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Nitro vs nitrito linkage isomers

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In this experiment, the nature of linkage isomers will be observed on the example of nitritopentaamminecobalt(III) Chloride and nitropentaamminecobalt(III) Chloride. Their relative stability will be compared on standard conditions and the infrared spectroscopy of both the isomers will be obtained to analyze the characteristic absorption bands for the nitro and nitrito group. Yield for nitritopentamminecobalt(III) Chloride was obtained to be = 1.9221 g(73.24 %) and yield for nitropentamminecobalt(III) Chloride = 0.76 g(76 %) Introduction

Linkage isomers may arise when one or more of the ligands can coordinate to the metal ion in more than one way.

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The first reported example of linkage isomerism had the formula [Co(NH3)5(NO2)]Cl2.1 This compound exists in two isomeric forms; one containing Co-NO2 linkage, the other the Co-ONO group.2 In this experiment, the synthesis of both of the isomers will be performed. The figure below illustrates the bonding in nitrito(on the right) and nitro(on the left)

Figure : Nitrito and Nitro linkage isomers.

For the nitrito isomer, the metal(Cobalt in this case) is connected to the oxygen of the ONO group, whereas for nitro isomer, the metal is connected to the nitrogen of the NO2 group.

One of the useful techniques identifying linkage isomers is infrared spectroscopy. The characteristic frequencies for both of the isomers are distinguishable in the infrared spectra of these compounds. There are only minor differences in the procedures for the preparation of the two linkage isomers. In both cases the reaction is represented by [Co(NH3)5Cl]2+ + NO2- = [Co(NH3)5NO2]2+ + Cl- (eqn 1.1) The mechanism of reaction will in both cases go through the O-bonded isomer(the nitrito-isomer) which is unstable and gradually converts to the N-bonded isomer in an equilibration process.3 The equilibrium in aqueous acid solution [Co(NH3)5ONO]2+  [Co(NH3)5NO2]2+ (eqn 1.2) lies very much to the right.

Synthesis of the nitritopentaamminecobalt(III) Chloride will be performed according to the following reaction [CoCl(NH3)5]Cl2 + NaNO2 = [Co(ONO)(NH3)5]Cl2 + NaCl (eqn 1.3) Small portion of the product will be used to make nitropentaamminecobalt(III) Chloride. Some portion of nitrito isomer will also be stored in the oven and the isomerization process will be observed(conversion of less stable isomer, the nitrito complex, to more stable isomer, nitro complex) over the week. Infrared spectra will be obtained for the three compounds synthesized in this experiment. The characteristical frequencies will be observed and the isomers will be identified according to their bands. Experimental

Preparation of [Co(ONO)(NH3)5]Cl2.
Chloropentaamminecobalt(III) Chloride(2.518 g) was dissolved in a solution containing 4 mL of concentrated ammonia and 40 mL of water. The solution was stirred and heated for ten minutes. The resulting solution was filtered by gravity and then cooled in an ice bath. Fifteen drops of 3 M HCl was added to adjust pH to 6 using pH meter. The solution was cooled and sodium nitrite (2.5 g) was dissolved in the resulting solution. Some concentrated HCl was used to dissolve sodium nitrite. The resulting solution was stirred and filtered to obtain crystals of [Co(ONO)(NH3)5]Cl2. The filtrate was washed with ice water(12 mL) and ethanol(12 mL). The product was in a form of salmon-red color crystal. Yield was 1.9221 g(73.24 %). An infrared spectrum was obtained for this product using KBr pellets.

Preparation of [Co(NO2)(NH3)5]Cl2
Nitritopentaamminecobalt(III) Chloride(1.0 g) was dissolved in 25 mL of hot water containing 1 mL of concentrated ammonia. The solution was cooled and 10 mL of concentrated HCl was added. The solution was filtered and washed with 12 mL of ethanol and allowed to dry. The product was in a form of dark-yellow crystal. The yield was 0.76 g(76 %). An infrared spectrum was obtained for this product using KBr pellets. A portion of the nitrito complex was placed in the oven at 110 degrees Celsius and was left for a week. The product was observed to have dark green color and the infrared spectrum was obtained for this compound as well using KBr pellets.

Theoretical yield calculations
For [Co(ONO)(NH3)5]Cl2
[CoCl(NH3)5]Cl2 + NaNO2 = [Co(ONO)(NH3)5]Cl2 + NaCl (eqn 1.4) n([CoCl(NH3)5]Cl2) = 2.518 g / 250.4456 = 0.010054 mol
n(NaNO2) = 2.5 g / 68.99 = 0.03624 mol
Thus, [CoCl(NH3)5]Cl2 is the limiting agent.
n([CoCl(NH3)5]Cl2) = n([Co(ONO)(NH3)5]Cl2) = 0.010054 mol
m([Co(ONO)(NH3)5]Cl2) = 0.010054 mol * 260.998 = 2.62407 g
%Yield = 1.9221 g / 2.62407 g = 73.24 %
For [Co(NO2)(NH3)5Cl2
[Co(ONO)(NH3)5]Cl2 = [Co(ONO)(NH3)5]Cl2 (eqn 1.5) Their molecular weight is same so
% Yield = 0.76 g / 1 g = 76 %

Infrared spectrum table

Table : Absorption bands of the products synthesized
Compound name
Peak assignment
Literature reference (cm-1)
Chloropentaamminecobalt(III) Chloride
3300.42 (s)
3267 1

