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UB CHE 102 - Color and Crystal Field Theory

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Chem 102 1st Edition Lecture 20Outline of Last Lecture I. IsomersII. Optical ActivityIII. Rules for EnantiomersOutline of Current LectureI. Complex ColorII. Crystal Field Theory A. OctahedralB. TetrahedralC. Square PlanarCurrent LectureI. Complex Color- Depends upon the metal involved as well as the ligands bound. Metal must have unpaired d-electrons. To have color it must also be able to absorb visible light.White light- contains all wavelengths of visible light and can be dispersed into a spectrum. -Each color has a specific wavelength, thus a different energy. The smaller the wavelength the more energy. -Objects can absorb and reflect light, we see the reflected color. If objects absorb all colors but orange, then it reflects only orange colors, we see orange. If object absorbs only orange, then it reflects all other colors, we see blue. Complementary colors are opposite each other on an artist’s color wheel. -Gemstones- color comes from trace amounts of transition metal ions. Red ruby-Cr, Blue Sapphire- Fe and Ti, Violet Amethyst- Cr and Ti, Yellow Topaz- Fe, Green Emerald- Cr, Aquamarine- Fe. These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.II. Crystal Field Theory- Bonding model for transition metal complexes. The lone pair of the ligand is donated to a d-orbital in the transition metal. Assumes ligands are negative point charges. The metal has 5 d-orbitals, all orbitals are degenerate (same energy). -Positively charged metal will attract negative charged ligand. Creating a sphere of negative charge around the metal. The attraction stabilizes the complex vs.the free components. Increase the energy of the d-orbitals. They are still degenerate. -Reality- Ligands don’t approach equally from all sides due to geometry. When incoming negative ligand lines up with an electron in a metal d-orbital, repulsion occurs. Results in orbitals having different energies (splitting). A. Octahedral- 6 ligands all approach ON the X, Y, Z axis, energies of d-orbitals ON axis increases. Energy gap=amount of splitting. Magnitude depends upon the metal present, surrounding ligands. Spectrochemical Series-lists ligands in order of ability to split d-orbitals. Strong field ligands: interact strongly with metals, causing much repulsion. Weak field ligands: interact weakly with metal, causing little repulsion. Spin-pairing energy- energy required to pair electrons in a d-orbital. High spin complex- easy to jump gap (weak field). In a high spin complex, field ½ fill bottom, then ½ fill top then repeat process. Low spin complex- hard to jump gap (strong field). In a low spin complex, fill ½ bottom and then fill bottom again then repeat with top. B. Tetrahedral- 4 ligands all approach OFF the x,y,z axis. Energies of d-orbitals OFF axis increases. Tetrahedral gap is ~44% smaller than octahedral gap due to few ligand point charges, and not metal lobes pointed directly at the ligand point charges. ALL tetrahedralcomplexes are high spin, fill ½ bottom and then ½ top. C. Square Planar- 4 ligands all approach ON the x,y axis. Energies of d-orbitals on flat increases. Almost always low spin, regardless of ligand fill ½ bottom and then ½ bottom and then repeat with the top. Characteristic of d8


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UB CHE 102 - Color and Crystal Field Theory

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