Octahedral complexes energies

For tungstates more values of the Stokes shift are known and these confirm quantitatively the statement made above for the titanate group (see Sect. 3.7). Due to the smaller Stokes shift in the case of the octahedral complexes, energy transfer between octahedral complexes is more efficient than between tetrahedral complexes. This will be discussed further in Chap. 2. At this stage of the discussion this fact is nevertheless important to realize, because the occurrence of energy transfer phenomena complicates the study of isolated centres considerably. 

Figure 2.5. Splitting of the d energy level in an octahedral complex.

Fig. 2. Simplified molecular orbital diagram for a low spia octahedral complex, such as [Co(NH3 )g, where A = energy difference a, e, and t may be antisymmetric (subscript ungerade) or centrosymmetric (subscript, gerade) symmetry orbitals. See text.

Thc Crystal l-ield Siabili2ation Energy (CFSl ) is the additional stability which accrues to an ion in a complex, as compared to the free ion, because its d-orbitals are split In an octahedral complex a l2 electron increases the stability by 2/5Ao and an Cf, electron decreases it by 3/5Ao- In a tetrahedral complex the orbital splitting is reversed and an e electron therefore increases the stability by 3/5At whereas a t2 electron decreases it by 2/5Ai. 

As six ligands approach a central metal ion to form an octahedral complex, they change the energies of electrons in the d orbitals. The effect (Figure 15.10, p. 419) is to split the five d orbitals into two groups of different energy. 

In octahedral complexes, the e -orbitals (dz< and dx2 -yi) lie higher in energy than the t2 -orbitals (dxy, dyz, and dzx). The opposite is true in a tetrahedral complex, for which the ligand field splitting is smaller. 

Figure 20-12 summarizes the electrical interactions of an octahedral complex ion. The three orbitals that are more stable are called 2 g orbitals, and the two less stable orbitals are called Sg orbitals. The difference in energy between the two sets is known as the crystal field splitting energy, symbolized by the Greek letter h. 

C20-0049. For an octahedral complex of each of the following metal ions, draw a crystal field energy... 

Figure 3 Crystal field states (left-hand panel) and potential energy surfaces (right-hand panel) for an octahedral complex of nickel(II) in the 3Tig/1Eg energy range. Calculated spectra for the transition to each electronic state are shown in the central panel. Lines with markers connect electronic states and their corresponding calculated spectra. The total calculated spectrum (calc.) is obtained as the sum of the four individual spectra and is compared to the experimental spectrum of Ni(H20)62+ measured at 5K336 (reprinted with permission from ref. 336 1998, American Chemical Society).

Fig. 29. a) Level shift upon abstraction of one ligand from an octahedral complex, b) Energies of the levels of a square pyramid M (CO) 5. Abscissa scale in degrees. The vertical bar is 1 eV on the ordinate energy scale. 

---