Preparation of various Cobalt (III) complexes
Abstract
A starting product of [Co (NH3)5CL].Cl2 this was completed dissolving cobalt (II) chloride hexahyrdate in a solution of ammonium chloride. Once synthesised an IR spectrum of the compound was collected showing peaks present in the compound.
Reacting the starting product with concentrated or diluted ammonia created two linkage isomers. Red/Orange crystals of [Co(NH3)5NO2]Cl2 with yield of 56% and yellow/orange crystals of [Co(NH3)5ONO]Cl2 with a yield of 69%. Once synthesised a UV-Vis spectrum was created for both isomers, a shift in the wavelength from 452nm to 498nm was shown, meaning a different donor atom was bonded to the cobalt ion in each isomer.
Introduction
The aim of this experiment is to successfully made two isomers from the original product and test them to see if the predication made using crystal field theory and colour changes can be verified once producing the products and testing using UV-Vis spectrometer.
This is to do with crystal field theory. This theory describes the breaking of orbital degeneracy in metal complexes due to ligands. It describes the strength of the bonds between the metal and ligand, and based on this bond the energy of the system is altered which can lead to changes in colour of compound and magnetic properties. Crystal field theory is based on the symmetry of ligand around the central ion in this case the Cobalt. Once the ligands’ electrons interact with the electrons of the D-orbitals, the electrostatic interaction causes the energy levels to change depending on the orientation of the ligand.
Both isomers are an octahedral arrangement with orbital lobes in the dx2-y2 and dz2 with cobalt ion pointing directly at the ligands. These orbitals for both complex have greater repulsion than the electrons in the dxy, dxz, and dyz orbitals. The cobalt ion has a 3d6 orbital electron configuration for both isomers so they can occupy high or low spin arrangement dependant on the bonding to a strong or weak ligand donor. Strong field donors push the electron closer to the cobalt ion which causes a bigger crystal field splitting as there is a greater repulsion.
In this experiment, equation will be used to determine the molar absorption. The beer lambert law being rearranged to give; equation [4] where E is the molar absorption. A is the absorbance of sample, C is the concentration of the solution in moldm-3 and L is the length of solution the light passes through in cm.
Overall equation of Pentaamminechlorocobalt;
2 CoCl2.6H2O + 8 NH3 + 2NH4Cl + H2O2 2[Co(NH3)5Cl]Cl2 + 14 H20
Overall equations of the two isomers;
[Co(NH3)5Cl].Cl2 + NaNO2 [Co(NH3)5ONO].Cl2 + NaCl
[CoNH3)5Cl].Cl2 + NaNO2 (H+) [Co(NH3)5NO2].Cl2 + Nacl
Equation [1]; mass of product made theoretical mass x 100=percentage yeild
Equation [2]; mass of product x 1molmolar mass=moles in the product
Equation [3]; moles in solutevolume in dm3=concentration of product
Equation [4]; Ac x l =molar absortion
Experimental
2.5g of cobalt (II) chloride was dissolved in 2.4cm3 of deionized water in a small conical flask, creating a dark pink solution. In a separate flaks while in the fume cupboard dissolve 5.01g of ammonium chloride in 15cm3 of concentrated ammonia solution and 5cm3 of deionized water. This solution was transferred to the cobalt solution creating a dark brown solution. Additions of 1cm3 of 30% hydrogen peroxide was added repeatedly until a total of 4cm3 was added, done in a fume cupboard until a dark red solution was made. A rubber bung was placed loosely on top of the flask and left for 15 minutes. A total of 18cm3 of concentrated hydrochloric acid was added drop-wise to the solution over a period of 30 minutes. The solution now a lighter red was heated on a hot plate and a dark pink colour was observed. After the solution was cooled to room temperature was washed firstly with 7cm3 of 4M of hydrochloric acid and the with 5cm3 of absolute ethanol. Once dried the powder was weighed for the final mass and collected an IR spectrum of the compound to assess if the compound was indeed the target metal.
Results and discussion
Results of synthesising [Co (NH3)5CL] Cl2
The compound was synthesised with a percentage yield of 88%, the cobalt complex is red crystals. The yield being under 100% indicated some purity was lost, this could be due to product being lost. Product could be lost when transferring between flasks and transferred into the product bottle in the end. When filtered some product could have been left on filter paper. Also, due to the time to complete the experiment the compound could have been wet still when recording the final mass, meaning some other impurities could have been in the compound.
Once made an IR was run of the product to see what peaks were shown to see what bonds had been made. In the product made peaks were shown at; 3148.1cm-1, 1556.4cm-1, 1306.2cm-1 and 804.1cm-1. These peaks at these wavelength indicate bonds being in the compound such as halide group Co-Cl bond (804.1 peak) as well as Nitrogen to Hydrogen bond present at 3148.1cm-1.
When compared to an IR from the exact product that was desired to make, similar peaks were observed in the same places indicating the same bonds being present. (1) The bonds in the exact product IR gave a broad peak at 3162.8cm-1 this indicated a Nitrogen – Hydrogen bond which is also present in the IR for the product made. A large sharp peak is also present at 1307.1cm-1.
