Octahedral complexes VSEPR

The molecular geometry of a complex depends on the coordination number, which is the number of ligand atoms bonded to the metal. The most common coordination number is 6, and almost all metal complexes with coordination number 6 adopt octahedral geometry. This preferred geometry can be traced to the valence shell electron pair repulsion (VSEPR) model Introduced In Chapter 9. The ligands space themselves around the metal as far apart as possible, to minimize electron-electron repulsion. 

The VSEPR model for predicting structure does not work for complex ions. However, we can safely assume that a complex ion with a coordination number of 6 has an octahedral arrangement of ligands, and that complexes with two ligands are linear. On the other hand, complex ions with a coordination number of 4 can be either tetrahedral or square planar there is no reliable way to predict which will occur in a particular case. 

The amended VSEPR model predicts two forms of five-coordination, and experimental chemistry has clearly identified many examples of both forms. These limiting structures are square-based pyramidal (or, simply, square pyramidal) and trigonal bipyramidal (Figure 4.13). The classical square-based pyramidal shape is formed simply by cleaving off one bond from an octahedral shape, which leaves the metal in the same plane as the four square-based ligands. In reality, almost no complexes exhibit this shape, but rather adopt a distorted... 

When using VSEPR to predict the shape of a molecule, we often refer to a molecule with four repulsion axes as adopting a tetrahedral arrangement, even if the four repulsion axes are not identical. To belong to Td, however, the molecule or ion must have four identical bonds arranged tetrahedrally. Many metal complexes are tetrahedral or octahedral, and if you go on to study the chemistry of the transition elements, you will find these two groups play an important role. 

Ni(H20)g] (d ) and [Zn(H20)g]" (d ) to vary as the electronic configuration of the metal ion changes. However, each of these species has an octahedral arrangement of ligands (19.1). Thus, it is clear that VSEPR theory is not applicable to rZ-block metal complexes. 

The two complex ions can be classified as AB4 and ABg stractures (no unshared electron pairs on Al and 4 or 6 attached atoms, respectively). Their VSEPR geometries are tetrahedral and octahedral. 

The isomers described to this point have had octahedral or square-planar geometry. In this section, we describe other geometries. Explanations for some of the shapes are consistent with VSEPR predictions (Chapter 3), with the general assumption that the metal d electrons are stereochemically inactive. In these cases, 3-coordinate complexes have a trigonal-planar shape, 4-coordinate complexes are tetrahedral, and so forth, assuming that... 

Finally, it should be noted that the XeF molecule exhibits a definite tendency to donate a fluoride ion and form the XeFj cation, which is isoelectronic and isasmic-tural with IF] as expected from the VSEFR model. The structure of solid XeF is complex, wilh 144 molecules of XeF per unit cell however there are no discrete XeF molecules. The simplest way to view the solid is as pyramidal XeF cations extensively bridged by free fluoride ions. Obviously, these bridges must contain considerably covalent character. They cause the xenon-containing fragments to duster into tetrahedral and octahedral units (Fig. 6.ISa,b). There are 24 tetrahedra and eight octahedra per unit cell, packed very efficiently as pseudospheres into a CujAu structure (Fig, 6.ISc)."-i The structure thus provides us with no information about molecular XeF, but it does reinforce the idea that the VSEPR-correct, square pyramidal XeF] is structurally stable. 

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