Square planar crystal field

Octahedral crystal field Square planar crystal field... 

Although Chapter 25 does not address directly why some compounds with coordination 4 are tetrahedral and some are square planar, it is possible to surmise that the answer lies with (1) Crystal Field Theory and the energies of the d orbitals involved bonding and (2) how many unpaired electrons the metal complex has. 

The above discussion has considered the stabilization of complexes in terms of the crystal field theory. It is desirable to consider the same topic in terms of modern molecular orbital theory. Although the development and sophisticated consideration of the MO treatment is far beyond the scope of this chapter, an abbreviated, qualitative picture will be presented, focusing again on the energy levels of the highest occupied and lowest empty orbitals and again using the square planar d case. 

Figure 1.18 Crystal field splitting for d orbitals A = square-planar field B = octahedral field C = tetrahedral field.

Fio. 26. Energy level diagram of 3cP configuration (Cu" " ) in square planar complex or tetragonal crystal field (CF). Effect of bonding of the 3d-electrons with ligands is shown. 

The coordination geometry of these Cu(III) complexes is presumed to be square planar, indicative of high field d complexes. This has been demonstrated in the crystal structure of deprotonated tri-a-aminobutyric acid, Cu "(H 2Aib3), in which the copper-donor atom bonds were found to be 0.12-0.17 A shorter than for the corresponding Cu(II) complex... 

Square planar d paramagnetic complexes are extremely rare (see Footnote 9). Account for this observation with a crystal field argument. 

On a theoretical basis, Pd(III) and Pd(I) ions are probably located in a distorted octahedral or trigonal crystal field (Si or Si/, Sir sites) rather than in a square planar environment. The ESR spectra of adsorbed CO CO /... 

The [Ni(CN)4]2 anion is one of the most stable nickel(II) complexes and an overall formation constant as high as about 1030 has been determined.627,62 The structure of the complex is square planar with the nickel(II) bound to carbon atoms of cyanides and with linear Ni—C—N linkages (Table 37).629 630 The planar [Ni(CN)4]2 units are stacked in columns in the crystal lattice with Ni—Ni interlayer distances as short as 330 pm. C-bonded CN- is a strong field donor and the electronic spectrum of [Ni(CN)4]2 shows two weak d-d bands at 444 and 328 nm. 

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. 

Draw a crystal field energy-level diagram for a square planar complex, and explain why square planar geometry is especially common for ds complexes. 

Fig. l.i. Crystal field splitting diagrams for (a) octahedral MLh, (b) tetrahedral ML4, (c) square planar ML4, (d) square pyramidal MLS and (e) trigonal bipyramidal ML5. 

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