Transition Metals
Definitions
• Complex: A metal ion surrounded by
ligands.
• Ligand: an electron pair donor i.e. a
molecule or ion joined onto the metal ion
by a dative covalent bond to the metal.
• Coordination number: The number of
atoms directly joined to a transition metal
by a coordinate (dative covalent) bond.
Typical ligands
• Unidentate H2O: :NH3 :Cl• Bidentate C2O42- H2NCH2CH2NH2
• Multidentate EDTA4• Watch out for the chelate effect.
Shapes
• Six-coordinate complexes are octahedral.
eg Cu(H2O)62+ Co(NH3)63+
• Four-coordinate complexes are usually
tetrahedral eg CoCl42- but a few are
square planar eg Ni(CN)42-.
• Two-coordinate complexes are linear eg
AgCl2-.
Colour
• Most transition metal colours are due to
d-d electron transitions. The energy gap
between the split d-orbitals corresponds to
visible light (E=hf).
• Some of the really strong colours are due
to charge transfer (such as in MnO4-).
Changing colour
• Anything that changes the energy
difference between the d-orbitals causes a
change in colour:
• Oxidation state
• Ligand
• Coordination number
Measuring concentration by colour
A measurement of colour intensity (using a
colorimeter) can measure the
concentration of a solution.
Sometimes a special complex is formed so
that the colour is more intense.
This is known as UV/visible spectroscopy.
eg Fe(H2O)63+ + SCN-
Fe(H2O)5(SCN)2+ + H2O
Reaction types
• The reactions of the transition metals
come under three headings (but
sometimes more than one occurs at the
same time!).
• Ligand substitution
• Hydrolysis (the acidity reaction)
• Redox
Ligand substitution
• One kind of ligand is replaced by another.
For example,
Cu(H2O)62+ + 4Cl- → CuCl42- + 6H2O
Hydrolysis (the Acidity reaction)
• One or more hydrogen ions is removed.
For example,
Cu(H2O)62+(aq) + 2OH-(aq) →
Cu(H2O)4(OH)2(s) + 2H2O(l)
Redox
The oxidation state of the metal is changed
by adding or removing one or more
electrons
For example,
Co(NH3)62+ → Co(NH3)63+ + e-
Titanium
• Mainly known for its chloride, TiCl4, as part
of the Kroll process for extraction of
titanium from its ores.
• The chloride is covalently bonded.
• Titanium oxide, TiO2 , is used as a white
pigment.
Vanadium
• Most stable form is V(IV) eg as VO2+.
• V(V) can be reduced in acidic conditions
(eg Zn / HCl)
VO2+ → VO2+ → V(H2O)63+ → V(H2O)62+
V(H2O)62+ is unstable in air since it is
oxidised by oxygen.
Chromium
• Cr(H2O)63+
Cr(H2O)3(OH)3
Cr(OH)63-
>>>>Increasingly alkaline conditions>>
Cr2O72- CrO42<<<<Increasingly alkaline conditions<<<<
Manganese
• Mn(H2O)62+
II
v. pale pink
MnO2
MnO42- MnO4IV
VI
VII
brown
deep purple
MnO4- is a very strong oxidising agent.
Use acidic conditions. For titrations, make sure you add
sufficient sulfuric acid to take Mn all the way to Mn2+.
Otherwise you get incomplete reduction
(the brown solid, MnO2 is formed, rather than Mn2+).
Iron
• Fe(H2O)62+ is non-acidic in water (green).
• Pure Fe(H2O)63+ is a lilac colour but on
contact with water goes rusty brown.
• Fe(H2O)62+ forms Fe(OH)2 (a green solid)
with NaOH but it goes brown (forming
Fe(OH)3 on standing in air.
Cobalt
add ammonia solution
add hydrogen peroxide (or air)
Co(H2O)62+ → Co(NH3)62+ → Co(NH3)63+
pink
Add
conc
HCl
brown / yellow
Add NaOH or NH3 (aq)
CoCl42blue
Co(OH)2
blue-grey
orange
Nickel
add ammonia solution
Ni(H2O)62+
Ni(NH3)62+
green
Add
conc
HCl
blue
Add
NaOH
or NH3 (aq)
NiCl42-
Ni(OH)2
blue
pale green
Add
NaCN (aq)
Ni(CN)42a square planar complex!
Copper
add ammonia solution
Cu(H2O)62+
Co(NH3)4(H2O)22+
blue
Add
conc
HCl
deep blue / violet
Add NaOH or NH3 (aq)
CuCl42-
Cu(OH)2
yellow-green
pale blue
Silver
Small quantity of NaOH, NH3(aq)
Ag+(aq)
excess NH3 (aq)
Ag(NH3)2+
Ag2O(s)
Cl-
AgCl(s)
excess
Cl-
AgCl2-
NaCN(aq)
Na2S2O3(aq)
Ag(S2O3)23-
Ag(CN)2-
Uses of transition metals
and their complexes
• V2O5 is used as a catalyst in the Contact
Process (manufacture of sulfuric acid).
• Cis-platin (a platinum complex) is a very
effective anti-cancer drug.
• Fe2+ is an important part of haemoglobin.
• Ag(NH3)2+ is Tollen’s reagent (test for
aldehydes).
• Ag(S2O3)23- is formed during photographic
processing.
• Ag(CN)2- is used in electroplating.