Protocole 420
Utilisation de la spectroscopie Raman pour la
caractérisation de complexes de cobalt (structure,
isomérisation, ligands) 1
Section 420a) Structure des complexes de cobalt
Experiment 1. Synthesis of Hexaamminecobalt(lII) chloride
Experimental Procedure
Add 5.0g of CoCl2. 6H2O and 3.3 g of NH4Cl to 30 mL of DI water in a 250-mL
Erlenmeyer flask. In the hood add 1.0 g activated charcoal and 45mL conc. aqueous
ammonia. Cool the brown slurry in an ice bath to 0°C, then add 4.0 mL 30% H2O2 from
a buret. Do not allow the temperature to rise above 10°C. Heat the resulting red-brown
solution to 60°C, and maintain this temperature for 30min. (The incubation is needed to
ensure complete displacement of all aqua ligands.) Cool the mixture to 0°C; the product
will precipitate from the solution. Collect the product and the charcoal by filtration.
Recrystallization is necessary to separate the product from the activated charcoal. Place
the solid in a 250-mL Erlenmeyer flask, and add 40 mL hot water and 1.0 mL conc. HCl
(test the solution with pH paper, if necessary, add a few more drops of HCl). Heat the
mixture to 70°C, and filter while still hot. Place the filtrate in an ice bath, and add 1.0 mL
cold conc. HCl (it may be precipitated by common-ion effect). Collect the orange solid
by filtration, wash with 25 mL ice-cold ethanol, and allow to air-dry.
Experiment 2. Synthesis of ChloropentaamminecobaIt (III)
chloride
Experimental procedure
In a fume hood, add 5 g of ammonium chloride to 30 mL concentrated aqueous ammonia
in a 250-mL Erlenmeyer flask. (The combination of NH4Cl and NH3(aq) guarantees a
large excess of the NH3 ligand.) Stir the ammonium chloride solution vigorously using a
magnetic stirring plate while adding 10 g finely divided CoCl2 .6H2O add 8 mL 30%
hydrogen peroxide to the brown Co slurry, using a burette that has been set up in the
hood. An addition rate of about 2 drops per second is usually sufficient, but care should
be taken to avoid excessive effervescence in this exothermic reaction. (If the reaction
shows signs of excessive effervescence, turning off the magnetic stirrer momentarily will
usually prevent overflow of the solution.) When the effervescence has ceased, add 30 mL
conc. HCl with continuous stirring, pouring about 1-2 mL at a time. At this point, the
1
Protocole 420
reaction may be removed form the hood. Use a heater to heat the solution to 60°C with
occasional stirring. Hold the temperature between 55 °C and 65 °C for 15 min.; this
incubation period is necessary to allow complete displacement of all aqua ligands. Add
25 mL distilled water, and allow the solution to cool to room temperature. Collect the
purple product by filtration through a Buchner funnel; wash it three times with 7.5 mL
cold distilled water and twice with 7.5 mL ice-cold ethanol. (The solutions must be cold
to prevent undue loss of product by redissolving). Transfer the product to a crystallizing
dish, loosely cover with aluminum foil, and allow to dry.
Section 420b) Isomérisation des complexes de cobalt
Experiment 3. SYNTHESIS of nitritopentaamminecobalt (III)
Chloride
Introduction:
The equations for the preparation of [Co(NH ) ONO]Cl and [Co(NH ) NO ]Cl can be
3 5
2
3 5
2
2
written as follows:
2+
3+
-
3+
-
[Co(NH ) Cl] + H O → [Co(NH ) H O] + Cl [Co(NH ) H O] + NO →
3 5
2
3 5
2
3 5
2+
2
2
[Co(NH ) ONO] + H O
3 5
2+
2
2+
[Co(NH ) ONO] → [Co(NH ) NO ]
3 5
3 5
2
Experimental Procedure:
Start to heat a solution of 6.4 mL of concentrated, aqueous ammonia in 64 mL of water
on the stirrer-hot plate. The surface temperature of the hot plate is not critical. While
heating and stirring this solution, add 4 g of [Co(NH ) Cl]Cl (if 4 g of the
3 5
2
chloropentaamminecobalt (III) chloride was not obtained in the previous experiment
(420a), adjust the reactants to the amount you obtained.) Continue heating and stirring
until the colored product dissolves. If a dark brown to black precipitate of cobalt oxide
o
forms, filter it off. Cool the filtrate which should be a clear solution to about 10 C. Add
2M HCl slowly while keeping the solution cold until it is just neutral to slightly basic.
Add 4 g of sodium nitrite followed by 4 mL of 6M HCl. After the solution has been in an
ice bath for an hour, filter the precipitated salmon pink crystals of [Co(NH ) ONO]Cl
3 5
2
using a Buchner funnel. Wash with 20 mL of ice water, wash with 20 mL of alcohol, and
then allow it to dry on the lab bench for one hour. The product is not stable and will
slowly isomerize to the nitro compound.
2
Protocole 420
Experiment 4. SYNTHESIS of nitropentaaminecobalt (III)
Chloride
Experimental Procedure:
The nitritopentaaminecobalt (III) chloride in the previous experiment is isomerized to the
nitro compound by heating. The nitrito compound prepared before can be utilized before
it is dried. Bring 20 mL of water to a boil, add a few drops of aqueous ammonia, and add
2.0 g of the [Co(NH ) ONO]Cl . As this solution cools, add 20 mL of conc. HC1. After
3 5
2
cooling the solution, the [Co(NH ) NO ]CI will crystalize from the solution. Filter the
3 5
2
2
product in a Buchner funnel, wash the product with 13 mL of alcohol, and allow it to dry
in air for two hours.
Raman Spectral Data
Species
Symmetry
NH3 Streching
NH3 Symmetric
deformation
Metal ligand
Co-N strech
N-Co-N bend
[Co(NH3)6]Cl3
Oh
3280 vw,br
3235 vw, br
3210 vw, br
1324 m
503 vs
489 m
447 s
1
1
5
327 wb
8
[Co(NH3)5Cl]Cl2
C4V
3287 wsh
3260 vw
3207 vw
1311 sh
1279 s
490 vs
466 m
444 m
330 wb
279 m
Co-Cl strech
3 , 8
2
5
4 , 7
1
------Reference : T. W. Swaddle, P. J. Craig. Spectrochimica Acta,. 26A, 1559-1565 (1969)
3
Protocole 420
Species
symmetry
[Co(NH3)5NO2]Cl2
[Co(NH3)5ONO]Cl2
N-O asymmetric stretch
N-O symmetric strech
1424 w
1333 m
1450 m
1367 vw
NH3 Symmetric
deformation
1288 s
1284 s
O-N-O bending
827 s,p
----
482 vs
448 w
429 w
330 sh
490 s
454 w
287 m
---
--lower than 250 m
Metal-ligand
Co-N
Co-N’
Co-O
337w
N = NH3, N’= NO2
K. Kanamori, T. Morikawa, K. Kawai. Bull.Chem. Soc. Jpn., 53, 2787-2791 (1980)
References for experiments 1 et 2
Schlessinger, Gert G. Inorganic Syntheses (1967), 9, 160.
Williams, G. M.; Olmsted, J. III; Preksa, A. P. III. Journal of Chemical
Education (1989), 66(12), 1043.
References for experiments 3 et 4
Williams, G. M.; Olmsted, J. III; Preksa, A. P. III. Journal of Chemical
Education (1989), 66(12), 1043.
4