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Appendix 8.
Complex ion stability, entropy changes and stability constants (Kstab)
(1)
Summary of ideas about complex ion stability constants and
equilibrium expressions
This page discusses the
relative stability of a wide range of transition metal ion complexes.
The
stability is measured in terms of the stability constant Kstab for the
equilibrium expression for the formation of complex ion on interaction
of the hydrated metal ion and another ligand with which it undergoes a
ligand substitution reaction.
Apart from a variety of the 3d
block transition metals and particular ligands, other factors dealt with
include change in ion charge due to oxidation state, monodentate,
bidentate and polydentate ligands (chelation) are considered for a particular
transition metal ion.
Equilibrium expressions
-
Equilibrium expression
for transition metal complex ion formation are readily written out, but
as the case of weak acid ionisation Kc expressions, the
concentration of the solvent water is considered a constant and omitted
from the expression e.g.
-
 Since
[ ] used in depicting complex ion structure, I've used [ ] as
well to denote concentration.
-
[Fe(H2O)6]3+(aq) + SCN–(aq)
 [Fe(H2O)5SCN]2+(aq) + H2O(l)
-
| |
[[Fe(H2O)5SCN]2+(aq)] |
|
Kstab = |
------------------------------------------ |
| |
[[Fe(H2O)6]3+(aq)]
[SCN–(aq)] |
-
On this page I've often
quoted the equilibrium expression and value in one line e.g.
-
Kstab
= [[Fe(H2O)5SCN]2+(aq)]
/ [[Fe(H2O)6]3+(aq)]
[SCN–(aq)]
= 1.4 x 102 mol–1dm3
-
BUT, you do need to be able
to write out the mathematical equilibrium expression as clearly
as possible and note that Kstab is used instead of
the usual Kc when dealing with molar concentration
terms.
-
Note that the water
concentration terms is effectively constant in aqueous solutions
and incorporated into the value of Kstab.
-
In this
section [] = concentration, because [] used in expressing the complex formulae.
-
The Kstab units -
[concentration] / [concentration]2.
-
The bigger the value of
Kstab the more stable the complex i.e. the equilibrium is more
favoured to the right.
-
Kstab
values vary considerably and are often quoted as
lg Kstab
= log10 (Kstab).
-
i.e. for the
above equilibrium: log10 (1.4 x 102) =
lg Kstab = 2.1
(2) Complex formation by
polydentate ligands is sometime called chelation or sequestration.
| |
Ligand
substitution reaction to give the new complex ion for Zn2+ |
Kstab |
lg Kstab |
|
a |
[Zn(H2O)4]2+(aq)
+ 4CN–(aq)
 Zn(CN)4]2–(aq) + 4H2O(l) |
5.0 x 1016 |
16.7 |
|
b |
[Zn(H2O)4]2+(aq)
+ 4NH3(aq)
Zn(NH3)4]2+(aq)
+ 4H2O(l) |
3.8 x 109 |
9.58 |
|
c |
[Zn(H2O)4]2+(aq)
+ 4Cl–(aq)
 [ZnCl4]2–(aq) + 4H2O(l) |
1.0 |
0.0 |
|
d |
[Zn(H2O)4]2+(aq)
+ 4Br–(aq)
[ZnBr4]2–(aq) + 4H2O(l) |
10–1 |
–1.0 |
|
e |
[Zn(H2O)4]2+(aq)
+ 4I–(aq)
[ZnI4]2–(aq) + 4H2O(l) |
10–2 |
–2.0 |
The stability constant
equilibrium expression for these equilibria
-
|
for a |
[[Zn(CN)4]2-(aq)] |
|
Kstab = |
------------------------------------------ |
| |
[[Zn(H2O)4]2+(aq)]
[CN-(aq)]4 |
- All the Kstab units are [concentration] /
[conentration]5
-
a: Kstab = [[Zn(CN)4]2-(aq)]
/ [[Zn(H2O)4]2+(aq)] [CN-(aq)]4
= 5.0 x 1016 mol–4dm12
-
b: Kstab = [[Zn(NH3)4]2-(aq)]
/ [[Zn(H2O)4]2+(aq)] [NH3(aq)]4
= 3.8 x 109 mol–4dm12
-
c: Kstab = [[ZnCl4]2-(aq)]
/ [[Zn(H2O)4]2+(aq)] [Cl-(aq)]4
= 1.0 mol–4dm12
-
d: Kstab = [[ZnBr4]2-(aq)]
/ [[Zn(H2O)4]2+(aq)] [Br-(aq)]4
= 10–1 mol–4dm12
-
e: Kstab = [[ZnI4]2-(aq)]
/ [[Zn(H2O)4]2+(aq)] [I-(aq)]4
= 10–2 mol–4dm12
The very high Kstab value for the tetracyanozincate(II) in reflects the strong of central
metal ion (Zn2+) – ligand (CN) bond.
