Electronic spectra of transition metal complexes

When spectral bands are weak there is a reason. Often it means that they are bands which are forbidden but—obviously—not totally forbidden. The weak bands with which we are concerned are of this type. Electronic transitions which correspond to strong bands are electric dipole in type. Classically, the electric vector associated with the incident light beam behaves like a pair of alternating -I- and — charges across the molecule. These oscillating charges induce an oscillating dipole in the molecule when the 

that the metal electrons will be partially delocalized onto the ligands  

that the effective positive charge on the transition metal will be smaller 

Both of these effects mean that the metal electron cloud will be more diffuse in the complex than in the free ion, and repulsion between the electrons making up this cloud is therefore reduced. This conclusion is confirmed by a more detailed analysis. 

Here we follow the usual spectroscopic practice of giving the excited state first. 

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Schmidtke H-H, Degan J (1989) A Dynamic Ligand Field Theory for Vibronic Structures Rationalizing Electronic Spectra of Transition Metal Complex Compounds. 71 99-124 Schneider W (1975) Kinetics and Mechanism of Metalloporphyrin Formation. 23 123-166... 

M. C. Zerner. Intermediate neglect of differential overlap calculations on the electronic spectra of transition metal complexes. In N. Russo D. R. Salahub, editors. Metal-ligand interactions structure and reactivity, pages 493-531, Dordrecht, 1996. NATO ASI Workshop, Kluwer. 

Zerner, M. C. 1996. Intermediate Neglect of Differential Overlap Calculations on the Electronic Spectra of Transition Metal Complexes , in Metal-Ligand Interactions, Russo, N. and Salahub, D. R., Eds., Kluwer Dordrecht, 493. 

Up to this point we have considered two central issues involved in interpreting electronic spectra of transition metal complexes—the number and intensities of spectral lines. There is a third important spectral feature, the widths of observed bands, which we have not yet discussed. Consider again the visible spectrum for... 

D. Sutton, Electronic Spectra of Transition Metal Complexes , McGraw-Hill, London, 1968. 

P. V. Bernhardt and P. Comba, Inorg. Chem., 32,2798 (1993). Prediction and Interpretation of Electronic Spectra of Transition Metal Complexes via the Combination of Molecular Mechanics and Angular Overlap. Model Calculations. 

In discussing spectra, it will be particularly useful to be able to relate the electronic spectra of transition metal complexes to the ligand field splitting, A for octahedral complexes. To do this it will be necessary to introduce two special types of diagrams, correlation diagrams and Tanabe-Sugano diagrams. 

Gas-phase Photoelectron Spectroscopy Electronic Spectra of Transition Metal Complexes Natural and Magnetic Optical Activity Magnetic Susceptibility Measurements. 

A Dynamic Ligand Field Theory for Vibronic Structures Rationalizing Electronic Spectra of Transition Metal Complex Compounds... 

Zerner, M. C. 1995. Neglect of differential overlap calculations of the electronic spectra of transition metal complexes. Metal-Ugand interactions Structure and reactivity. NATO ASI Sen, Sen C. 474. 

When relaxation times are short, the Uncertainty Principle becomes important, because the lifetime t of an excited state and the uncertainty in its energy, A , are related by rA (where S = A/2 n). Spectra consist of lines representing transitions, and if the uncertainty in the upper state becomes large, these hnes could be broadened, so that resolution is lost and, in extreme cases, the whole spectrum might become just a single, extremely broad hump. The constant Ti is very small, about 10 " J s, but for electronic spectra of transition-metal complexes in solution, relaxation times are typically around 10 s, so that A is of the order of 10 J per molecule, or 60kJ mol. This is comparable with the transition energies involved. Most such electronic spectra therefore consist of a few broad hnes, and much potentially useful information is lost. 

The selection rules that govern the intensities of valence electronic transitions are derived from the quantum-mechanical equations that describe the processes involved. They depend on many factors, and it is not possible to give a full account of them here you will find details in [8] and [9]. It is important, however, to understand some aspects of the rules to make sense of the electronic spectra of transition-metal complexes. This is covered in Section 9.6.2. 

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