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Brown's Chemistry
Advanced
Level Inorganic Chemistry Periodic Table
Revision Notes – Transition Metals
Part 10. Transition Metals 3d–block:
10.9 Cobalt
Chemistry
The chemistry of the
transition metal cobalt (most common oxidation states +2 and +3) is dominated by the stability of the cobalt(II)
ion which forms a wide variety of stable complexes with most ligands
such as water, ammonia, chloride ion etc. The cobalt(III) state can be
stabilised by a suitable ligand and cobalt(III) complexes are usually
made by oxidising a cobalt(II) salt in the presence of the stabilising
ligand.
principal oxidation states of
cobalt, redox reactions of cobalt, ligand substitution displacement
reactions of cobalt, balanced equations of cobalt chemistry, formula of
cobalt complex ions, shapes colours of cobalt complexes, formula of
compounds
GCSE/IGCSE
Periodic Table Revision Notes *
GCSE/IGCSE Transition Metals Revision Notes
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INORGANIC
Part 10 The 3d block TRANSITION METALS
sub–index: 10.1–10.2
Introduction 3d–block Transition Metals * 10.3
Scandium
* 10.4 Titanium * 10.5
Vanadium * 10.6 Chromium
* 10.7 Manganese * 10.8
Iron * 10.9 Cobalt
* 10.10 Nickel
* 10.11 Copper * 10.12
Zinc
* 10.13 Other Transition Metals e.g. Ag and Pt * Appendix 1.
Hydrated salts, acidity of
hexa–aqua ions * Appendix 2. Complexes
& ligands * Appendix 3. Complexes and isomerism * Appendix 4.
Electron configuration & colour theory *
Appendix 5. Redox
equations, feasibility, Eø * Appendix 6.
Catalysis * Appendix 7.
Redox
equations
* Appendix 8. Stability Constants and entropy
changes *
Appendix 9. Colorimetric analysis
and complex ion formula * Appendix 10 3d block
– extended data
* Appendix 11 Some 3d–block compounds, 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
10.9.
Chemistry
of Cobalt Co, Z=27, 1s22s22p63s23p63d74s2
data comparison of cobalt
with the other members of the 3d–block and transition metals
|
Z
and symbol |
21
Sc |
22
Ti |
23
V |
24
Cr |
25
Mn |
26
Fe |
27
Co |
28
Ni |
29
Cu |
30
Zn |
|
property\name |
scandium |
titanium |
vanadium |
chromium |
manganese |
iron |
cobalt |
nickel |
copper |
zinc |
|
melting
point/oC |
1541 |
1668 |
1910 |
1857 |
1246 |
1538 |
1495 |
1455 |
1083 |
420 |
|
density/gcm–3 |
2.99 |
4.54 |
6.11 |
7.19 |
7.33 |
7.87 |
8.90 |
8.90 |
8.92 |
7.13 |
|
atomic
radius/pm |
161 |
145 |
132 |
125 |
124 |
124 |
125 |
125 |
128 |
133 |
|
M2+
ionic radius/pm |
na |
90 |
88 |
84 |
80 |
76 |
74 |
72 |
69 |
74 |
|
M3+
ionic radius/pm |
81 |
76 |
74 |
69 |
66 |
64 |
63 |
62 |
na |
na |
|
common oxidation
states |
+3
only |
+2,3,4 |
+2,3,4,5 |
+2,3,6 |
+2,3,4,6,7 |
+2,3,6 |
+2,3 |
+2,+3 |
+1,2 |
+2
only |
|
outer electron config. |
3d14s2 |
3d24s2 |
3d34s2 |
3d54s1 |
3d54s2 |
3d64s2 |
3d74s2 |
3d84s2 |
3d104s1 |
3d104s2 |
|
Electrode
potential M(s)/M2+(aq) |
na |
–1.63V |
–1.18V |
–0.90V |
–1.18V |
–0.44V |
–0.28V |
–0.26V |
+0.34V |
–0.76V |
|
Electrode
potential M(s)/M3+(aq) |
–2.03V |
–1.21V |
–0.85V |
–0.74V |
–0.28V |
–0.04V |
+0.40 |
na |
na |
na |
|
Electrode
potential M2+(aq)/M3+(aq) |
na |
–0.37V |
–0.26V |
–0.42V |
+1.52V |
+0.77V |
+1.87V |
na |
na |
na |
Extended data table for COBALT
|
property of cobalt/unit |
value for Co |
|
melting
point Co/oC |
1495 |
|
boiling
point Co/oC |
2870 |
|
density Co/gcm–3 |
8.90 |
|
1st
Ionisation Energy/kJmol–1 |
760 |
|
2nd
IE/kJmol–1 |
1646 |
|
3rd
IE/kJmol–1 |
3232 |
|
4th
IE/kJmol–1 |
4950 |
|
5th
IE/kJmol–1 |
7670 |
|
atomic
radius Co/pm |
125 |
|
Co2+
ionic radius/pm |
74 |
|
Relative polarising power Co2+ ion |
2.7 |
|
Co3+
ionic radius/pm |
63 |
|
Relative polarising power Co3+ ion |
4.8 |
|
oxidation
states of Co |
+2, +3 |
|
simple electron
configuration of Co |
2,8,15,2 |
|
outer electrons of Co |
[Ar]3d74s2 |
|
Electrode potential Co(s)/Co2+(aq) |
–0.28V |
|
Electrode potential Co(s)/Co3+(aq) |
+0.40 |
|
Electrode potential Co2+(aq)/Co3+(aq) |
+1.87V |
|
Electronegativity of Co |
1.88 |

The
Chemistry of
COBALT

-
The
electrode potential chart highlights the values for various
oxidation states of cobalt.