The lower Kstab value for
ammonia indicates on average a weaker dative covalent bond, but still
relatively strong.
The ligand bonds are even weaker
for the halide ions possibly due to their larger radius, since there is a
steady decrease in Kstab as the halide ion radius increases (down
the group), making the Zn–X
dative covalent bond longer and weaker (X = halogen).
Generally speaking, the stronger the
metal ion-ligand bond, the greater with be Kstab, moving the
equilibrium to the right.
However,
above, all the ligands considered were monodentate, so, in the next
section (3) we
also compare bidentate and polydentate and significant differences i.e.
increases are observed in the values of the stability constants.
(3) Kstab values and the nature of the ligand in
terms of mono, bi or polydentate ligands
(4)
Examples for all ten of the 3d-block element complex ion formation and values of Kstab
NOTE
(i) By convention,
the term [ H2O(l)
]n is omitted from equilibrium expressions, and is
incorporated into Kstab, because water is the
medium, and the bulk of the solution, therefore its concentration effectively remains
constant.
(ii) I'm afraid again, Kstab values
do differ
depending on the source!
(iii) log10 = lg and
Kstab = 10Kstab
Scandium
chemistry page
Titanium
chemistry page
Vanadium
chemistry page
Chromium chemistry page
Manganese chemistry page
-
The hexa–aqua
manganese(II) ion readily forms complexes with polydentate
ligands.
-
(i)
[Mn(H2O)6]2+(aq)
+ 3en(aq)
 [Mn(en)3]2+(aq)
+ 6H2O(l) (en = H2NCH2CH2NH2)
-
(ii)
[Mn(H2O)6]2+(aq)
+ EDTA4–(aq)
[Mn(EDTA)]2–(aq)
+ 6H2O(l)
-
The higher Kstab
value for EDTA reflects the greater entropy change. A
simplistic, but not illegitimate argument, shows that in (i) a
net gain of 3 particles, but in (ii) 5 more particles are
formed.
-
–
Iron chemistry page
-
Complex with
thiocyanate ion
-
Fe(H2O)6]3+(aq) + SCN–(aq)
[Fe(H2O)5SCN]2+(aq) + H2O(l)
-
Initial
monosubstituted complex with the fluoride ion ligand
-
[Fe(H2O)6]3+(aq)
+ F–(aq)
[Fe(H2O)5F]2+(aq) + H2O(l)
-
Kstab
= [[Fe(H2O)5F]2+(aq)]
/ [[Fe(H2O)6]3+(aq)]
[F–(aq)]
-
Kstab
= 2.4 x 105 mol–1dm3
-
lg(Kstab) = 5.38
-
The fluoride ion is more
strongly bonded than the thiocyanate ion.
-
–
-
Fe3+ ions
give an anionic complex in concentrated chloride ion solutions
-
[Fe(H2O)6]2+(aq)
+ 4Cl–(aq)
[FeCl4]–(aq)
+ 2H2O(l)
-
formation of the
tetrachlroroferrate(III) anion.
-
Kstab = [[FeCl4]–(aq)]
/ [[Fe(H2O)6]2+(aq)] [Cl–(aq)]4
-
Kstab = 8
x 10–1 mol–4 dm12
-
lg(Kstab)
= –0.097
-
Both the hexa–aqua
ions of iron(II) and iron(III) readily complex with EDTA
-
[Fe(H2O)6]2+(aq)
+ EDTA4–(aq)
[Fe(EDTA)]2–(aq)
+ 6H2O(l)
-
[Fe(H2O)6]3+(aq)
+ EDTA4–(aq)
[Fe(EDTA)]–(aq)
+ 6H2O(l)
-
Note that the more
highly charged Fe3+(aq) ion complexes
more strongly than the Fe2+(aq) ion,
reflecting the effect of the higher charge on the central metal ion
more strongly attracting the lone pairs of same ligand.
-
Iron(III) ions
complex with the bidentate ligand, the 1,2–diaminoethane molecule (en)
-
[Fe(H2O)6]3+(aq)
+ 3en(aq)
[Fe(en)3]3+(aq)
+ 6H2O(l)
-
Kstab = [[Fe(en)3]3+(aq)]
/ [[Fe(H2O)6]3+(aq)]
[en(aq)]3
-
Kstab
= 3.98 x 109 mol–3 dm9
-
lg(Kstab)
= 9.6
-
–
Cobalt
chemistry page
-
Both the hexa–aqua ions of
cobalt(II) and cobalt(III) readily complex with EDTA
-
[Co(H2O)6]2+(aq)
+ EDTA4–(aq)
[Co(EDTA)]2–(aq)
+ 6H2O(l)
-
[Co(H2O)6]3+(aq)
+ EDTA4–(aq)
[Co(EDTA)]–(aq)
+ 6H2O(l)
-
Note that the more
highly charged Co3+(aq) ion complexes
more strongly with the EDTA hexadentate ligand than the lower
charged Co2+(aq) ion
(see also below with the ammonia complexes).
