no 3d electrons. Zn is also not a transition metal because it only form Zn2+
which has a full 3d subshell.
- Properties of transition metals:
o High densities
o High melting and boiling points
o Hard/Rigid
o Ductile
o Malleable
o Good conductors of heat and electricity
o Form coloured compounds (e.g., Cu2+(aq) is blue)
o Good catalysts (e.g., Fe in the Haber process)
o Variable oxidation states
o Form complex ions
23.2 Complex Formation and the Shape of Complex Ions:
- A compound or ion with a lone pair that can donate its electrons to a transition metal ion creating a
dative bond is called a ligand.
- A complex is a central metal atom or ion surrounded by ligands.
- The co-ordination number is the number of co-ordinate bonds to the central metal.
- Complex ions are written with a square bracket around them, the metal ion is written first followed by
any neutral ligands and then any negative ligands. The overall charge on the complex ion is then shown
outside the square brackets.
- H2O, NH3 and Cl- are all monodentate ligands, the Cl- ligand is bigger than the other two so only 4 can
fit around a metal ion, leading to a change in co-ordination number.
- Ions with co-ordination number 6 are usually octahedral e.g., [Fe(H2O]2+
- Ions with co-ordination number 4 are usually tetrahedral e.g. [CuCl4]2+
- Compounds with Ni or Pt with a co-ordination number of 4 tend to be square planar e.g., [Ni(CN)4]2
- Some complexes are linear such as [Ag(NH3)2]+. This is tollens reagent
- Ligands that form one dative covalent bond are called monodentate. Ligands that form two are called
bidentate and ligands that form more than two are called polydentate.
- Bidentate ligands:
o Ethane-1,2-diamine
o Ethandioate ion
o Benzene-1,2-diol
- EDTA is a ligand that binds in multiple places, if the original ion was aqueous, it will replace all of the
water molecules.
- [Cu(H2O)]2+(aq) + EDTA4-(aq) à [CuEDTA]2- + 6H2O(l)
- This is a very favourable reaction as there is a significant increase in entropy. Complexes with
polydentate ligands are favoured over monodentate ligands. This is called the chelate effect
, - Haem is an iron(II) complex with a multidentate ligand. Oxygen forms a co-ordinate bond to Fe(II) in
haemoglobin, enabling oxygen to be transported around the body. Carbon monoxide is toxic because it
replaces the oxygen datively bonded to Fe.
- Complexes show both cis/trans isomerism. A pair of cis/trans isomers have different chemical
properties. Cisplatin is a successful cancer drug while transplatin has no effect.
- They also show optical isomerism – where two isomers are non-superimposable mirror images of each
other. They have identical chemical properties, but one will rotate a plane of light clockwise, and the
other anti-clockwise.
23.3 Coloured Ions:
- Most transition metal compounds are coloured.
- The colour that a solution looks, is the colours that are not absorbed by the solution.
- Transition metals have partially filled d-orbitals, electrons are able to move from one d-orbital to
another of a higher energy level (this is called an excited state). The difference in energy between the
different d-orbital corresponds to a specific colour of light.
- The energy difference between the ground state and the excited state is given using the Planck’s
constant equation:
- ∆𝐸 = ℎ𝑣 E is the energy, v is the frequency and h is Planck’s constant
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- ∆𝐸 = c is the velocity of light and 𝜆 is the wavelength
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- Purple is high energy and frequency, and red is low energy and frequency
- The colour of the transition metal depends on the energy gap, ∆𝐸. This can be altered by changes in
oxidation state, co-ordination number and ligand.
- A colorimeter can be used to show the concentration of coloured ions in solution, and a calibration
graph is produced by measuring solutions of known concentrations. This can then be used to determine
the concentration of unknown substances.
23.4 Variable Oxidation States of Transition Elements:
- Transition metals can have a wide range of oxidation states. Oxidation Species Colour
+
- Tollen’s reagent [Ag(NH3)2] , does not react with ketones as Number
they can’t be oxidised. But Ag+ is reduced to Ag when it 5 VO2+(aq) Yellow
2+
oxidises aldehydes to carboxylic acids. 4 VO (aq) Blue
3+
- How easily an oxidation state of a transition metal is 3 V (aq) Green
2+
changed depends on the pH and the ligands present. 2 V (aq) Violet
- Generally, it is easier to reduce a transition metal in acidic conditions and to oxidise a transition metal in
alkaline conditions.
- Where M is the transition element:
o [M(H2O)6]2+ in acidic conditions
o [M(H2O)4(OH)2] in neutral conditions
o [M(H2O)2(OH)4]2- in alkaline conditions
- Solutions of Fe2+ are often made in acid to slow the oxidation to Fe3+
- Fe2+/MnO4- Titration:
o 8H+ + MnO4- + 5Fe2+ à 5Fe3+ + Mn2+ + 4H2O
o MnO4- goes in the burette, the Fe2+ is put in the conical flask with sulfuric acid.
o An indicator is not required because there is a colour change due to a redox reaction.
o The purple MnO4- becomes colourless when it reacts, when all of the Fe2+ has reacted a slight
pink colour appears.
- Fe /C2O42- Titration:
2+
o 5C2O42- + 2MnO4- + 16H+ à10CO2 + 2Mn2+ + 8H2O
