Analysis. Analysis and ranking of the ligands into the spectrochemical series can be done at three different levels. The most basic level involves ranking the ligands by the color of the cobalt compound they produce. This qualitative ranking of ligands is appropriate for the high school level and is accomplished using the color transmission wheel in Figure 1. Solutions will display the color opposite the wavelength absorbed. This method of ranking ligands is valid given the direct correlation of the wavelength of light absorbed to the energy of the electron transition in the molecule (D0).
For the introductory college chemistry level where UV–visible spectrophotometers are available, ligands can be quantitatively ranked based on the lowest energy wavelengths of maximum absorption (lmax). The lmax of lowest energy can be used to generate the spectrochemical series, as it is directly proportional to, though not equal to, D0.
For high-spin d6 Co(III) complexes, two eg ← t2g transitions are seen in the UV–visible region due to the fact that half of the of the six possible orbital changes (for example dz2←dxz) give rise to orbital transitions that result in a net increase in electron–electron repulsion. Note that other spin-forbidden transitions are possible but have such low probability that the associated molar absorptivities are negligible. In addition, ligand-to-metal charge transfer bands (LMCT) are seen in the ultraviolet region in almost all cases, but do not provide information about metal dorbitals and need not be considered in the present analysis.
For more advanced college chemistry courses, true values for D0 are calculated using Tanabe–Sugano diagrams as described below. In addition, in classes in which metal-to-ligand bonding interactions are discussed in detail, the spectrochemical series not only can be produced, but the ranking of ligands can be justified/explained in terms of ligand-bonding interactions, such as s and p-acceptor/donor.
Analysis. Analysis and ranking of the ligands into the spectrochemical series can be done at three different levels. The most basic level involves ranking the ligands by the color of the cobalt compound they produce. This qualitative ranking of ligands is appropriate for the high school level and is accomplished using the color transmission wheel in Figure 1. Solutions will display the color opposite the wavelength absorbed. This method of ranking ligands is valid given the direct correlation of the wavelength of light absorbed to the energy of the electron transition in the molecule (D0).For the introductory college chemistry level where UV–visible spectrophotometers are available, ligands can be quantitatively ranked based on the lowest energy wavelengths of maximum absorption (lmax). The lmax of lowest energy can be used to generate the spectrochemical series, as it is directly proportional to, though not equal to, D0.For high-spin d6 Co(III) complexes, two eg ← t2g transitions are seen in the UV–visible region due to the fact that half of the of the six possible orbital changes (for example dz2←dxz) give rise to orbital transitions that result in a net increase in electron–electron repulsion. Note that other spin-forbidden transitions are possible but have such low probability that the associated molar absorptivities are negligible. In addition, ligand-to-metal charge transfer bands (LMCT) are seen in the ultraviolet region in almost all cases, but do not provide information about metal dorbitals and need not be considered in the present analysis.For more advanced college chemistry courses, true values for D0 are calculated using Tanabe–Sugano diagrams as described below. In addition, in classes in which metal-to-ligand bonding interactions are discussed in detail, the spectrochemical series not only can be produced, but the ranking of ligands can be justified/explained in terms of ligand-bonding interactions, such as s and p-acceptor/donor.
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