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Electronic spectra of atoms

المؤلف:  Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong

المصدر:  Shriver and Atkins Inorganic Chemistry ,5th E

الجزء والصفحة:  ص487-488

2025-09-28

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Electronic spectra of atoms

Key point: Electron-electron repulsions result in multiple absorptions in the electronic spectrum. Figure 20.22 sets the stage for our discussion by showing the electronic absorption spectrum of the d³ complex [Cr (NH₃)₆]³⁺ in aqueous solution. The band at lowest energy (longest wavelength) is very weak; later we shall see that it is an example of a ‘spin-forbidden’ transition. Next are two bands with intermediate intensities; these are ‘spin-allowed’ transitions between the t₂g and eg orbitals of the complex, which are mainly derived from the

Figure 20.22 The spectrum of the d3complex [Cr (NH₃)₆]³⁺, which illustrates the features studied in this section, and the assignments of the transitions as explained in the text.

metal d orbitals. The third feature in the spectrum is an intense charge-transfer band at short wavelength (labelled CT, denoting ‘charge transfer’), of which only the low-energy tail is evident in the illustration. One problem that immediately confronts us is why two absorptions can be ascribed to the apparently single transition t₂g² eg¹ ← t₂g³. This splitting of a single transition into two bands is in fact an outcome of the electron-electron repulsions mentioned above. To understand how it arises, and to extract the information it contains, we need to consider the spectra of free atoms and ions.