Electron Configurations in Octahedral Complexes

For which d electron configurations in octahedral complexes is it possible to distinguish between high-spin and low-spin arrangements ... 

It is not clear why this luminescence does not appear in solution. Complexes of d metals of the second and third transition row do not seem to show an LF emission. Such metal ions exist only in low-spin configurations. In octahedral complexes they then possess an electron hole in their dn subshell. It follows that such compounds are often characterized by low-energy LMCT transitions [8]. Accordingly, LF emissions are not likely to occur. 

We shall now consider in some detail the ground-state electronic configurations of octahedral complexes containing metal ions with more than one valence electron. Referring back to Fig. 9-5, we see that metal ions with one, two, and three valence electrons will have the respective ground-state configurations iKd, d = S =... 

In a complex that possesses a center of symmetry, all states arising from a dn configuration have the g character inherent in the d orbitals. Since the dipole moment vectors belong to odd representations, all of the integrals such as / y/gXi//g dr are identically zero because the direct product of two g functions can never span any u representations. On this basis alone, we would predict that transitions between the various states arising from dn configurations in octahedral environments would have zero absorption intensity. In fact, these transitions do take place but the absorption bands are only —lO"3 times the intensity expected for symmetry allowed electronic transitions. Thus the prediction we have made is substantially correct, but at the same time there is obviously some intensity giving mechanism that has been overlooked. 

In octahedral complexes, the choice between high-spin and low-spin electron configurations arises only for dA-d7 complexes. Explain. 

Copper(II) has a 3d9 electronic configuration. In principle, pure octahedral and tetrahedral symmetries can never be observed because Jahn-Teller distortions (see Section 3.3.1) remove the orbital degeneracy of the ground state. The separation of the electronic energy levels depends on the coordination number and stereochemistry, as well as on the nature of the ligands. However, the ground state orbital is always well isolated from the excited states, and therefore the electronic relaxation mechanisms are relatively inefficient. Copperfll) complexes have thus relatively sharp EPR signals, and it is often possible to record these spectra at room temperature. 

The octahedral complex ions [FeClg] and [Fe(CN)g] are both paramagnetic, but the former is high spin and the latter is low spin. Identify the d-electron configurations in these two octahedral complex ions. In which is the octahedral field splitting greater. How does the CFSE differ between the complexes ... 

---