Steric numbers

CyclooctatetraenylCompounds. Sandwich-type complexes of cyclooctatetraene (COT), CgH g, are well known. The chemistry of thorium—COT complexes is similar to that of its Cp analogues in steric number and electronic configurations. Thorocene [12702-09-9], COT2Th, (16), the simplest of the COT derivatives, has been prepared by the interaction of ThCl [10026-08-1] and two equivalents of K CgHg. Thorocene derivatives with alkyl-, sdyl-, and aryl-substituted COT ligands have also been described. These compounds are thermally stable, air-sensitive, and appear to have substantial ionic character. 

Figure 9-16 shows the molecular shapes of methane, ammonia, and water, all of which have hydrogen ligands bonded to an inner atom. These molecules have different numbers of ligands, but they all have the same steric number. 

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

The steric number identifies how many groups of electrons must be widely separated in three-dimensional space. In ammonia, for example, the nitrogen atom bonds to three hydrogen atoms, and it has one lone pair of electrons. How are the three hydrogen atoms and the lone pair oriented in space Just as in methane, the four groups of electrons are positioned as far apart as possible, thus minimizing electron-electron repulsion. 

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

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. 

From the Lewis structure, we see that oxygen bonds to three hydrogen ligands and has one lone pair. The sum of the lone pairs and the ligands yields a steric number of 4. 

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. 

Notice that the zinc atom is associated with only four valence electrons. Although this is less than an octet, the adjacent carbon atoms have no lone pairs available to form multiple bonds. In addition, the formal charge on the zinc atom is zero. Thus, Zn has only four electrons in the optimal Lewis structure of dimethyizinc. This Lewis stmcture shows two pairs of bonding electrons and no lone pairs on the inner atom, so Zn has a steric number of 2. Two pairs of electrons are kept farthest apart when they are arranged along a line. Thus, the C—Zn—C bond angle is 180°, and linear geometry exists around the zinc atom. 

The carbon atom in CO2 has two groups of electrons. Recall from our definition of a group that a double bond counts as one group of four electrons. Although each double bond includes four electrons, all four are concentrated between the nuclei. Remember also that the VSEPR model applies to electron groups, not specifically to electron pairs (despite the name of the model). It is the number of ligands and lone pairs, not the number of shared eiectrons, that determines the steric number and hence the molecular shape of an inner atom. 

A1 (CH2 0113)3 1 indicates that aluminum bonds to three CH2 CH3 fragments. There are 42 valence electrons, all of which are used to complete the bonding framework. Each of the six carbon atoms in triethylaluminum has an octet of electrons and a steric number of 4. Thus, each ethyl group of A1 (CH2 0113)3 described exactly... 

The elements beyond Row 2 of the periodic table can accommodate more than four groups of electrons, and this results In steric numbers greater than 4. 

Unlike the geometries for other steric numbers, the five positions in a trigonal bipyramld are not all equivalent, as shown in Figure 9-21a. Three positions lie at the comers of an equilateral triangle around the phosphorus atom, separated by 120° bond angles. Atoms in the trigonal plane are In equatorial positions. The other two positions... 

The difference between equatorial and axial positions determines the arrangement of bonding pairs and lone pairs around an atom with a steric number of 5. An example is provided by sulfur tetrafluoride, a colorless gas that has industrial uses as a potent fluorinating agent. The Lewis structure of SFq shows four S—F bonds and one lone pair of electrons on the sulfur atom. These five pairs of electrons are distributed in a trigonal bipyramid around the sulfur atom. 

The trigonal bipyramid (PCI5) and the seesaw (SF4) are two of the four geometries for an atom with steric number 5. Example introduces the other two, T-shaped and linear. 

Use the Lewis structure of CIF3 to determine the steric number of the chlorine atom. Obtain the molecular shape from the orbital geometry after placing lone pairs in appropriate positions. 

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

When the steric number is 5, there are two distinct positions, equatorial and axial. Placing lone pairs in equatorial positions always leads to the greatest stability. Thus, CIF3 is T-shaped with two equatorial lone pairs. 

C09-0023. The fourth molecular shape arising from a steric number of 5 is represented by the triiodide anion I3. Determine the molecular geometry and draw a three-dimensional picture of the triiodide ion. 

Figure 9-23 summarizes the characteristics of atoms with steric number 5. 

Three common molecular shapes are associated with octahedral electron group geomehy. Most often, an inner atom with a steric number of 6 has octahedral molecular shape with no lone pairs. Example uses a compound of xenon, whose chemical behavior is described in the Chemical Milestones Box, to show a second common molecular shape, square planar. 

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. 

The square pyramidal geometry of CIF5 completes our inventory of molecular shapes. Figure 9-26 summarizes the characteristics of atoms with steric number 6. 

Each of the steric numbers described in Sections 94 and 94 results in electron groups separated by well-defined bond angles. If the VSEPR model is accurate, the actual bond angles found by experimental measurements on real molecules should match the optimal angles predicted by applying the model. 

Experimental results agree with the predictions of the model. For instance, measurements show bond angles of 109.5° in CH4, 120° in A1 (C2 115)3 > Moreover, when the steric number of an atom changes,... 

To interpret bond angles, we must constmct a model of the molecule by using Lewis structures and steric numbers. 

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 ... 

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. 

This relatively small catalog of molecular shapes accounts for a remarkable number of molecules. Even complicated molecules such as proteins and other polymers have shapes that can be traced back to these relatively simple templates. The overall shape of a large molecule is a composite of the shapes associated with its inner atoms. The shape around each inner atom is determined by steric numbers and the number of lone pairs. 

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