Crystal field theory square planar

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. 

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. 

We will begin our discussion of crystal field theory with the most straightforward case, namely, complex ions with octahedral geometry. Then we will see how it is applied to tetrahedral and square-planar complexes. 

The first two entries are examples of HTMCC with a planar metal framework. In the [AuFe4(CO)i6]" n = 1,2,3) system only the parent monoanion has so far been investigated by X-ray diffraction (Fig. la) whereas the corresponding di- and trianions have only been characterized spectroscopically. The [AuFe4(CO)i6] anion can be considered as a square-planar d Au- + complex stabilized by two bidentate [Fe2(CO)8] ligands, isolobal with diphosphine. Crystal-field theory... 

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. 

Richard Fenske arrived as Assistant Professor at the University of Wisconsin in 1961 after having completed his PhD, with Donald Martin at Iowa State University on applying crystal-field theory to square-planar platinum complexes [19]. Fenske was interested in developing a method more closely tied to the ab initio molecular orbital method described so beautifully by Roothaan [20]. Building on some previous suggestions [21], he and his first students, especially Ken Caulton and Doug Radtke, developed an approximate self-consistent field method that had no empirical or adjustable parameters. With some later refinements by this author, the method became widely known as the Fenske-Hall method [22], and in this form it is still being used today [23]. 

Obtain the distribution of d electrons in the complex ions listed below, using crystal field theory. Each ion is either tetrahedral or square planar. On the basis of the number of unpaired electrons (given in parentheses), decide the correct geometry. 

A6.13 It is important to note that, within the crystal field theory, A1 (for square planar MX4) is... 

This completes a description of the basic CFT as it applies to octahedral fields. We proceed now to briefer treatments of tetragonaUy distorted, square planar, and tetrahedral fields. Following these sections, we will be in a position to investigate the consequences and applications of crystal field theory. 

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