Experiment 8 - EXPERIMENT 8 The Spectrochemical Series Objective In the first week of this experiment your group is to synthesize one of two

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EXPERIMENT 8 The Spectrochemical Series Objective: In the first week of this experiment, your group is to synthesize one of two unknown cobalt complexes (Complex A or B). In the second week of this experiment, your group is to purify the cobalt complex synthesized in week one and measure the wavelengths of maximum absorbance of five cobalt complexes: the cobalt complex synthesized by your group (Complex A or B), the cobalt complex not synthesized by your group (Complex B or A), and three other cobalt complexes (Complexes C, D, and E) provided by your instructor. You are to use these data and the spectrochemical series to determine the identities of each of the five cobalt complexes. Introduction: One of the most remarkable properties of transition metal complexes is the variety of colors they come in; red, blue, yellow, orange, and green are just a few of the colors transition metal complexes display. These colors arise because transition metal complexes absorb light in the visible spectrum. The perceived color of a sample is the color complementary to that which is most strongly absorbed (Figure 8.1). 650 nm 600 nm If a substance absorbs here 800 nm 560 nm 400 nm it appears as this color. 430 nm 490 nm Figure 8.1 The absorbed light increases the energy of the transition metal complex. In transition metal complexes, this energy increase corresponds to an electronic transition from one energy state to another (Figure 8.2). Orange Red Violet Blue Green Yellow
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Experiment 8: The Spectrochemical Series E 2 E E 1 Figure 8.2 The wavelength ( λ ) of the absorbed light is determined by the difference in energy ( Δ E ) between the two states: E 2 " E 1 = # E = h $ = hc % (1) The model that explains how the different electronic energy levels of transition metal complexes arise is called crystal field theory, developed by Hans Bethe, so-called because it was first applied to transition metal ions in ionic crystals. According to crystal field theory, the bonding in a transition metal complex is due to the electrostatic attraction between the positively charged transition metal ion and the ligands (the groups bonded to the transition metal ion), each of which is negatively charged or possessed of a dipole moment. The ligands, being of the same charge, repel one another, causing the complex to adopt the geometry that separates them from one another as much as possible. The complex as a whole is stable because the metal-ligand attractions are greater than the ligand-ligand repulsions. Different electronic energy levels in transition metal complexes occur because of the different amounts of repulsion between ligands and the electrons in d orbitals of the transition metal ions. In a transition metal ion in free space, the energies of electrons in the five d -orbitals are degenerate, or the same, as one another. When ligands are brought near the transition metal, the energies of electrons in the five d -orbitals increase because of the repulsive “field” induced
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This note was uploaded on 06/10/2011 for the course CHEM 2090 taught by Professor Zax,d during the Spring '07 term at Cornell University (Engineering School).

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Experiment 8 - EXPERIMENT 8 The Spectrochemical Series Objective In the first week of this experiment your group is to synthesize one of two

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