-
Comparison of the stability
of the hexaammine complexes formed from the monodentate ligand ammonia
-
The cobalt(II) ion complexes
with 1,2–diaminoethane, a bidentate ligand
-
–
Nickel chemistry page
-
[Ni(H2O)6]2+(aq)
+ 6NH3(aq)
[Ni(NH3)6]2+(aq)
+ 6H2O(l)
-
The hexaaquanickel(II) ion
also forms complexes with other amine ligands
-
e.g. the bidentate
ligand 1,2–diaminoethane (H2N–CH2–CH2–NH2,
often abbreviated to en)
-
[Ni(H2O)6]2+(aq)
+ 3en(aq)
[Ni(en)3]2+(aq)
+ 6H2O(l)
-
The complex with EDTA is
also readily formed.
-
[Ni(H2O)6]2+(aq)
+ EDTA4–(aq)
[Ni(EDTA)]2–(aq)
+ 6H2O(l)
-
Note that Kstab
for the same ion tend to increase the greater the chelating power of
an individual ligand in terms of the ligand bond formed – mainly due
to the increase in entropy as more particles are formed by the
polydentate ligands
-
e.g. for the same
nickel(II) ion Kstab(EDTA) > Kstab(en)
> Kstab(NH3)
-
though this argument does ignore
the different stoichiometry of the equations.
-
Ni2+ forms
the tetrachloronickelate(II) ion, [NiCl4]2–, a
tetrahedral anionic complex
with the chloride ion (Cl–).
-
Ni2+ forms
the tetracyanonickelate(II) ion, [Ni(CN)4]2–, a
square planar anionic complex
with the cyanide ion (CN–).
-
[Ni(H2O)6]2+(aq) +
4CN–(aq)
[NiCN4]2–(aq) + 6H2O(l)
-
Kstab
= [[NiCN4]2–(aq)]
/ [[Ni(H2O)6]2+(aq)]
[CN–(aq)]4
-
Kstab
= 2 x 1031 mol4 dm–12
-
lg(Kstab) = 31.3
-
Its likely that the more
bulky chloride ion (radius Cl > C) 'forces' the formation of the
tetrahedral shape rather than a square planar shaped complex.
-
–
Copper chemistry page
Zinc
chemistry page
|
INORGANIC Part
10 3d block TRANSITION METALS sub–index:
10.1–10.2
Introduction to 3d–block Transition Metal
chemistry
10.3
Chemistry of Scandium
* 10.4
Chemistry of Titanium
10.5
Chemistry of Vanadium
* 10.6
Chemistry of Chromium
10.7
Chemistry of Manganese
* 10.8
Chemistry of Iron
10.9
Chemistry of Cobalt
* 10.10
Chemistry of Nickel
10.11
Chemistry of Copper
* 10.12
Chemistry of Zinc
10.13
Selected chemistry of other Transition Metals e.g. Ag and Pt
Appendix 1.
Hydrated salts, acidity of
hexa–aqua ions
Appendix 2. Complexes
and ligands
Appendix 3. Complexes and isomerism
Appendix 4.
Electron configuration and colour theory
Appendix 5.
Redox
equations, feasibility of reaction, Eø
calculations
Appendix 6.
Catalysis - types and
effectiveness
Appendix 7.
Redox
equations - construction and balancing
Appendix 8. Stability
constants
of complexes and entropy
changes
Appendix 9. Colorimetric analysis
and determining a complex ion formula
Appendix 10 3d block
– extended data table
Appendix 11
3d–block transition metal complexes, oxidation states
& electrode potentials
Appendix 12
Hydroxide complex precipitate 'pictures',
formulae and equations
Advanced
Level Inorganic Chemistry Periodic Table Index: Part 1
Periodic Table history
* Part 2
Electron configurations, spectroscopy,
hydrogen spectrum,
ionisation energies *
Part 3
Period 1 survey H to He *
Part 4
Period 2 survey Li to Ne * Part
5 Period 3 survey Na to Ar *
Part 6
Period 4 survey K to Kr and important trends down a
group *
Part 7
s–block Groups 1/2 Alkali Metals/Alkaline Earth Metals *
Part 8
p–block Groups 3/13 to 0/18 *
Part 9
Group 7/17 The Halogens *
Part 10
3d block elements & Transition Metal Series
*
Part 11
Group & Series data & periodicity plots * All
11 Parts have
their own sub–indexes near the top of the pages
|
Periodic
Table - Transition Metal Chemistry - Doc
Brown's Chemistry. Revising
Advanced Level Inorganic Chemistry Periodic Table
Revision Notes.
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Revision Notes stability constants and entropy changes for the formation
of 3d block transition metal complex ions
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Index
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Advanced level organic chemistry revision notes
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(1)
a monodentate ligand e.g. ammonia