CHEM 404 Spring 2021 Discussion Exercise 12 April 19, 2021 1

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CHEM 404 Spring 2021 Discussion Exercise 12 April 19, 2021 1. Wavenumber, or cm–1, are essentially energy units, as energy is proportional to the reciprocal of wavelength. If you take the reciprocal of the wavelength of a uv-vis absorption in cm you will get the wavenumber in cm–1. ?o values are often reported in cm–1 because they are measured by spectroscopy. Figure out the wavelength, in nm, and the energy, in eV, for ?o = 18,000 cm–1. h = 6.626 x 10–34 J s c = 2.998 x 108 m/s 1 J = 1 V x 1 C e = 1.602 x 10–19 C/e– 2. a. Below are the ?o values for four octahedral complexes of Co, in wavenumbers. Put them in order from smallest ?o to largest ?o. Complex ?o (cm–1) [Co(en)3] 3+ 23,160 [Co(CN)6] 3– 33,700 [CoF6] 3– 13,100 [Co(NH3)6] 3+ 23,000 [Co(H2O)6] 3+ 18,300 3. a. Which two of these complexes would be best explained by a sigma-only molecular orbital diagram? b. Sketch with a molecular orbital energy-level diagram all the interactions that result in ?o for a sigma-only octahedral metal complex. c. Explain why these are sigma-only complexes and explain the differences in ?o between these complexes. 4. a. Which of these complexes would be best explained by a pi-donor molecular orbital diagram? b. Using a molecular orbital energy-level diagram, show the interactions that result in ?o for a pi-donor octahedral metal orbital. (You can start with a sigmaonly diagram to the left). c. Explain what orbitals and/or electrons on the ligands contribute to the pi donation, and explain any differences in ?o between the complexes you have chosen as being pi-donor complexes. 5. a. Which of these complexes would be best explained by a pi-acceptor molecular orbital diagram? b. Using a molecular orbital energy-level diagram, show the interactions that result in ?o for a pi-acceptor octahedral metal orbital. (You can start with a sigmaonly diagram to the left). c. Explain what orbitals and/or electrons on the ligands contribute to the pi accepting, and explain any differences in ?o between the complexes you have chosen as being pi-accepting complexes.