and Results Discussion: Transition metal complexes come in a variety of colors which arise from the absorption of visible light by the complexes. The color of the light absorbed is complementary to the color that is seen. The wavelength of the colors of the absorbed light can be calculated by the difference in energy (E) using the equation E = hc/ . The different energy levels that arise from transition metal complexes can be explained by the crystal field theory. The crystal field theory explains that when the negatively charged ligands are brought near the transition metal, the energies of the five degenerate d orbitals change by different amounts. The difference in energy of the orbitals is called the crystal field splitting (). The magnitude of the crystal field splitting depends on the identity of the transition metal and the ligand. If the ligand is kept constant, the crystal field splitting varies according to a certain order called the spectrochemical series. The series increases in the following order... Br- < Cl- < F- < H2O < NH3 An unidentified sample of one of the previous complexes (Complex B) was synthesized and purified in this lab by my group. The other (Complex A) was synthesized and purified by other groups in the lab. The wavelengths of maximum absorbance of these two complexes along wit the remaining (Complexes C, D & E) were measured. The complexes all have the general formula Co(NH3)5Xy+, where X is one of the five unknown ligands and y is the magnitude of the positive charge of the complex: 2 for Br,Cl-, and F-; 3 for H2O and NH3. Based on the maximum absorbance wavelengths of the complexes, their identities were determined. Procedure for Synthesizing Complex B 1. Vial was placed in oven at 110C for on hour. - Mass of vial = 16.460 grams 2. 20ml of acetone and a distilled water bottle was placed in an ice bath. 3. 4.017g of NH4Cl was dissolved in 10 ml of distilled water in a 50 ml beaker. 4. The solution was warmed using a hotplate. 5. 5.009g of cobalt salt was added to the boiling solution. 6. 0.302g of activated charcoal was added to the solution then chilled in the ice bath. 7. 15ml of NH4OH was added to the beaker then cooled to 10C. 8. 8ml of 30% H2O2 was added slowly to minimize the production of O2. 9. The solution was heated on a steam bath at 60C for 30 minutes. 10. The mixture was cooled to room temperature then in an ice bath. 11. Precipitate formed at the bottom. 12. The precipitate was vacuum filtered using a Buchner funnel. 13. The product was washed with three 5ml portions of ice cold distilled water and three 5 ml portions of acetone.
The final product after filtration was a brownish greenish solid.
Procedure for Purification of Complex B 1. 20 ml of acetone was placed in a beaker and chilled in an ice bath. 50ml 2. of 0.5M HCl was boiled and the synthesized solid was added to the boiling HCl and re-boiled. 3. The solution was filtered using fluted filter paper and cooled to room temperature. - The solution was red after filtering 4. 6ml of concentrated HCl was added to the solution. - The solution turned orange 5. The orange solution was put in an ice bath. 6. Solid crystals formed at the bottom of the solution. 7. The solution was vacuumed filtered and washed with three 5ml portions of distilled water and three 5ml portions of acetone. 8. The precipitate was put in the vial and baked at 110C.
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Mass of vial & precipitate = 17.822 grams
Mass of vial & precipitate Mass of vial = Mass of precipitate 17.822 grams 16.460 grams = 1.362 grams
Visible Absorbance Measurements A solution of 15ml of water and a very small amount of each complex was made and the maximum absorbance wavelength was measured for each. The maximum absorbance wavelength of complexes C, D, & E was measured as well. Complex A B C D E Wavelength (nm) 534.93 470.89 513.21 540.30 499.70
Based on the recorded wavelengths, the identities of the complexes can be found. The stronger ligands produce a greater and therefore a greater E. Using the Equation E = hc/, the ligands with shortest wavelength will have the largest E. E = hc/ E complex A = (6.626 x 10-34 J . S)(3.00 x 1017nm/s) / (534.93nm) = 3.7160 x 10-19J
E complex B = 6.626 x 10-34 J. S) (3.00 x 1017nm/s) / (470.89nm) = 4.2214 x 10-19J E complex C = (6.626 x 10-34 J. S) (3.00 x 1017nm/s) / (513.21nm) = 3.8733 x 10-19J E complex D= (6.626 x 10-34 J . S) (3.00 x 1017nm/s) / (540.30nm) = 3.6791 x 10-19J E complex E = 6.626 x 10-34 J . S) (3.00 x 1017nm/s) / (499.70nm) = 3.9780 x 10-19J
Complex A Complex B Complex C Complex D Complex E
= = = = =
[Co(NH3)5Cl]2+ [Co(NH3)5NH3]3+ [Co(NH3)5F]2+ [Co(NH3)5Br]2+ [Co(NH3)5H2O]3+
Conclusion: The identity of the compounds was found using the maximum absorbance wavelength. Using the wavelength of each unknown complex, the crystal field splitting, which corresponds to the change in energy, was found using the equation E = hc/. The complex with the smallest wavelength has the largest and therefore corresponds to the strongest ligand (NH3), the one with the second smallest wavelength has the second largest and corresponds to the second strongest ligand (H2O) and so forth. Using these observations Complex A which had the second smallest was found to be [Co(NH3)5Cl]2+. Complex B which my group synthesized and created was found to be [Co(NH3)5NH3]3+ with the largest . Complex C was found to be [Co(NH3)5F]2+ . Complex D with the smallest was found to be [Co(NH3)5Br]2+ and Complex E was [Co(NH3)5H2O]3+.
The Spectrochemical Series By Camielle Headlam
Lab Instructor: Haitao Wang November 15, 2005