Trigonal planar hybridization

This structure is somewhat unusual in that the two bonds in axial positions are longer than those in the equatorial positions. The hybrid bond type for the trigonal bipyramid is sp3d (or dsp3 in some cases). However, in reality this is a combination of dp (linear) and sp2 (trigonal planar) hybrids so the orbitals used in the axial and equatorial positions are not equivalent, and this is apparent from the bond lengths shown on the preceding structure. 

Fig. 10.9 (a) Trigonal planar hybridization is obtained when an s and two p orbitals are hybridized. The three lobes lie in a plane and make an angle of 120° to each other, (b) The remaining p orbital in the valence shell of an sp -hybridized atom lies perpendicular to the plane of the three hybrids. 

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

To ensure that the arrangement of four atoms in a trigonal planar environment (e.g., a sp -hybridized carbon atom) remains essentially planar, a quadratic term like V(0) = (fe/2) is used to achieve the desired geometry. By calculating the angle 9 between a bond from the central atom and the plane defined by the central... 

Boron tnhahdes, BX, are trigonal planar molecules which are sp hybridized. The X—B—X angles are 120°. Important physical and thermochemical data are presented in Table 1 (8—14). Additional thermodynamic and spectroscopic data may be found in the hterature (1 5). 

We use different hybridization schemes to describe other arrangements of electron pairs (Fig. 3.16). For example, to explain a trigonal planar electron arrangement, like that in BF, and each carbon atom in ethene, we mix one s-orbital with two /7-orbitals and so produce three sp2 hybrid orbitals ... 

Answer Three cr-bonds formed from F2/7c-orbitals and B2s/r hybrids in a trigonal planar arrangement]... 

C atom trigonal planar, so 120° bond angle sp2 hybridized. 

Three equivalent p -hybridized orbitals achieve maximum distance from one another when they arrange in a trigonal planar structure ... 

Inspection of the second resonance structure reveals that this nitrogen atom is actually sp hybridized, not sp. It might look like it is sp hybridized in the first resonance structure, but it isn t. Here is the general rule a lone pair that participates in resonance must occupy ap orbital. In other words, the nitrogen atom in the compound above is sp hybridized. And as a result, this nitrogen atom is trigonal planar rather than trigonal pyramidal. 

For example, let s look at the stereochemistry of SnI reactions. We already saw that Sn2 reactions proceed via inversion of configuration. But SnI reactions are very different. Recall that a carbocation is sp hybridized, so its geometry is trigonal planar. When the nucleophile attacks, there is no preference as to which side it can attack, and we get both possible configurations in equal amounts. Half of the molecules would have one configuration and the other half would have the other configuration. We learned before that this is called a racemic mixture. Notice that we can explain the stereochemical outcome of this reaction by understanding the nature of the carbocation intermediate that is formed. 

The vinylic carbon atoms, highhghted above, are both sp hybridized, and therefore trigonal planar. As a result, aU four groups (connected to the vinylic positions) are... 

The oxygen atom on the left is sp hybridized and trigonal planar. The oxygen atom on the right is sp hybridized and bent. 

Compounds like triethylaluminum and dimethylzinc that have metal-carbon bonds are rather uncommon. Nevertheless, trigonal planar geometry (.s and linear geometry (.S p) occur frequently in nature. As we show in Section 10-1. these geometries and their corresponding hybridizations occur in molecules with double bonds and triple bonds. 

An inner atom with a steric number of 3 has trigonal planar electron group geometry and can be described using s p hybrid orbitals. 

C13-0116. The nitrogen atom of a peptide linkage has trigonal planar geometry. What is the hybridization of the nitrogen atom in a peptide linkage Explain why nitrogen adopts this form of hybridization. 

Trimethylboron is an example of one type of Lewis acid. This molecule has trigonal planar geometry in which the boron atom is s hybridized with a vacant 2 p orbital perpendicular to the plane of the molecule (Figure 21-11. Recall from Chapter 9 that atoms tend to use all their valence s and p orbitals to form covalent bonds. Second-row elements such as boron and nitrogen are most stable when surrounded by eight valence electrons divided among covalent bonds and lone pairs. The boron atom in B (CH ) can use its vacant 2 p orbital to form a fourth covalent bond to a new partner, provided that the new partner supplies both electrons. Trimethyl boron is a Lewis acid because it forms an additional bond by accepting a pair of electrons from some other chemical species. 

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

Crystal structure analysis of 43 revealed that the radical is perfectly trigonal planar, implying the sp -hybridization of the central Si atom and, hence, the localization of the unpaired electron on its 3pz-orbital. The remarkable planarity of the radical 43 was explained by the great steric bulk of the voluminous t-Bu2MeSi groups, which prefered to move away from each other as far as possible to avoid the steric repulsion between them. On the contrary, the significant electron donation of the positive silyl substituents... 

It should be very important to reveal the structural features of heavy ketones and to make a systematic comparison with those of a carbonyl compound such as the bond shortening and the trigonal planar geometry which result from the sp2 hybridization between the carbon and oxygen atoms. 

It goes from a trigonal planar geometry and sp hybridization to a tetrahedral... 

Additional bonds are thus donor bonds, and to accept electron pairs from neutral and anionic ligands, zinc uses the two remaining 4p orbitals to form sp2 and sp3 hybrids. In the absence of steric effects, discrete, homoleptic, anionic tri- and tetraorganozinc compounds (zincates) have almost always ideal trigonal-planar and tetrahedral geometries, respectively. 

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