Lecture #9, Friday, April 17, 2009

UV-Vis spectroscopy was introduced and I collected a spectrum of [Ni(H₂O)₆]²⁺, [Co(H₂O)₆]²⁺, and [Zn(H₂O)₆]²⁺. The one thing I want you to definitely keep in mind when you are looking at these spectra is that the each peak corresponds to an electronic excitation from one molecular orbital to another. In order to promote and electron to from the ground state (the lowest energy configuration of the electrons) to an excited state we need to provide some sort of energy for the transition to happen. In many transition metal complexes photons of visible light have enough energy to promote an electron from the ground state to an excited state.

Once I provided that background on UV-Vis I discussed the electromagnetic spectrum and looked into the relationship between Energy and wavelength. I then went over how absorbance relates to the color we physically see with our eyes. If absorbance occurs in only one region of the color wheel, then our eyes see its complimentary color. If multiple wavelengths are absorbed a material will generally take on a color in the middle of the colors not absorbed. So if wavelengths of Violet, Blue, and Green light are absorbed a material will appear Orange and if Violet and Red light is absorbed the material will be a Yellow/Green color. In the first REEL lab next week you will observe the color of five transition metal complexes and you will have to identify which spectra matches with which complex ion.

In order to really understand the energies of the molecular orbitals, we need to review the shapes of various transition metal complexes and analyze how the orbitals overlap to form their respective bonds. Always keep in mind that a molecule will form in order to lower the overall energy of the electrons with respect to the individual atoms. In order to see this effect you need to be sure you first know the shapes of the orbitals like the back of you hand. IF YOU DO NOT KNOW THE ORBITAL SHAPES YOU ARE SCREWED FROM HERE ON OUT! We will need to visualize how these orbitals interact with each other to determine the resulting energies. Remember, the color of a material is dictated by the difference in energy of the molecular orbitals, so if we want to manipulate the color of a pigment, we need to know how to manipulate the energies of the molecular orbitals. This is an extremely important concept in the REEL project.

I ended class by talking about the molecular orbital diagram for an H₂ molecule and I will open lecture with that on Monday, then continue to the molecular orbital diagram of O₂, and finish up with molecular orbital diagrams involving transition metal complex ions.

For homework over the weekend read the Chem 123 REEL Orbitals, Bonding, MO Diagrams, and Color handout posted on carmen. Also, if you feel like you need a refresher I have also posted my Chem 121 lecture notes on electronic structure/electron configurations and orbital overlap/molecular orbital theory. It would be good to review that.

I hope everyone is enjoying the wonderful weather and you are having a great weekend, Dr. Fus