Practica 1
Practica 1
Practica 1
Introduction
Linkage isomerism is a type of isomerism where the central atom is bonded to the same
ligands, but this can be linked by different atoms. This, occurs in ligands that are
ambidentates: they have more than one atom with which can coordinate. Due to this
difference in the metal-ligand bond, the isomers have different properties, such as color
or bond strengths that will be studied with the characterization with IR and UV-Vis
spectroscopy.
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Objectives
The aim of this practice is to prepare the two isomers [Co(NH3)5(NO2)]Cl2 and
[Co(NH3)5(ONO]Cl2 and study their different properties (color, absorbance, IR bands).
Also, we are going to study the process of isomerization of the nitrite isomer to nitro
isomer to determinate the rate constant of the process.
Experimental process
X X X
Hydrochloric
acid
Ammonia X X
Hydrogen X
peroxyde
Sodium X X
nitrite
CoCl2 . 6H2O
NH4Cl X
Tabla 1. Reagent hazards.
For the study of the IR, we use the spectrophotometer Nicolet iS5.
For the spectra of UV-Vis, we use the Spectrophotometer Shimazdu UVProve.
The kinetics has been carried out using a spectrophotometer UV-Vis Shimadzu UV-160
with a thermostat Haake model DC-1 at 50oC.
pH-metre Crison MicropH 2000.
In the UV-Vis, we use a quartz cell with two clear sides.
Data treatment was analyzed by Excel and Origin.
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Procediment and incidences
1) [CoCl(NH3)5]Cl2.
First, we prepare the started compound of the complexes that we studied in this report,
the [CoCl(NH3)5]Cl2.
We followed the procedure described in the guideline.
During all this process, the mixture was stirring.
We weighed 10.05 g of CoCl2·6H2O and then added to a solution of 5,11 g of NH4Cl
(that gives the acid medium) in 30 mL of concentrated NH3. We observe that the color
of the solution change to orange and then to brown (because the Co(ll) change the
ligands at which is coordinated, H2O to NH3).
2) [Co(NH3)5(ONO)]Cl2
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Reduction of the peroxide. H2O2 → O2 + H++2e-
2
We washed in this order because we are increasing the volatility of the solvent. As a result, we
obtained a drier precipitate.
3
We added HCl 1:1 slowly until we get a pH of 7.09.
We added 2.5g of NaNO2 and 2.5 mL of HCl 1:1. This step must be done
in the hood because NO2 gas it forms (orange smoke). Also, we observe
a formation of an orange precipitate (the nitrite isomer,
Co(NH3)5(ONO)]Cl2). We filtered it and washed it with cold water and
ethanol.
We obtained 1.82g of Co(NH3)5(ONO)]Cl2). and the yield was 69.89 %.
Ilustración 3. Isomer
Co(NH3)5(ONO)]Cl2.
3) [Co(NH3)5(NO2)]Cl2
Ilustración 4. Isomer
Co(NH3)5(NO2)]Cl2).
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Discussion of the results
Reactions
CoCl2 ·6H2O → [Co(H2O)6]2+ + 2Cl- (1)
UV-Vis Characterization
In this section will de study the UV-Vis spectrum of the two isomers
prepared. We can make the spectrum of the two isomers because both
absorbs energy at a wavelength that it is in the visible range. More
specifically, those responsible for this absorption are the chromophore
groups3. The nitro group absorbs at a wavelength corresponding to the blue
and we see it yellow and on the other hand, the nitrite group, which absorbs
a wavelength corresponding to green, we see it orange. Ilustración 5.
Quartz cell with
two clear sides.
To know how many and which bands are expected to be observed in the spectrum, it is
necessary to use the Tanabe-Sugano diagrams (Appendix). We have that for the central
metal of the complex Co(ll) is a d6. According to the Tanabe-Sugano diagram for an
octahedral complex, there are 4 allowed spin bands. But there is only one band that
corresponds to the visible, which is the one observed in the spectrum.