-
COBALT(II) chemistry
-
In aqueous solution,
in the absence of complexing agents,
-
cobalt forms the stable pink hexaaqua cobalt(II) ion, [Co(H2O)6]2+(aq)
-
Aqueous solutions of
cobalt(II) sulfate CoSO4(aq) or cobalt(II) chloride CoCl2(aq)
are suitable for laboratory experiments investigating the aqueous
chemistry of the cobalt(II) ion.
-
With alkalis sodium
hydroxide and ammonia,
cobalt(II) ions produce the hydrated
cobalt(II) hydroxide
blue ppt. which turns pink on standing. There is no further reaction with excess
of NaOH or Na2CO3, but see further down for excess NH3.
-
With alkaline aqueous
sodium carbonate solutions
cobalt(II) ions produces a precipitate of
pink/blue? cobalt(II) carbonate.
-
When excess ammonia
is added to
a cobalt(II) salt solution, the hexamine complex is formed BUT this is
unstable in the presence of dissolved oxygen and is oxidised to the
cobalt(III) complex. This change in cobalt's oxidation state from +2
to +3 via an oxidising agent is quite common if a complexing agent is
present too.
-
[Co(H2O)6]2+(aq) + 6NH3(aq)
==> [Co(NH3)6]2+(aq) + 6H2O(l)
-
pink
hexaaquacobalt(II) ion == oxygen ==> brown hexaamminecobalt(II) ion.
-
The uncharged ligand
molecules ammonia NH3 and water H2O are similar in
size and ligand exchange occurs without change in co–ordination number
(stays at 6).
-
Oxidation then follows from dissolved oxygen, or you can add hydrogen
peroxide for a more efficient job!
-
(i) 4[Co(NH3)6]2+(aq) + O2(g/aq) + 4H+(aq)
==> 4[Co(NH3)6]3+(aq) + 2H2O(l)
-
(ii) 2[Co(NH3)6]2+(aq) +
H2O2(g/aq) + 2H+(aq)
==> 2[Co(NH3)6]3+(aq) + 2H2O(l)
-
brown ==>
colour? hexaamminecobalt(III) ion.
-
Oxidation state changes:
in both (i) & (ii) Co from +2 to +3, (i) O from 0 to
–2, (ii)
O
from –1 to –2.
-
Comparison of the
stability of the hexammine complexes irrespective of redox stability
-
[Co(H2O)6]2+(aq) + 6NH3(aq)
==> [Co(NH3)6]2+(aq) + 6H2O(l)
-
[Co(H2O)6]3+(aq) + 6NH3(aq)
==> [Co(NH3)6]3+(aq) + 6H2O(l)
-
Note that the more
highly charged Co3+(aq) ion complexes
more strongly than the Co2+(aq) ion
i.e. forms a more stable complex
-
VIEW ppts. with OH–, NH3
and CO32–, & complexes,
if any, with
excess reagent.
-
When hydrogen peroxide is
added to an alkaline cobalt(II) solution, oxidation occurs to give cobalt(III)
complexes.
-
If e.g. sodium chloride
or hydrochloric acid is added to cobalt(II) sulphate solution the
blue tetrachlorocobaltate(II) complex ion is formed.