This shows that the compound made in the start of the experiment was reliable to use as it was the desired product that was made to make the isomers from. Checking this before carrying on the experiment was crucial as if the wrong product was made in the first section would give inaccurate result in the end of the experiment.
[Co(NH3)5Cl]Cl2 has an outer electron arrangement of 3d6, as the oxidation state of the cobalt is +3. The 3d orbitals can therefore take a high or low spin configuration. Both chlorine and nitrogen are bonded to the cobalt, chorine is a weak field ligand and the 5 NH3 are strong field ligands. Therefore, this could give a large crystal field splitting because this energy is greater than the paring energy the electron are arranged in low spin configuration.
Results of [Co(NH3)5(ONO)].Cl2
The mass of final product that was synthesised weighed 0.29g which gave a colour of a red/orange. The percentage yield for this product works out at 56%.
To workout the percentage yield you work out the theoretical yield using moles of the original product used [CO(NH3)5Cl].Cl2, 0.001996 and multiply this by the molar mass of the product to give you 0.52g theoretical mass.
The mass of product made is the divide by the theoretical mass and multiplied by 100 to give the percentage yield.
10.290.52×100=56%
The final product is then tested in a UV-VIS spectrum to see what peaks is present and to compare to the other isomer. A mass of 0.0254g is dissolved in 10cm3 of water. The sample one dissolved is then placed in a sample tube containing 1cm of the product. The sample is then run through the UV-VIS machine.
A peak is shown on at 498nm with an absorbance of 0.624. This peak shows is the absorbed colour of cyan. The colour of the product is a red colour but once tested the absorbed colour give the peak for cyan.
To work out molar absorption the concentration of the solution used in the UV-VIS spectrometer needs to be determined, this can be done using equation [2] and [3].
[2] 0.0254g x 1 mol250.4gmol=1.014×10-4 moles in the solute used.
[3] 1.014 x10-4 0.01=0.0101 moldm3
With this concentration, it is inputted into a rearranged beer lamberts law equation to work out the molar absorption.
[4] 0.6240.101 x 1 =6.178
Results of [Co(NH3)5(NO2)].Cl2
The mass of final product that was synthesised weighed 0.71g which gave a colour of a yellow/orange. The percentage yield for this product works out at 69%.
To workout the percentage yield you work out the theoretical yield using moles of the original product used [CO(NH3)5Cl].Cl2, 0.003993mol and multiply this by the molar mass of the product to give you 1.04g theoretical mass.
The mass of product made is the divide by the theoretical mass and multiplied by 100 to give the percentage yield.
0.711.04×100=69%
The final product is then tested in a UV-VIS spectrum to see what peaks is present and to compare to the other isomer. A mass of 0.0266g is dissolved in 10cm3 of water. The sample one dissolved is then placed in a sample tube containing 1cm of the product. The sample is then run through the UV-VIS machine.
A peak is shown on at 452nm with an absorbance of 0.703. This peak shows is the absorbed colour of blue. The colour of the product is a yellow colour but once tested the absorbed colour give the peak for blue.
To work out molar absorption the concentration of the solution used in the UV-VIS spectrometer needs to be determined, this can be done using equation [2] and [3].
[2] 0.0266g x 1 mol250.4gmol=1.062×10-4 moles in the solute used.
[3] 1.062 x10-4 0.01=0.0106 moldm3
With this concentration, it is inputted into a rearranged beer lamberts law equation to work out the molar absorption.
[4] 0.7030.106 x 1 =6.632
Conclusion
As different ligands generate crystal fields of different strengths and different colours being produced. The weaker field ligand create complexes with a smaller energy difference which absorbs light for longer wavelengths and lowers the frequency. A strong field ligand creates a larger energy difference which absorbs light of shorter wavelengths and giving a higher frequency.
Both being linkage isomers which differ in the atom of a ligand bonded to a metal in the complex and both being octahedral structured. In [Co(NH3)4(NO2)Cl]+ the N atom of the nitrite group bonds to the Co atom as the nitrogen is the donor atom. Whereas, [Co(NH3)4(ONO)Cl]+ the O atom of the nitrite group is bonded to the Co atom as the oxygen is the donor atom. This mean each isomer produced will have different magnitude of crystal field splitting which is linked to the colours observed.
Using the UV-VIS color spectrum wheel, it shows that the peak shown for isomer NO2 at 498nm should be an absorbed colour of cyan giving the product colour before testing a reddish/orange colour which was correct. The isomer ONO gave a peak of 452nm which should give the absorbed colour of blue meaning the complementary colour giving to the product would be predicted to be lighter orange/yellow colour which is also the colour the product turned out. Meaning the results received were accurate and the colours produced in the compound and absorbed once tested were correct.
The slight change in the wavelength and colour of both compound is due to the bonding and ligands. Nitrogen in NO2 on the spectrochemical series is a low spin and a stronger field ligand which will absorb higher energy of light. Oxygen in ONO is higher on the spectrochemical series meaning a higher spin and is a weaker field ligand which will absorb lower energy of light. (3)