Square planar complex crystal field splitting

Note that, of course, the cubic crystal field splitting dzi, dx2 y2 (eg) at 6Dq and dxz, dyz, dxy at -4Dq is reproduced if Ds and Dt are both zero. Note also that the centre of gravity of neither of the eg nor of the t2g sets is maintained as the symmetry departs from cubic. This means that, in low symmetry, the concept of the cubic field splitting is not clearly defined. For small departures from cubic symmetry the lack of definition is not serious in practice, but to maintain the concept in, say, a square-planar complex, as is sometimes done, requires care. 

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. 

One always begins with the monomer. What are its frontier levels The classical crystal field or molecular orbital picture of a square planar complex (Fig. 2) leads to a 4 below 1 splitting of the d block.11 For 16 electrons we have z2, xz, yz, and xy occupied and x2-y2 empty. Competing with the ligand field-destabilized x2-y2 orbital for being the lowest unoccupied molecular orbital (LUMO) of the molecule is the metal z. These two orbitals can be manipulated in understandable ways x acceptors push z down, x donors push it up. Better a donors push x2-y2 up. 

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. 

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. 

In Section 20.3, we discussed the increase in crystal field splitting on progressing down group 10, and explained why Pd(II) and Pt(II) complexes favour a square planar arrangement of donor atoms (but see Box 20.7). In this section, the discussion of Pd(II) and Pt(II) compounds reiterates these points. 

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. 

Crystal-field theory also applies to tetrahedral and square-planar complexes, which leads to different d-orbital splitting patterns. In a tetrahedral crystal field, the splitting of the d orbitals results in a higher-energy t2 set and a lower-energy e set, the opposite of the octahedral case. The splitting by a tetrahedral crystal field is much smaller than that by an octahedral crystal field, so tetrahedral complexes are always high-spin complexes. 

PdCl4] and [PtCl4] (also d ) are square planar and diamagnetic. This dilference is a consequence of the larger crystal field splitting observed for second and third row metal ions compared with their first row congener Pd(II) and Pt(II) complexes are invariably square planar (but see Box 21.6). 

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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