-
[Co(H2O)6]2+(aq) + 4Cl–(aq)
[CoCl4]2–(aq) + 6H2O(l)
-
This particular
ligand substitution/exchange reaction involves several changes (L
to R):
-
the larger
chloride ion ligand leads to a change in co–ordination number
from 6 to 4,
-
the complex ion
shape changes from octahedral to tetrahedral
-
the colour of the
complex changes from pink to blue,
-
the complex
changes from a cationic to an anionic ion.
-
There is no oxidation
state change at all.
-
This is quite a good
reaction to demonstrate Le Chatelier's equilibrium principles:
-
dilution shifts
the equilibrium to the left, more pink,
-
increasing the
chloride ion concentration shifts the equilibrium to the
right, more blue,
-
increasing the
solution temperature shifts the equilibrium to the right, more
blue
-
or if prepared at
higher temperature, with just enough chloride to turn the
solution blue, on cooling it becomes pink,
-
this shows that
left to right is endothermic and right to left is exothermic.
-
Summary of some
complexes–compounds & oxidation states of cobalt compared to other
3d–block elements
-
–
-
COBALT(III) chemistry
-
As we have seen
above the hexaaquacobalt(III) cation is unstable in aqueous solution but
can be stabilised by a suitable ligand.
-
The formation of
[Co(NH3)6]3+ is described above and two
other stable complex anions are with the ...
-
(i)
,
(ii)

-
(i) nitrate(III) ion (nitrite, ion
NO2–) it
forms the anionic octahedral complex [Co(NO2)6]3–
-
(ii) cyanide
ion CN– it forms the anionic octahedral complex
hexacyanocobaltate(III) ion [Co(CN)6]3–
-
Isomerism in
cobalt(III) complexes e.g. with the ligands ammonia and chloride (i)–(iii)
-
(i) crystalline
[Co(NH3)6]3+(Cl–)3
is orange–yellow, no isomers possible
-
(ii) crystalline
[Co(NH3)5Cl]2+(Cl–)2
is violet, no isomers possible
-
(iii) crystalline
[Co(NH3)4Cl2]+Cl–
is violet or green – there are two geometrical E/Z isomers (trans/cis)
-
(iii)
-
Geometrical
isomerism diagrams: The Z and E (cis and trans
geometrical isomers) isomeric octahedral complexes of the dichlorotetraamminecobalt(III) complex ion
-
(1) is the cis or Z
isomer, (2) is the trans or E isomer
-
(iv) –
-
More examples of the
complexes of cobalt(II) and cobalt(III)
-
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 than the Co2+(aq) ion.
-
The cobalt(II) ion complexes
with 1,2–diaminoethane, a bidentate ligand
-
[Co(H2O)6]2+(aq)
+ 3en(aq) ==> [Co(en)3]2+(aq)
+ 6H2O(l)
-
Kstab = {[Co(en)3]2+(aq)}
/ {[Co(H2O)6]2+(aq)}
[en(aq)]3
-
Kstab = 6.3 x
1013 mol–3 dm9 [lg(Kstab) =
13.8]
-
Note: en is an
abbreviation for the ligands old name ethylenediamine
-
–
-
An example of heterogeneous catalysis:
-
Cobalt(II)
ions catalyse the oxidation of the 2,3–dihydroxybutandioate
ion (acid/salt, old name 'tartaric/tartrate') to water, methanoate
ion and carbon dioxide with hydrogen peroxide solution.
The likely scheme of events is outlined below, the
equations are NOT
meant to be balanced.
-
Starting
with the
pink hexa–aqa Co2+ ion, which is a Co(II)
complex
-
[Co(H2O)6]2+(aq)
==> [Co(OOCCH(OH)CH(OH)COO)3]4–(aq)
-
the
pink Co(II) complex changes ligand
from water to the organic acid, but no change in oxidation
state or co–ordination number, and I
don't know its colour?, but it perhaps it doesn't
exist long enough to be seen?
-
[Co(OOCCH(OH)CH(OH)COO)3]4–(aq) ==via
H2O2==>
[Co(OOCCH(OH)CH(OH)COO)3]3–(aq)
-
[Co(OOCCH(OH)CH(OH)COO)3]3–(aq) ==> [Co(H2O)6]2+(aq),H2O(l),HCOO–(aq),CO2 (aq/g)
-
the
green Co(III) complex then breaks down to
give the products,
-
and
you see the bubbles of carbon dioxide and the 'return'
of the
pink hexa–aqa Co2+ complex ion.
-
In
the above sequence, the change in ligand affects the
relative stability of the oxidation states. The CoII–acid
complex is stable as regards 'breakdown', but is readily
oxidised to the CoIII–acid complex, which is
NOT stable to breakdown.
