Grouping geometries

Linear Linear coordination structures are formed by ligand-metal-ligand, or l M-L, bonding in a line. They are usually associated with CN of 2 and +1 cations, such as Cu, Ag, Au, and Hg2. Sometimes they occur for other transition metals when the ligands are extremely bulky, leaving room for only two ligands. Examples include BeH2, CO2, and HCN. 

Square planar Square planar structures form with CN of 4, and all the 

Tetrahedral Tetrahedral structures are also associated with CN of 4 and are more common than square planar structure. In tetrahedral or tetragonal structures the four ligands and central atom are not in the same plane. The tetragonal shape can form for all the nontransition metals, and some transition metals. Examples include CH, CCl, and SO . (The T-metals on the right side of the d-block have no electron in the valence shell and can only form sigma bonds to the ligands. For more about a (sigma) and k (pi) bonds, see Chapters 6 and 7.) 

Octahedral Octahedral structures usually form with CN of 6. These complexes form an octahedron, with near perfect symmetry that can be elongated if a variety of ligands are used. This is important because nearly all cations form 6-coordinate complexes, so it s the most common geometric shape. Examples include SFg, and SiFg.  

Trigonal Trigonal structures form with CN 3 and CN 5. These complexes can form planar trigonal structures, or trigonal pyramidal shapes. These are less common because coordination numbers of 3 and 5 are r ire. Examples include BF3, COg, and CoClg. 

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Electron group geometries and molecular shapes for steric number of 4. 

An inner atom with a steric number of 4 has tetrahedral electron group geometry. 

Our approach to these molecules illustrates the general strategy for determining the electron group geometry and the molecular shape of each inner atom in a molecule. The process has four steps, beginning with the Lewis structure and ending with the molecular shape. 

Follow the four-step process described in the flowchart. Begin with the Lewis structure. Use this stracture to determine the steric number, which indicates the electron group geometry. Then take into account any lone pairs to deduce the molecular shape. 

A steric number of 4 identifies four electron groups that must be separated in three-dimensional space. Four groups are as far apart as possible in tetrahedral electron group geometry. 

Both atoms have tetrahedral electron group geometry. 

Many elements of the periodic table, from titanium and tin to carbon and chlorine, exhibit tetrahedral electron group geometry and tetrahedral molecular shapes. In particular, silicon displays tetrahedral shapes in virtually all of its stable compounds. 

Quartz, a common form of silica, is a network of Si—O bonds. Silicon and oxygen both have tetrahedral electron group geometry. All the silicon atoms have tetrahedral shapes and all the oxygen atoms have bent shapes. 

Tetrahedral geometry may be the most common shape in chemistry, but several other shapes also occur frequently. This section applies the VSEPR model to four additional electron group geometries and their associated molecular shapes. 

An Inner atom with a sterlc number of 5 has trigonal blpyramldal electron group geometry. 

The steric number for chlorine is 5, leading to a trigonal bip3Tamidal electron group geometry. 

Stable noble gas compounds are restricted to those of xenon. Most of these compounds involve bonds between xenon and the most electronegative elements, fluorine and oxygen. More exotic compounds containing Xe—S, Xe—H, and Xe—C bonds can be formed under carefully controlled conditions, for example in solid matrices at liquid nitrogen temperature. The three Lewis structures below are examples of these compounds in which the xenon atom has a steric munber of 5 and trigonal bipyramidal electron group geometry. 

Follow the usual procedure. Determine the Lewis stmcture, then use it to find the steric number for xenon and to deduce electron group geometry. Next, use the number of ligands to identify the molecular shape. 

Table 9 3 summarizes the relationships among steric number, electron group geometry, and molecular shape. If you remember the electron group geometry associated with each steric number, you can deduce molecular shapes, bond angles, and existence of dipole moments. 

C09-0124. The inner atom of a triatomic molecule can have any of four different electron group geometries. Identify the four, describe the shape associated with each, and give a specific example of each. 

C09-0140. Determine the Lewis stmctures, electron group geometries, and molecular shapes of the following compounds, which contain odd numbers of valence electrons. 

According to the VSEPR model developed in Chapter 9, an inner atom with a steric number of 4 adopts tetrahedral electron group geometry. This tetrahedral arrangement of four electron groups is very common, the only important exceptions being the hydrides of elements beyond the second row, such as H2 S and PH3. Thus,... 

Use the strategies from Chapter 9 to determine the Lewis structure, steric number of the inner atom, and electron group geometry. The steric number also determines the hybridization. 

Both inner atoms have steric numbers of 4 and tetrahedral electron group geometry, so both can be described using s p hybrid orbitals. All four hydrogen atoms occupy outer positions, and these form bonds to the inner atoms through 1 s-s p overlap. The oxygen atom has two lone pairs, one in each of the two hybrid orbitals not used to form O—H bonds. 

We generate hybrid orbitals on inner atoms whose bond angles are not readily reproduced using direct orbital overlap with standard atomic orbitals. Consequently, each of the electron group geometries described in Chapter 9 is associated with its own specific set of hybrid orbitals. Each type of hybrid orbital scheme shares the characteristics described in our discussion of methane ... 

To describe bonding, start with a Lewis structure, determine the steric number, and use the steric number to assign the electron group geometry and hybridization of the inner atom. Treat each molecule separately. 

With a steric number of 6, xenon has octahedral electron group geometry. This means the inner atom requires six directional orbitals, which are provided by an. s p d hybrid set. Fluorine uses its valence 2 p orbitals to form bonds by overlapping with the hybrid orbitals on the xenon atom. The two lone pairs are on opposite sides of a square plane, to minimize electron-electron repulsion. See the orbital overlap view on the next page. 

The hybridization schemes match the shapes that we describe in Chapter 9. Notice that the steric numbers of inner atoms uniquely determine both the electron group geometry and the hybridization. This makes sense, because the steric number describes how many electron groups must be accommodated around an inner atom. 

For each of the following Lewis structures, name the electron group geometry and the hybrid orbitals used by the inner atoms. 

The steric number of an inner atom determines the electron group geometry, each of which is associated with one specific type of hybrid orbital. 

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