Tetrahedral complexes crystal field splitting

Tetrahedral complexes arc also common, being formed more readily with cobali(II) than with the cation of any other truly transitional element (i.e. excluding Zn ). This is consistent with the CFSEs of the two stereochemistries (Table 26.6). Quantitative comparisons between the values given for CFSE(oct) and CFSE(let) are not possible because of course tbc crystal field splittings, Ao and A, differ. Nor is the CFSE by any means the most important factor in determining the stability of a complex. Nevertheless, where other factors are comparable, it can have a decisive effect and it is apparent that no configuration is more favourable than d to the adoption of a tetrahedral as opposed to... 

C20-0014. Draw crystal field splitting diagrams that show the electron configurations for the following complex ions (a) [Cr (H2 (b) [IrCle] (c) [V (en)3] and (d) [NiCl4] (tetrahedral). 

FIGURE 20.30 Energies of the d orbitals in tetrahedral and square planar complexes relative to their energy in the free metal ion. The crystal field splitting energy A is small in tetrahedral complexes but much larger in square planar complexes. 

We may use exactly similar arguments to obtain total CFSE terms for the various d electron configurations within a tetrahedral crystal field. It is quite possible to construct crystal field splitting diagrams for any of the other geometries commonly adopted in transition-metal complexes, and to calculate the appropriate CFSE terms. 

However, since CFT is a rather crude model, such an exact relationship is seldom of use. Instead, it is instructive and convenient to realize that, with all conditions being equal, the crystal field splitting for a tetrahedral complex is about half of that for an octahedral complex. 

For tetrahedral complexes, with their crystal field splitting At being only about half of A0, the high-spin configuration is heavily favored. Indeed, low-spin tetrahedral complexes are rarely observed. 

The first-order JT effect is important in complexes of transition metal cations that contain nonuniformly filled degenerate orbitals, if the mechanism is not quenched by spin-orbit (Russell-Saunders) coupling. Thus, the JT effect can be expected with octahedrally coordinated and high spin d cations, and tetrahedrally coordinated and d cations. The low-spin state is not observed in tetrahedral geometry because of the small crystal field splitting. Also, spin-orbit coupling is usually the dominant effect in T states so that the JT effect is not observed with tetrahedrally coordinated d, d , d, and d ions. 

We will briefly consider the crystal field splitting of the rf-orbitals in four-coordinate, tetrahedral complexes. The cube, octahedron and tetrahedron are related geometrically. Octahedral coordination results when ligands are placed in the centers of cube faces, while tetrahedral coordination results when ligands are placed on alternate comers of a cube, as shown in Fig. 10.9. 

FIGURE 22.23 Crystal field splitting between d orbitals in a tetrahedral complex. 

FIGURE 22.24 Energy-level diagram for a square-planar complex. Because there are more than two energy levels, we cannot define crystal field splitting as we can for octahedral and tetrahedral complexes. 

For the same type of ligands, explain why the crystal field splitting for an octahedral complex is always greater than that for a tetrahedral complex. 

The [Ni(CN)4] ion, which has a square-planar geometry, is diamagnetic, whereas the [NiC ] ion, which has a tetrahedral geometry, is paramagnetic. Show the crystal field splitting diagrams for those two complexes. 

Crystal Field Splitting in Tetrahedral and Square Planar Complexes Four ligands around a metal ion also cause d-orbital splitting, but the magnitude and pattern of the splitting depend on whether the ligands are in a tetrahedral or a square planar arrangement. 

Nickel(II) complexes in which the metal coordination number is 4 can have either square-planar or tetrahedral geometry. [NiC ] is paramagnetic, and [Ni(CN)4] is diamagnetic. One of these complexes is square planar, and the other is tetrahedral. Use the relevant crystal-field splitting diagrams in the text to determine which complex has which geometry. 

Which of these crystal-field splitting diagrams represents (a) a weak-field octahedral complex of Fe, (b) a strong-field octahedral complex of Fe, (c) a tetrahedral complex of Fe , (d) a tetrahedral complex of Ni (The diagrams do not indicate the relative magnitudes of A.) [Section 23.6]... 

The use of magnetic data to assist in the assignments of coordination geometries is exemplified by the difference between tetrahedral and square planar species, e.g. Ni(II), Pd(n), Pt(n), Rh(I) and Ir(I). Whereas the greater crystal field splitting for the second and third row metal ions invariably leads to square planar complexes (but see Box 21.6), nickel(n) is found in both tetrahedral and... 

Figure 2.13 Crystal field splittings and electron distributions for some metal complexes. The struetures of the first two complexes are oetahedral, and the others (left to right) are tetragonal, square planar, and tetrahedral (see Figure 2.10).

We have considered the distortions to octahedral structure that result from the presence of d electrons. Tetrahedral structures are also observed in metal complexes however, they are less common than octahedral and distorted octahedral configurations. If four ligands surround a metal atom, a tetrahedral structure is expected. Two exceptions must be noted. As we have seen, four-coordinated low-spin (f complexes are square planar, as are many four-coordinated (f and high-spin d complexes. Tetrahedral cf, d, cf, and (f systems should exhibit marked Jahn Teller distortions however, very few examples of this type of eompound exist. Low-spin tetrahedral complexes need not be discussed, sinee there are no examples of such complexes. The tetrahedral crystal field splitting (A,) is apparently too small to cause spin pairing. 

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