As to which band is more energetic (of the nitrite or he nitro one), we have that, according
to the spectrochemical series, ONO ligand is a weaker ligand than the nitro, that is, the
field crystal theory, the energy when the complex is formed is smaller for the nitrite than
for the nitro. Therefore, the absorption band of the complex with nitrite ligand should
appear at a higher wavelength than for the nitro complex.
3
The chromophore is a region in the molecule where the energy difference between
two separate molecular orbitals falls within the range of the visible spectrum.
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Also, the absorption of the nitro isomer must be appeared at a lower wavelength than
the nitrite because the group nitro has a plus of stability because it has more resonant
forms, so the bond is more strong.
Figure 2. Absorption spectra UV-Vis of the ONO. Figure 1. Absorption spectaUV-Vis of NO2.
In the spectrum obtained for both isomers, we observe that, indeed, the ONO ligand
absorbs at a higher wavelength than the NO2 ligand.
In our case, the nitrite compound was isomerized a little before we made the UV-Vis
spectrum. Therefore, the spectrum obtained for the nitrite, we didn’t have 100% nitrite
isomer and the concentration was not exactly 0.01M.
So, we can observe that the absorbance of the nitrite is shifted to smaller wavelength
due to this earlier isomerization.
Also, as we explained before, we observe that, indeed, the ONO ligand absorbs at a
higher wavelength than the NO2 ligand.
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• Study of isomerization process
0,88
Absorbance (ua)
0,68
Abs t=0 min
Abs t=64 min
0,48
0,28
0,08
380 400 420 440 460 480 500 520 540 560
-0,12
Wavelength (nm)
Graphic 1. Absorption vs Wavelength at t=0 and t=64 minutes for the ONO and NO2 isomers.
We observe that there is almost no difference between the maximus between the
absorption at t=0, where mostly we have the nitrite isomer, and the absorbance at t=64
minutes, where mostly we have the nitro isomer. This is because when we stats the
characterization with the nitrite isomer, the isomerization reaction was already started as
we explain before, and then, at t=0 we already had enough nitro isomer formed.
The Wavelength at which there is the maximum difference between the molar extinction
coefficients (which is directly proportional to the absorbance) of the two isomers is
approximately 461,5nm.
In the graph we observe that there are two isosbestic points. Isosbestic points are points
where at a determinate wavelength, the compounds have the same absorption. We
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observe two isosbestic points, that give us an indication of the mechanism by which it
occurs the reaction of the isomerization. There is only one reactant, and one product, so
nitrogen binds first and then breaks the bond with oxygen. The reaction of isomerization,
therefore, is an intramolecular reaction.
From this maximum of the two curves of the graph 1 at 461,5 nm, which corresponds to
the maximum of absorbance at t=0 ant t=64 minutes respectively, the concentration of
the solution(0,01M) and the optical distance (b=1 cm), the values of the molar extinction
coefficient have been calculated with the Lambert-Beer law:
𝐴 =𝜀·𝑏·𝑐
Once we calculated the ε values and with the absorptions as a function of time, we
calculated the variation of the concentration of each isomer as a function of time.
Concentration vs t
0,012
0,01
Concentration (mol/L)
0,008
0,006
0,002
0
0 10 20 30 40 50 60 70
-0,002
Time (minutes)
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We can observe that, the concentration of the ONO isomer is decreasing and the
concentration of the NO2 isomer is increasing. This is because the isomerization reaction
is happening.
We know that the isomerization reaction follows a first-order law:
v=k[Co(NH3)5(ONO)]Cl2] which on integration becomes At=Ainf + (A0-Ainf)·exp·(-Kobs·t).
At for the graphic 2 is the Abs(t) of the nitrite and for the graphic x, is the Abs(t) of the
nitro.
So, to demonstrate this, it has been determined if the data obtained fits this model or not.
To do it, we used the program of Origin and Excel.
In Origin, we represent the absorbance of the nitro isomer and also the absorbance of
the nitrite isomer as a function of time. And then, we fitted the data to the model.
Figure 4. First-order law model for ONO isomer. Figure 3. First-order law model for NO2 isomer. Time
is in minutes not seconds.
We observe that, the absorbance is decreasing in case of the nitrite isomer and
increasing in case of another isomer. The data fit the model quite well in both isomers.
