Download Transition metals 2

AN INTRODUCTION TO
TRANSITION METAL
COMPLEXES
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
STABILITY CONSTANTS
Definition
The stability constant, Kstab, of a complex ion is the equilibrium constant
for the formation of the complex ion in a solvent from its constituent ions.
STABILITY CONSTANTS
Definition
The stability constant, Kstab, of a complex ion is the equilibrium constant
for the formation of the complex ion in a solvent from its constituent ions.
In the reaction
[M(H2O)6]2+(aq) + 6X¯(aq)
[MX6]4–(aq) + 6H2O(l)
STABILITY CONSTANTS
Definition
The stability constant, Kstab, of a complex ion is the equilibrium constant
for the formation of the complex ion in a solvent from its constituent ions.
In the reaction
the expression for the
stability constant is
[M(H2O)6]2+(aq) + 6X¯(aq)
Kstab =
[MX6]4–(aq) + 6H2O(l)
[ [MX64–](aq) ]
[ [M(H2O)6]2+(aq) ] [ X¯(aq) ]6
STABILITY CONSTANTS
Definition
The stability constant, Kstab, of a complex ion is the equilibrium constant
for the formation of the complex ion in a solvent from its constituent ions.
In the reaction
the expression for the
stability constant is
[M(H2O)6]2+(aq) + 6X¯(aq)
Kstab =
[MX6]4–(aq) + 6H2O(l)
[ [MX64–](aq) ]
[ [M(H2O)6]2+(aq) ] [ X¯(aq) ]6
The concentration of X¯(aq) appears to the power of 6
because there are six of the ions in the equation.
Note that the water isn’t included; it is in such overwhelming quantity that its
concentration can be regarded as ‘constant’.
STABILITY CONSTANTS
Because ligand exchange involves a series of equilibria, each step in the process has a
different stability constant…
[Co(H2O)6]2+(aq) + NH3(aq)
[Co(NH3)(H2O)5]2+(aq)
+ H2O(l)
Kstab / dm3 mol-1
K1 = 1.02 x 10-2
[Co(NH3)(H2O)5]2+(aq) + NH3(aq)
10-2
[Co(NH3)2(H2O)4]2+(aq)
+ H2O(l)
K2 = 3.09 x
[Co(NH3)2(H2O)4]2+(aq) + NH3(aq)
10-1
[Co(NH3)3(H2O)3]2+(aq)
+ H2O(l)
K3 = 1.17 x
[Co(NH3)3(H2O)3]2+(aq) + NH3(aq)
10-1
[Co(NH3)4(H2O)2]2+(aq)
+ H2O(l)
K4 = 2.29 x
[Co(NH3)4(H2O)2]2+(aq) + NH3(aq)
[Co(NH3)5(H2O)]2+(aq)
+ H2O(l)
K5 = 8.70 x 10-1
etc
STABILITY CONSTANTS
Because ligand exchange involves a series of equilibria, each step in the process has a
different stability constant…
[Co(H2O)6]2+(aq) + NH3(aq)
[Co(NH3)(H2O)5]2+(aq)
+ H2O(l)
Kstab / dm3 mol-1
K1 = 1.02 x 10-2
[Co(NH3)(H2O)5]2+(aq) + NH3(aq)
10-2
[Co(NH3)2(H2O)4]2+(aq)
+ H2O(l)
K2 = 3.09 x
[Co(NH3)2(H2O)4]2+(aq) + NH3(aq)
10-1
[Co(NH3)3(H2O)3]2+(aq)
+ H2O(l)
K3 = 1.17 x
[Co(NH3)3(H2O)3]2+(aq) + NH3(aq)
10-1
[Co(NH3)4(H2O)2]2+(aq)
+ H2O(l)
K4 = 2.29 x
[Co(NH3)4(H2O)2]2+(aq) + NH3(aq)
[Co(NH3)5(H2O)]2+(aq)
+ H2O(l)
K5 = 8.70 x 10-1
etc
The overall stability constant is simply the equilibrium constant for the total reaction.
It is found by multiplying the individual stability constants... k1 x k2 x k3 x k4 ... etc
Kstab or pKstab?
For an easier comparison,
the expression pKstab is often used…
pKstab = -log10Kstab
STABILITY CONSTANTS
Because ligand exchange involves a series of equilibria, each step in the process has a
different stability constant…
[Co(H2O)6]2+(aq) + NH3(aq)
[Co(NH3)(H2O)5]2+(aq)
+ H2O(l)
Kstab / dm3 mol-1
K1 = 1.02 x 10-2
[Co(NH3)(H2O)5]2+(aq) + NH3(aq)
10-2
[Co(NH3)2(H2O)4]2+(aq)
+ H2O(l)
K2 = 3.09 x
[Co(NH3)2(H2O)4]2+(aq) + NH3(aq)
10-1
[Co(NH3)3(H2O)3]2+(aq)
+ H2O(l)
K3 = 1.17 x
[Co(NH3)3(H2O)3]2+(aq) + NH3(aq)
10-1
[Co(NH3)4(H2O)2]2+(aq)
+ H2O(l)
K4 = 2.29 x
[Co(NH3)4(H2O)2]2+(aq) + NH3(aq)
[Co(NH3)5(H2O)]2+(aq)
•
•
•
•
+ H2O(l)
K5 = 8.70 x 10-1
Summary
The larger the stability constant, the further the reaction lies to the right
Complex ions with large stability constants are more stable
Stability constants are often given as pKstab
Complex ions with smaller pKstab values are more stable
etc
REACTION TYPES
The examples aim to show typical properties of transition metals and their compounds.
