The square planar crystal field

Worked example 20.1 Square planar and tetrahedral complexes 

The complexes [Ni(CN)4] and [NiCl4] are square planar and tetrahedral respectively. Will these complexes be paramagnetic or diamagnetic  

Consider the splitting diagrams shown in Fig. 20.11. For [Ni(CN)4] and [NiC ], the eight electrons occupy the d orbitals as follows  

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GO FIGURE Why is the orbital the highest-energy orbital in the square-planar crystal field ... 

Generation of the square planar crystal field splitting pattern by starting with an octahedral crystal field and removing the ligands along the z-axis by way of a tetragonal distortion. 

Figure 6.9 The effect of a square planar crystal field on the energies of d orbitals...

We used crystal field theory to order the energy-level splittings induced in the five d orbitals. The same procedure could be applied to p orbitals. Predict the level splittings (if any) induced in the three p orbitals by octahedral and square-planar crystal fields. 

A FIGURE 23.34 Energies of the d orbitals in a square-planar crystal field. 

Use Figure 4.6 to determine the position of the barycenter for a square planar crystal field. Hint Recall that the net increase in energy from the barycenter must equal the net decrease.)... 

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 4 Crystal field splitting patterns for the common coordination geometries tetrahedral and square planar. For the square-planar arrangement, the z-axis is conventionally taken to be perpendicular to the square plane...

The oxidation of dimethyl sulfide to the corresponding sulfoxide on different zeolites has been reported recently, using zeolite entrapped Cu-ethylenediamine ([Cu(en)2]2") complexes. Spectroscopic comparison between the neat and the NaY, KL, and NaBETA entrapped complexes, shows that the square planar complex undergoes distortion in the zeolite crystal [54-56], Changes in redox properties of the complexes in the zeolites are due to decrease of the HOMO / LUMO levels of the metal complexes upon encapsulation under influence of the electric field existing inside the zeolite [56]. The high activity in ZSM-5, however, points to the existence of extra-pore complexes, probably strongly adsorbed at the external surface. 

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. 

If one cuts the macrocycle of porphyrins by oxidation, helical structures become dominant. The most spectacular case has been found with zinc octaethylformyl-biliverdinate. At neutral pH, the central zinc ion is hydrated and a helix is formed with a disturbed square planar ligand field and one axial water molecule. Upon acidification, however, the water molecule is removed and the zinc ion enforces a tetrahedral ligand field by binding to two different formyl-biliverdinate molecules, rearranging to form a double-helix. Upon neutralization, hydration takes place again and the planar, monomeric monohydrate is reformed in quantitative yield. Both the helical monomer and dimer structures have been solved by single crystal analysis (Fig. 6.2.16). 

Crystal field splitting pattern for the square planar geometry. 

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