-
–
Scandium
* Titanium * Vanadium
* Chromium
* Manganese * Iron * Cobalt
* Nickel
* Copper *
Zinc
* Silver & Platinum
keywords redox reactions ligand
substitution displacement balanced equations
formula complex ions complexes ligand exchange reactions redox reactions ligands
colours oxidation states: cobalt ions Co(0) Co2+ Co(+2) Co(II) Co3+ Co(+3)
Co(III) CoCl2
CoSO4 Co2+ + 2OH– ==> Co(OH)2 Co2+ + CO32– ==> CoCO3 [Co(H2O)6]2+ + 6NH3
==> [Co(NH3)6]2+ + 6H2O 4[Co(NH3)6]2+ + O2(g/aq) + 4H+ ==> 4[Co(NH3)6]3+ + 2H2O (ii) 2[Co(NH3)6]2+ +
H2O2(g/aq) + 2H+ ==> 2[Co(NH3)6]3+ + 2H2O +1.82V for [Co(H2O)6]3+ + e–
[Co(H2O)6]2+ +0.10V for [Co(NH3)6]3+ + e– [Co(NH3)6]2+ [Co(H2O)6]2+ + 6NH3 ==>
[Co(NH3)6]2+ + 6H2O Kstab = {[Co(NH3)6]2+} / {[Co(H2O)6]2+} [NH3]6 Kstab = 7.7 x
104 mol–6 dm18 [lg(Kstab) = 4.9] [Co(H2O)6]3+ + 6NH3 ==> [Co(NH3)6]3+ + 6H2O
Kstab = {[Co(NH3)6]3+} / {[Co(H2O)6]3+} [NH3]6 Kstab mol–6 dm18 [lg(Kstab) =
33.7] [Co(H2O)6]2+ + 4Cl– [CoCl4]2– + 6H2O [Co(NH3)6]3+ [Co(NO2)6]3– [Co(CN)6]3–
[Co(NH3)6]3+(Cl–)3 [Co(NH3)5Cl]2+(Cl–)2 [Co(NH3)4Cl2]+Cl– [Co(H2O)6]2+ + EDTA4–
===> [Co(EDTA)]2– + 6H2O Kstab = {[Co(EDTA)3]2–} / {[Co (H2O)6]2+} [EDTA4–]
Kstab = 2.0 x 1016 mol–1 dm3 [lg(Kstab) = 16.3] [Co(H2O)6]3+ + EDTA4– ===>
[Co(EDTA)]– + 6H2O Kstab = {[Co(EDTA)3]–} / {[Co(H2O)6]3+} [EDTA4–] Kstab = 1.0
x 1036 mol–1 dm3 [lg(Kstab) = 36.0] [Co(H2O)6]2+ + 3en ==> [Co (en)3]2+ + 6H2O
Kstab = {[Co(en)3]2+} / {[Co(H2O)6]2+} [en]3 [Co(H2O)6]2+ + 3en ==> [Co(en)3]2+
+ 6H2O Kstab = {[Co(en)3]2+} / {[Co(H2O)6]2+} [en]3 [Co(OOCCH(OH)CH(OH)COO)3]4–
==via H2O2==> [Co(OOCCH(OH)CH(OH)COO)3]3– [Co(OOCCH(OH)CH(OH)COO)3]3– ==>
[Co(H2O)6]2+,H2O,HCOO–,CO2 oxidation states of cobalt, redox reactions of
cobalt, ligand substitution displacement reactions of cobalt, balanced equations
of cobalt chemistry, formula of cobalt complex ions, shapes colours of cobalt
complexes Na2CO3 NaOH NH3
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Alphabetical Index for Science
Pages Content
A
B C D
E F
G H I J K L M
N O P
Q R
S T
U V W
X Y Z
Scandium
* Titanium * Vanadium
* Chromium
* Manganese * Iron * Cobalt
* Nickel
* Copper *
Zinc
* Silver & Platinum
Introduction 3d–block Transition Metals * Appendix
1.
Hydrated salts, acidity of
hexa–aqua ions * Appendix 2. Complexes
& ligands * Appendix 3. Complexes and isomerism * Appendix 4.
Electron configuration & colour theory *
Appendix 5. Redox
equations, feasibility, Eø * Appendix 6.
Catalysis * Appendix 7.
Redox
equations
* Appendix 8. Stability Constants and entropy
changes *
Appendix 9. Colorimetric analysis
and complex ion formula * Appendix 10 3d block
– extended data
* Appendix 11 Some 3d–block compounds, complexes, oxidation states
& electrode potentials * Appendix 12
Hydroxide complex precipitate 'pictures',
formulae and equations
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