Square planar shape

It is clear that simple cyclobutadienes, which could easily adopt a square planar shape if that would result in aromatic stabilization, do not in fact do so and are not aromatic. The high reactivity of these compounds is not caused merely by steric strain, since the strain should be no greater than that of simple cyclopropenes, which are known compounds. It is probably caused by antiaromaticity. ... 

Chlorine pentafluoride and xenon tetrafluoride appear in Figure 9-26. Each has an inner atom with a steric number of 6, but their electron group arrangements include lone pairs. As a result, CIF5 has a square pyramidal shape, whereas XeF4 has a square planar shape. Pictures can help us determine whether or not the bond polarities cancel ... 

The result here is quite satisfactory because XeF4 does in fact exhibit square planar geometry. It is worth noting, however, that a square planar shape for XeF4 is also predicted by VSEPR theory. Despite the fact that the molecular orbital method has made some inroads as of late, VSEPR is still the best approach available for rationalizing the molecular geometries of noble gas compounds. 

The above treatment considers the ligands in an octahedral geometry (i.e. with the ligands placed at the centre of the faces of the cube). The square planar case is simply an extension of the octahedral where two ligands are removed from the z-axis. The repulsion of electrons in d.i and dx2 2 orbitals will not be the same and the result is a square planar shape. 

The xenon atom in XeF4 is bonded to four atoms and has two lone pairs. As you might expect, the lone pairs orient as far away from each other as possible to minimize electronic repulsions, giving the molecule a square planar shape ... 

Furthermore, inorganic compounds present coordination geometries different from those found for carbon. For example, although 4-coordinate carbon is nearly always tetrahedral, both tetrahedral and square planar shapes occur for 4-coordinate compounds of both metals and nonmetals. When metals are the central atoms, with anions or neutral molecules bonded to them (frequently through N, O, or S), these are called coordination complexes when carbon is the element directly bonded to metal atoms or ions, they are called organometaUic compounds. 

If we examine the two five-coordinate shapes from a crystal field perspective, the d orbitals split in a different way to that found for octahedral, tetrahedral and square planar shapes since the d orbitals find the ligands in clearly different locations in space. The crystal field splitting pattern for the two is shown in Figure 4.14. From this pattern, crystal field stabilization energies can be calculated, and favour the square pyramidal geometry in all cases (apart from the trivial situations d° and d10) except for high spin d5. This prediction differs from the outcome from the electron pair repulsion model. 

The absence of any plane of symmetry is a key requirement for a diastereomer having an enantiomer (and hence being chiral). Thus flat molecules, such as those of square-planar shape, do not exist as optical isomers octahedral complexes may be chiral, depending on the type and arrangement of ligands. 

When a molecule has four bonded atoms and two lone pairs, however, the lone pairs always lie at opposite vertices to avoid the stronger 90° lone pair-lone pair repulsions. This positioning gives the square planar shape (AX4E2), as in xenon tetrafluoride (XeF4) ... 

Other experiments were consistent with square-planar Pt(II) compounds, with the four ligands at the comers of a square. Werner found only two isomers for [Pt(NHj)2Cl2]. These isomers conceivably could have different shapes (tetrahedral and square-planar are just two examples (Figure 9.4)), but Werner assumed they had the same shape. Because only one tetrahedral structure is possible for [Pt(NH3)2Cl2], he argued that the two isomers had square-planar shapes with cis and trans geometries. His theory was correct, although his evidence could not be conclusive. 

In addition to tetrahedral, another common shape for AB4 molecules is square planar. All five atoms lie in the same plane, with the B atoms at the corners of a square and the A atom at the center of the square. Which shape in Figure 9.3 could lead to a square-planar shape upon removal of one or more atoms ... 

The five-atom XY4 molecules and ligands commonly adopt tetrahedral and square-planar shapes. The normal modes of tetrahedral and square-planar XY4 are shown in Figure 5.5. Tetrahedral XY4 molecules show two normal modes that are infrared-active , while the square-planar XY4 molecules show three... 

Although coordination number 6 is certainly the most prevalent, 4 is also fairly common-The geometry associated with four ligands around a central metal is usually either tetrahedral or square planar. It makes sense that the larger the ligand, the fewer of them fit around a small metal cation. A detailed explanation of the reasons that a given metal takes on a coordination number of 4 rather than 6 and a square planar shape rather than tetrahedral, however, will have to await our discussion of bonding theories (Chapter 4) and, to some extent, solid-state structures (Chapter 7). For now, it is sufficient to say that square planar complexes are the most common in metals such as Ni(II), Pd(II), Pt(II), and Au(III), and in the d Cu(II). 

Figure 24.5 The complex ion formed between a transition metal ion and a larger ligand can only fit four ligands around the central ion. These are arranged in either a square planar shape (as in a [Ni(CN) ] ") or a tetrahedral shape (as in...

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