Octahedral complexes, energy difference

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.

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. 

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. 

From a consideration of the combination of ligand and metal orbitals, it should be apparent that the overlap is much more effective in an octahedral complex (in which orbitals are directed at ligands) than in a tetrahedral complex (where orbitals are directed between ligands). The result is that the energy difference between the e and t2 orbitals in a tetrahedral complex is much smaller than that between the t2g and eg orbitals in an octahedral complex. As we saw when considering the two types of complexes by means of ligand field theory, At is only about half as large as A0 in most cases. 

Figure 2.7 The ways in which Jt-bonding interactions with a ligand can influence the value of the energy difference, A for an octahedral complex. High energy, poorly populated jr-orbitals in the ligand increase the splitting (i.e. are Jt-acceptors), whereas filled, low-energy Jt-orbitals decrease the splitting (they are Tt-donors).

SA quite different approach (Biirgi and Dunitz, 1983), using atomic vibration tensors to estimate torsional energy barriers (assumed sinusoidal) about single bonds, is described by Trueblood and Dunitz (1983) and has been applied to pseudo-Jahn-Teller deformations of octahedral complexes by Stebler and Biirgi (1987). 

Cobalt(II) forms more tetrahedral complexes than any other transition metal ion. Also, because of small energy differences between the tetrahedral and octahedral complexes, often the same ligand forms both types of Co(II) complexes in equilibrium in solutions. 

The first intense (Laporte-allowed) electron transfer absorption band of a complexed ion is generally due to the transition of an electron essentially from a ligand orbital to an orbital of the central ion for an octahedral complex of a transition metal, for example, the transition is from the (tt + a) orbital set to the (d) t2g or (d) e. The energy of this absorption band has been correlated, by Jt rgensen (7), with the electronegativity difference between the surrounding ligands and the central ion such that... 

Table 11.17 summarizes this information for weak field octahedral and tetrahedral complexes. Octahedral complexes having d1. d. d. and d9 configurations and weak field ligands should each give one absorption corresponding to A,r Configurations dz, d. d7, and d in weak octahedral fields each have three spin allowed transitions. (In each case is the energy difference between adjacent A2l, and terms.) As we have... 

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