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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
-
Co
data table 1 summary
* extended cobalt data table 2 *
Cobalt & electrode potential chart
of 3d-block
-
Summary of some
complexes-compounds & oxidation states of cobalt compared to other
3d-block elements
-
Cobalt is
alloyed with chromium and tungsten to make a metal hard enough,
even at red heat, to be used for high speed cutting tools and valves for
internal combustion engines.
-
COBALT(II) chemistry
-
In aqueous solution,
in the absence of complexing agents,
-
The alkalis sodium
hydroxide and ammonia, 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.
-
Alkaline aqueous
sodium carbonate solutions 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.
-
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 ...
-
nitrate(III) ion (nitrite, ion
NO2-) it
forms the anionic octahedral complex [Co(NO2)6]3-
-
cyanide
ion CN- it forms the anionic octahedral complex [Co(CN)6]3-
-
Isomerism in
cobalt(III) complexes e.g. with the ligands ammonia + 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)
-
-
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]
-
-
Scandium
* Titanium * Vanadium
* Chromium
* Manganese * Iron * Cobalt
* Nickel
* Copper *
Zinc
* Silver & Platinum
| (german) Chemie
Periodensystem 10,9 Cobalt Chemie * |
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