With the values of the fit equation, we calculate the constant rate of the reactions:
𝑁𝑖𝑡𝑟𝑖𝑡𝑒 Nitro
k1=0.0558 min-1
k-1=0.0616 min-1
In excel, we calculate the same but, first, we linearized the equation, and we represent
𝐴 −𝐴
𝑙𝑛 𝐴 𝑡 −𝐴𝑖𝑛𝑓 as a function of time.
0 𝑖𝑛𝑓
𝐴𝑡 − 𝐴𝑖𝑛𝑓
𝑙𝑛 = −𝑘𝑜𝑏𝑠 · 𝑡
𝐴0 − 𝐴𝑖𝑛𝑓
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ln((At-Ainf)/(Ao-Ainf)) vs t
0
0 10 20 30 40 50 60 70
-1
ln((At-Ainf)/(Ao-Ainf))
-2 y = -0,0796x - 0,6079
R² = 0,9548
-3
-4
-5
-6
Time (minutes)
Figure 5. Firs-order law linearized model.
We observe that the data fit the model but there are some points that deviates a little.
We obtain a straight with a slope, that is the value of the k, of 0,0796 min-1. It is similar
to the value obtained with Origin.
The value of the k obtained with Origin is closer to the theoretical value that the value
obtained with Excel, that is a little bigger. This is due to that in Origin. We fit the data to
the real model and not to a linearized model as we do in Excel.
IR Characterization
This technique measures the vibrations of atoms, and based on this it is possible to
determine the functional group (remember that the bond of an determinate ligand
vibrates at a particular frequency, and this depends on the environment of the ligand).
Then, It has been possible to identify which isomer is each one.
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Figure 6. IR spectra for ONO isomer. Figure 7. IR spectra for NO2 isomer.
Also, we observe a broad band towards 3100 cm-1 that corresponds to the N-H stretching
of NH3 ligand.
We observe that in the spectrum of the two isomers are the same bands because the
other ligands (NH3) are the same for both. But we detect that there is a band at 1061,07
cm-1 that appears only in the spectrum of the nitrite isomer, which corresponds to the
stretching of Co-O.
This band disappears in the spectrum of the nitro isomer. That confirms that the
transformation of the nitrite to the nitro isomer was totally done.
Conclusions
We obtained the two compounds successfully. The ONO isomer was isomerizing before
we could measure his absorbance, and this affects later in the treatment data. We should
be more careful with this compound and kept it in the freezer after we synthetize it.
The ONO ligand absorbs at a higher wavelength than the NO2 ligand due to the more
resonates forms for the NO2 and because it is stronger than ONO ligand.
We obtained that the absorbance at 461,5 nm was 0.8246 for the ONO isomer and
0.8911 for the NO2 isomer.
The IR characterization has allowed us to differentiate the two isomers because of the
extra band that have the ONO isomer at 1061 nm.
In solid state, the reaction of isomerization is slower than in solution. We could observe
that the IR of the ONO isomer the first day, 4 days later kept in the fridge, and kept in
the room temperature (Appendix 2) there was a decrease in the intensity of the band
that characterize the ONO isomer.
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A difference, in solution, what happens is that the ONO isomer is directly converted into
the NO2 isomer. That is, reaction rate is faster in solution than in solid state.
Bibliography
WILLIAMS, G. M.; OLMSTED, J.; BREKSA, P. J. Chem. Ed. 66 (1989) 1043.
BASOLO, F.; HAMMAKER, G. S. Inorg. Chem. 1 (1962) 1.
HOHMAN, W. H. J. Chem. Educ. 51 (1974) 553.
PHILLIPS, W. M.; CHOI, S.; LARRABEE, J. A. J. Chem. Educ. 67 (1990) 267.
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Apèndix
1. For the calculation of the Abs(t) and concentration(t).
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2. IR spectra
We observe that the band for the Co-O starts disappearing, that means that the
sample was isomerizing when we kept at room temperature but slowly.
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3. Linearization of the first-order Law.
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4. Calculation of Yields
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