One typical properties of transition elements is their ability to form complex ions.
Complex ions consist of a central metal ion surrounded by co-ordinated ions or
molecules known as ligands. This can lead to changes in ...
• colour
• shape
Reaction
types
• co-ordination number
• stability to oxidation or reduction
ACID-BASE
A-B
LIGAND SUBSTITUTION
LS
PRECIPITATION
Ppt
REDOX
RED
OX
REDOX
REACTION TYPES
The examples aim to show typical properties of transition metals and their compounds.
LOOK FOR...
substitution reactions of complex ions
variation in oxidation state of transition metals
the effect of ligands on co-ordination number and shape
increased acidity of M3+ over M2+ due to the increased charge density
differences in reactivity of M3+ and M2+ ions with OH¯ and NH3
the reason why M3+ ions don’t form carbonates
amphoteric character in some metal hydroxides
(Al3+ and Cr3+)
the effect a ligand has on the stability of a particular oxidation state
REACTIONS OF COBALT(II)
CO32-
[Co(H2O)6]2+(aq)
+ CO32-(aq)
——>
CoCO3(s) + 6H2O(l)
Ppt
mauve ppt.
Hexaaqua ions of metals with charge 2+ precipitate a carbonate but
heaxaaqua ions with a 3+ charge don’t.
Cl¯
[Co(H2O)6]2+(aq) + 4Cl¯(aq)
——> [CoCl4]2-(aq) + 6H2O(l)
pink, octahedral
blue, tetrahedral
LS
• Cl¯ ligands are larger than H2O
• Cl¯ ligands are negatively charged - H2O ligands are neutral
• the complex is more stable if tetrahedral - less repulsion between ligands
• adding excess water reverses the reaction
REACTIONS OF COPPER(II)
Cl¯
[Cu(H2O)6]2+(aq) + 4Cl¯(aq)
——>
[CuCl4]2-(aq) + 6H2O(l)
yellow, tetrahedral
LS
• Cl¯ ligands are larger than H2O and are charged
• the complex is more stable if the shape changes to tetrahedral
• adding excess water reverses the reaction
I¯
2Cu2+(aq) +
4I¯(aq)
——>
2CuI(s) +
I2(aq)
off - white ppt.
• a redox reaction
• used in the volumetric analysis of copper using sodium thiosulphate
REDOX
REACTIONS OF MANGANESE(VII)
• in its highest oxidation state therefore Mn(VII) will be an oxidising agent
• occurs in the purple, tetraoxomanganate(VII) (permanganate) ion (MnO4¯)
• acts as an oxidising agent in acidic or alkaline solution
acidic
MnO4¯(aq) + 8H+(aq) + 5e¯ ——> Mn2+(aq) + 4H2O(l)
E° = + 1.52 V
N.B. Acidify with dilute H2SO4 NOT dilute HCl
alkaline
MnO4¯(aq) + 2H2O(l) + 3e¯ ——> MnO2(s) + 4OH¯(aq)
E° = + 0.59 V
VOLUMETRIC USE OF MANGANATE(VII)
Potassium manganate(VII) in acidic (H2SO4) solution is extremely useful for carrying out
redox volumetric analysis.
MnO4¯(aq) + 8H+(aq) + 5e¯ ——> Mn2+(aq) + 4H2O(l)
E° = + 1.52 V
It must be acidified with dilute sulphuric acid as MnO4¯ is powerful enough to oxidise
the chloride ions in hydrochloric acid.
It is used to estimate iron(II), hydrogen peroxide, ethanedioic (oxalic) acid and
ethanedioate (oxalate) ions. The last two titrations are carried out above 60°C due to
the slow rate of reaction.
No indicator is required; the end point being the first sign of a permanent pale pink
colour.
Iron(II)
MnO4¯(aq) + 8H+(aq) + 5Fe2+(aq) ——> Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)
this means that
moles of Fe2+
moles of MnO4¯
=
5
1
REACTIONS OF IRON(II)
Volumetric
Iron(II) can be analysed by titration with potassium manganate(VII)
in acidic (H2SO4) solution. No indicator is required.
MnO4¯(aq) + 8H+(aq) + 5Fe2+(aq) ——> Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)
this means that
moles of Fe2+
moles of MnO4¯
CONTENTS
=
5
1
AN INTRODUCTION TO
TRANSITION METAL
COMPLEXES
THE END
© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING