TMC - Exp 2 Linkage Isomerism PDF

Preview

Summary

Lab Report...

Description

CHM 3350 Transition Metal Chemistry

Experiment 2 Linkage Isomerism

Lab Date: September 30 – October 7, 2021 Date Experiment Submitted: T.A.: Lab Group: Lab partners:

Procedure1

(Richeson,2020)

Observations Initially, the chloropentaamminecobalt(III) Chloride was a purple-coloured powder and the sodium nitrite was a white powder. Upon dissolution into aqueous ammonia the solution became a similar purple colour to that of the Chloropentaamminecobalt(III) Chloride powder. Following intense stirring and mild heating the solution most of the powder dissolved into solution. Upon acidification white smoke was produced as the acid droplets contacted the air around the solution. As the solution was heated and stirred more precipitate dissolved and the solution became slightly darker red in colour. Following recrystallization, the product seemed to be an identical colour to the original chloropentaamminecobalt(III) Chloride, but it was difficult to compare. When the nitritopentaamminecobalt(III) Chloride was heated for several hours the red colour became much more apparent and no longer had traces of purple. The synthesis of the nitro isomer had identical observations to that of the nitrito isomer with one exception which was that the resulting compound was much lighter in colour but still purple.

Results Table 1. Summary of data for nitrito isomer synthesis Chemical [Cl(NH3)5Co]Cl2 NaNO2 [NO2(NH3)5Co]Cl2

Mass (g) 0.5013 0.5000 0.1968

Yield (%) 37.67

Colour Purple White Purple

Expected Colour1 Purple White Pink

Colour Purple White Purple

Expected Colour1 Purple White Yellow-Brown

Table 2. Summary of data for nitro isomer synthesis Chemical [Cl(NH3)5Co]Cl2 NaNO2 [NO2(NH3)5Co]Cl2

Mass (g) 0.2514 0.2513 0.1096

Yield (%) 41.83

Table 3. IR data for Chloropentaamminecobalt(III) Chloride Shift (cm-1) 842 1305 1550 3000-3350

Assignment NH3: Rocking Vibration NH3: symmetric deformation vibration NH3: degeneration deformation vibration NH3: Stretching

Appearance Medium Strong

Reference2 840 1307

Medium

1620

Strong, Broad

3278

Table 4. IR data for Nitritopentaamminecobalt(III) Chloride Shift (cm-1) 842 1050 1305 1420 1550 3000-3350

Assignment NH3: Rocking Vibration Co – O stretch NH3: symmetric deformation vibration N=O stretch NH3: degeneration deformation vibration NH3: Stretching

Appearance Medium Strong Strong

Reference 8402 10603 13072

Medium Medium

14283 16202

Strong, Broad

32782

Table 5. IR data for Nitropentaamminecobalt(III) Chloride Shift (cm-1) 842 1305 1450 1550 3000-3350

Assignment NH3: Rocking Vibration NH3: symmetric deformation vibration N=O stretch NH3: degeneration deformation vibration NH3: Stretching

Appearance Medium Strong

Reference 8402 13072

Medium Medium

14603 16202

Strong, Broad

32782

Table 6. IR data for Nitritopentaamminecobalt(III) Chloride following heating at 150 °C for 2 hours Shift (cm-1) 842 1305

Assignment NH3: Rocking Vibration NH3: symmetric

Appearance Medium Strong

Reference 8402 13072

1450 1550 3000-3350

deformation vibration N=O stretch NH3: degeneration deformation vibration NH3: Stretching

Medium Medium

14603 16202

Strong, Broad

32782

Calculations Yield of Nitritopentaamminecobalt(III) Chloride [Cl(NH3)5Co]Cl2 (aq) + NaNO2 (aq) → [NO2(NH3)5Co]Cl (aq) + NaCl(aq) m ClCo = 0.5013 g m NaNO2 = 0.5000 g

Equivalent mols

∴ the theoretical yield of [NO2(NH3)5Co]Cl is 0.5224 g

%

∴ the percent yield of [NO2(NH3)5Co]Cl is 37.67%.

Yield of Nitropentaamminecobalt(III) Chloride [Cl(NH3)5Co]Cl2 (aq) + NaNO2 (aq) → [ONO(NH3)5Co]Cl (aq) + NaCl(aq) m ClCo = 0.2514 g

Equivalent mols

∴ the theoretical yield of [ONO(NH3)5Co]Cl is 0.2620 g

%

∴ the percent yield of [NO2(NH3)5Co]Cl is 41.83%.

Discussion Linkage Isomer Theory

A linkage isomer consists of two or more coordination compounds of which the donor atom of at least one of the ligands differs between compounds. This isomerism only occurs in ligand compounds which are capable of bonding to the metal in multiple different ways and are known as ambidentate ligands. Common examples of such

Figure 2. Nitro vs. Nitrito Linkage isomer

compounds include cyanide which binds at either with the carbon or nitrogen, the thiocyanate ion which binds either with the sulfur or nitrogen, and the nitrite compound which as seen in this experiment can bind with either the nitrogen or the oxygen. The nitrite ion has two possible resonance structures where one oxygen has a negative charge and the other is neutral and vice versa as seen in figure

Figure 1. Nitrite Resonance Structures

1. The lone pair found on both the nitrogen and oxygen atom allow for both of these atoms to bind to the metal. When attached via the nitrogen atom the ligand is referred to as nitro, and the ligand is referred to as nitrito when attached by the oxygen. The chemistry involved to make these two ligands bind to central metal ion is very similar with the main determinant being the pH conditions during the synthesis. Acidic conditions promote the formation of the nitro complex while basic or neutral conditions promote the formation of the nitrito ligand due to the formation of nitrous acid in the solution. NO2- will readily react with available protons in solutions to yield HNO2 with the hydrogen bound to the oxygen molecule. This compound is still able to react with the cobalt complex to yield a cobalt nitrite bond, however, the oxygen molecule is not available for bonding since it is bound to the hydrogen atom. This results in the nitrogen atom exclusively bonding with the cobalt and the hydrogen then being ejected shortly after.

Explanation of Observations The colour of the chloropentaamminecobalt(III) Chloride is due to the large amount of energy required to promote the lower-level electrons to the higher state from the split d-orbitals.

Since energy is directly coordinated to the colour observed this large energy results in light with a low wavelength which is perceived as violet. This also explains why the colour is still observed in the dissolved solution since the ammonia cobalt bonds are still present in the aqueous solution. During acidification white smoke was observed which is very likely ammonia chloride crystals since ammonia readily evaporates and forms a layer above the liquid. This gaseous layer will then react with the HCl drops as they fall into the solution to produce ammonia chloride crystals suspended in air. As the solution was heated and stirred following neutralization a darker red colour was observed which indicates the nitrite ligand was forming. However, upon recrystallization little colour change was observed between the product and the original chloropentaamminecobalt(III) chloride indicating very little nitrito chloropentaamminecobalt(III) Chloride. Additionally, during the synthesis of the nitro isomer the product observed was much lighter in colour indicating that a different compound had formed.

Analysis of IR Data 5 key peaks were observed during the IR of chloropentaaminecobalt(III) chloride at 3000 - 3350, 1550, 1305, 842 cm-1. The first peak between 3000 and 3350 cm-1 corresponds to the stretching vibration of the ammonia ligand. This occurs due to the tendency of the hydrogen bound to the central nitrogen group of the ammonia to vary in bond length in a sense ‘bouncing’ between a greater bond length and a shorter one. The peak at 1550 cm-1 can likely be attributed to the symmetric deformation of the ammonia however this typically occurs at 1620 cm-1. This type of vibration occurs when the hydrogen surrounding the nitrogen on the ammonia ligand expand and contract not only their bond length but also their bond angles. The 1305 cm-1 peak is the final peak which can be attributed to the ammonia ligand and is due to the rocking vibration of the hydrogen surrounding the nitrogen atom which is where the bond angle of the hydrogen changes, but the bond length remains constant. In a sense it can be thought that these three peaks are all caused by various ways the hydrogen jiggle around the nitrogen atom. The final peak observed at 842 cm-1 can be attributed to the cobalt chloride bond and the stretching which occurs between the chlorine and the central cobalt bond.

During the analysis of nitritopentaamminecobalt(III) chloride 2 additional peaks were observed one at 1050 and one at 1420 cm-1. The 1050 cm-1 peak correlates to the oxygen cobalt stretching bond and the 1420 peak correlates to the nitrogen oxygen stretching bond specifically the double bound nitrogen and oxygen. In the third analysis of nitropentaamminecobalt(III) chloride the 1050 peak was not observed indicating there was no cobalt oxygen bond and that the nitro ligand was in fact formed. Furthermore, the double bound nitrogen oxygen peak was observed at 1450 rather than 1420 due to the distance between the oxygen nitrogen bonds and the central cobalt atom. In the nitro ligand both atoms are closer to the central cobalt atom compared to the nitrito ligand. This results in a greater wavenumber since the wavenumber is directly correlated to the energy of the two atoms. During the analysis of the heated nitritopentaamminecobalt(III) chloride it was observed that the peak at 1050 cm-1 disappeared indicating that the cobalt oxygen bond had been replaced by a cobalt nitrogen one or in other words that the nitrito ligand had been converted into the nitro ligand. Reaction Scheme

Conclusion In conclusion despite the fact that the key colour change of this reaction was not observed likely due to a lack of the chloropentaamminecobalt(III) dissolving based upon the IR data it is clear that both the nitrito and nitro cobalt ligands were synthesized. This clearly

indicates that a significant amount of the original complex was present in the final product which inflated the percent yield. Despite this however enough of the desired nitro and nitrito cobalt complexes were synthesized to demonstrate the linkage isomer theory.

References (1) Richeson, D. CHM 3350 Inorganic Chemistry Laboratory. University of Ottawa: Ottawa, 2021; 8-9. (2) Abbas, N. K., Habeeb, M. A., & Algidsawi, A. J. K. (2015, February 15). Preparation of chloro penta amine cobalt(iii) chloride and study of its influence on the structural and some optical properties of polyvinyl acetate. International Journal of Polymer Science. Retrieved from https://www.hindawi.com/journals/ijps/2015/926789/. (3) Heyns, A. M., & Waal, D. de. (2001, December 3). An infrared study of the nitro-nitrito linkage isomerization in solid nitro- and nitritopentamminecobalt(iii) chloride. Spectrochimica Acta Part A: Molecular Spectroscopy. Retrieved from https://www.sciencedirect.com/science/article/pii/0584853989801461. (4) Sigma Aldrich. (n.d.). IR Spectrum Table & Chart. IR Spectrum Table. Retrieved from https://www.sigmaaldrich.com/US/en/technical-documents/technical-article/analyticalchemistry/photometry-and-reflectometry/ir-spectrum-table? fbclid=IwAR3oyKmGBAS7rhIM5Irudgcssv0ccXBI4zBwnk5sUq6yz8hN9xGH12WBO Fg.

Appendix I

FTIR of Chloro pentaaminecobalt(III) Chloride 110 100 Absorbance

90 80 70 60

4000

3500

3000

2500

2000

1500

1000

50 500

Wave Number (cm-1)

Graph 1. FTIR of chloropentaamminecobalt(III) chloride

FTIR of Nitrito pentaaminecobalt(III) Chloride 100 95 90 Absorbance

85 80 75 70 65

4000

3500

3000

2500

2000

Wave Number (cm-1)

Graph 2. FTIR of nitritopentaamminecobalt(III) chloride

1500

1000

60 500

Graph 3. FTIR of nitropentaamminecobalt(III) chloride

Absorbance

FTIR of Nitro pentaaminecobalt(III) Chloride

4000

3500

3000

2500

2000

1500

1000

100 90 80 70 60 50 40 30 20 10 500

Wave Number (cm-1)

Graph 4. FTIR of heated nitritopentaamminecobalt(III) chloride

Absorbance

FTIR of Heated Nitrito pentaaminecobalt(III) Chloride

4000

3500

3000

2500

2000

Wave Number (cm-1)

1500

1000

100 98 96 94 92 90 88 86 84 82 80 500...