3174 1

Vas(NH3) degenerate
1589 1

Vs(NH3) angle deformation
1309 1

850 1

485 1
Nitropentaamminecobalt(III) Chloride
3200 1

V(as(NH3) degenerate
1615 1

Vas(NO2) scissoring
1425 1

1316 1

845 1

625 1

486 1
Nitropentaamminecobalt(III) Chloride(in the oven)
1615.96 (s)
Vas(NH3) degenerate
1615 1

Vas(NO2) scissoring
1425 1

1316 1

845 1

625 1

486 1
Nitritopentaamminecobalt(III) Chloride
Vas(NH3) degenerate
1750 1

1460 1

Vs(NH3) deformation
1327 1

1135 1

850 1

In the synthesis of nitritopentaaamminecobalt(III) Chloride, the pH had to be stabilized around six so that the solution was not too basic for reaction to occur. If pH would have been higher, some ammonia could have escaped from the complex, resulting in lower yield and synthesis of the different compound. Nitritopentaamminecobalt(III) Chloride was synthesized first because it is much less stable than the nitro isomer, making it relatively easy to convert nitrito isomer into nitro under normal conditions. If nitrito isomer was synthesized at first, ultraviolet light would have been used to convert nitro isomer into nitrito, which is time-consuming and unnecessary. 2 Synthesis of nitritopentaamminecobalt(III) Chloride was done according to the following reaction : [CoCl(NH3)5]Cl2 + NaNO2 = [Co(ONO)(NH3)5]Cl2 + NaCl (eqn 1.6) This is not a redox reaction. It goes to completion under normal conditions with addition of some heating. The yield was 73.24 % for the synthesis of nitrito isomer. Some product might have been lost due to incorrect filtration technique. Nitro isomer was synthesized using the product from the synthesis above according to the following reaction: [Co(ONO)(NH3)5]Cl2  [Co(NO2)(NH3)5]Cl2 (eqn 1.7) This reaction is spontaneous, as nitro isomer is favored to produce under normal conditions. 3 Yield was 76 % for the synthesis of nitropentaamminecobalt(III) Chloride. Some product might have been lost when transferring the filtrate between the beakers. Under normal conditions, nitritopentamminecobalt(III) Chloride gradually converts into nitropentamminecobalt(III) Chloride. After placing a sample of the nitrito isomer in the oven for a week, the isomerization reaction can be observed.

The infrared spectra of the two isomers were taken and compared. There are two characteristic stretches for both isomers which can be used to distinguish the products. For nitrito isomer, these are the stretches of N=O bond which is observed at 1480-1380 cm-1, and the stretch of the N-O group that occurs at 1200-1050 cm-1.1 Both of these stretches can be seen on the infrared spectra of the product. Based on the results, it is clear that one of the products obtained is nitritopentaamminecobalt(III) Chloride, based on the characteristic stretches of the O-N=O group For nitro isomer, the characteristic stretches are the stretches of the NO2 group that occur at 1450-1350 cm-1 and 1340-1300 cm-1. In the table, these stretches are observable, on both the product in the oven and the one synthesized from nitritopentaamminecobalt(III) Chloride. This also proves the theory that nitro isomer is favored to form on standard conditions and that the isomerization does occur over time. One of the reasons that infrared spectroscopy can be used to identify these linkage isomers is that the nature of the bonding of the ONO and NO2 groups are fundamentally different. The figure below represents the bonding in both groups.

Figure : Nature of bonding in the ONO and NO2 groups
In the nitrito group, there is one N-O and one N=O bond, electrons are not shared equally and the bonds are different, thus in the infrared spectroscopy, the peaks are situated far from each other(1450 vs 1050 cm-1). In the nitro group there are two possible Lewis structures, which can be expressed as one resonance structure. The two N O bonds are similar, and are stronger than N-O single bonds but are weaker than N=O double bonds. This can also be explained in the infrared spectrum of our products. The two characteristic peaks are situated near each other(1450 vs 1350 cm-1). From observing the Lewis structures of both isomers, it is clear why infrared spectroscopy is successful in identifying this pair: it is because the nature of bonding in both groups(NO2 and ONO) are different, corresponding to different frequencies. From looking at infrared spectra of our starting material, it can be noted that asymmetric and symmetric stretching of either ONO or NO2 group is absent. There are no peaks in the 1450-1350 cm-1 region or the 1050-1150 cm-1 region that could correspond to either nitro or nitrito group.

In this experiment the synthesis of two linkage isomers, nitritopentaamminecobalt(III) Chloride and nitropentamminecobalt(III) Chloride was performed. The behavior and the difference in structure of linkage isomers were studied on this example. Infrared spectroscopy was used to identify each of the linkage isomer, using characteristic absorption bands for nitrito and nitro groups.

1. Nakomoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds., 4th Edition: Wiley Interscience 2. Housecroft, C.E.; Sharpe, A.G. Inorganic Chemistry, 3rd edition; Prentice Hall: 2008, 725-810. 3. Basolo,F.; Hammaker, G.S. Inorganic Chemistry, 1962,1,1.

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Nitro vs nitrito linkage isomers. (2016, Aug 13). Retrieved from https://graduateway.com/nitro-vs-nitrito-linkage-isomers/

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