Electronic Spectra of Coordination Compounds

NOTE For any configuration, the free-ion terms are the sum of those listed for example, for the configuration, the free-ion terms are. S + ) + G + + F. 

In the interpretation of spectra of coordination compounds, it is often important to identify the lowest-energy term. A quick and fairly simple way to do this is given here, using as an example a (f configuration in octahedral symmetry.  

Spin multiplicity of lowest-energy state Spin multiplicity = 3 -I- 1 = 4 = number of unpaired electrons -1-1.  

Determine the maximum possible value of Ml (sum of nil values) for the configuration as shown. This determines the type of free-ion term (e.g., S, P, D). 

Maximum possible value of Mi for three electrons as shown 2 -I- 1 -I- 0 = 3 therefore, F term 

Spin-oibit coupling can have significant effects on the electronic spectra of coordinations compounds, especially those involving fairly heavy metals (atomic number 40). 

Step 3 deserves elaboration. The maximum value of m/ for the first electron would be 2 (the highest value possible for a d electron). Because the electron spins are parallel, the second electron cannot also have ot = 2 (it would violate the exclusion principle) the highest value it can have is m/ = 1. Finally, the third electron cannot have m/ = 2 or 1, because it would then have the same quantum numbers as one of the first two electrons the highest m value this electron could have would therefore be 0. Consequently, the maximum value of Mi = 2+1+0 = 3. 

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In later chapters, symmetry will be a valuable tool in the construction of molecular orbitals (Chapters 5 and 10) and in the interpretation of electronic spectra of coordination compounds (Chapter 11) and vibrational spectra of organometallic compounds (Chapter 13). 

Electronic Spectra of Coordination Compounds 391 n-3-2 CORRELATION DIAGRAMS... 

Whether CFT or LFT was used, the one-electron MO diagrams for coordination compounds ignored the interelectron repulsions present whenever there was more than one valence electron. We cannot quantitatively predict the electronic spectra of coordination compounds until we include the electron-electron repulsions. 

Much remains to be understood about the structure of solvents in solution and the degree of local order caused by interaction with the solute. In particular, theoretical calculation of electronic spectra of coordination compounds in the presence of solvent has been limited to a narrow range of complexes, and mainly to water as a solvent. 

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