Trigonal planar molecules hybridization

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). 

D) BF3 is a trigonal-planar molecule because electrons can be found in only three places in the valence shell of the boron atom. As a result, the boron atom is sp hybridized, which leaves an empty 2p orbital on the boron atom. BF3 can therefore act as an electron-pair acceptor, or Fewis acid. It can use the empty 2p orbital to pick up a pair of nonbonding electrons from a Fewis base to form a covalent bond. BF3 therefore reacts with Lewis bases such as NH3 to form acid-base complexes in which all of the atoms have a filled shell of valence electrons. 

Hybridization. A satisfactory description of covalent bonding should also be able to account for molecular geometry, that is, for the mutual directions of bonds. Let us take for an example boron trifluoride, which is a trigonal planar molecule. Boron uses three orbitals to form three completely equivalent bonds to fluorine atoms. 

The localized 2c MO picture depends on hybrid AOs that point towards other atoms and provide directed valence. Combining s with onep orbital in a valence shell gives two sp hybrids directed at 180° apart. Twop orbitals with 5 make sp2 hybrids directed at 120° in a plane. These can be used to describe a trigonal planar molecule such as BF3. Combining 5 with all threep orbitals gives sp3... 

An sp orbital has three hybrid orbitals resulting in a trigonal planar molecule. 

Fig. 11.2. A the two coordinate systems (x, y) and 4,4 ) B Coordinate system for construction of sp hybrid orbitals on the metal atom pointing towards the ligating atoms in a trigonal planar molecule.

The three-dimensional structures of organic and biochemical molecules play an essential role in determining their physical and chemical behaviors. Because carbon has four valence electrons ([He]2s 2p ), it forms four bonds in virtually all its compounds. When all four bonds are single bonds, the electron pairs are disposed in a tetrahedral arrangement. (Section 9.2) In the hybridization model the carbon 2s and 2p orbitals are then sp hybridized. (Section 9.5) When there is one double bond, the arrangement is trigonal planar (sp hybridization). With two double bonds or a triple bond, it is linear (sp hybridization). Examples are shown in Figure 25.1 T. 

In finite dihedral point groups and dyy transform in the same manner as p, and Py and, for example, in trigonal planar and trigonal bipyramidal molecules hybrids of the type illustrated in Fig. 10 are generated. 

The model of sp hybridization can be used to describe the (T-bonding in trigonal planar molecules such as BH3. The valence state of the B atom is sp Y (i.e. three sp hybrid orbitals, each with one electron) and the equivalence of the B—H interactions follows by considering that each interaction is formed by the overlap of one B sp hybrid orbital with the I5 atomic orbital of an H atom (Fig. 5.5). Each H atom contributes one electron to the bonding scheme and, so, each B—H cr-bond is a localized 2c-2e interaction (see Section 2.2). A diagram similar to that shown in Fig. 5.3 can be constructed to show the formation of a valence state for the trigonal planar B atom. 

In this case, there are three equivalent hybrid orbitals, each called sp (trigonal hybridization). This method of designating hybrid orbitals is perhaps unfortunate since nonhybrid orbitals are designated by single letters, but it must be kept in mind that each of the three orbitals is called sp. These orbitals are shown in Figure 1.4. The three axes are all in one plane and point to the comers of an equilateral triangle. This accords with the known structure of boron trifluoride (BF3), a planar molecule with angles of 120°. 

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. 

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. 

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. 

In HON02, there are a total of 1 + 5 + (3 x 6) = 24 valence electrons, or 12 pairs. N is the central atom, and a plausible Lewis structure is shown on the right The molecule is trigonal planar around N which is sp2 hybridized. The O in the H—O—N portion of the molecule is sp3 hybridized. 

There is a recent review of two-coordinate phosphorus complexes.306 Malisch et a/.307 observed the reversible reaction (86), in which a metal-arsenic(III) double bond is formed, i.e. the M—As a bond is augmented by the arsenic lone pair to form a n bond system (since the cyclopentadienyl coligand is not coplanar with the M=As, the arsenic double bond is isolated). Complex (58) undergoes reactions typical of double bond molecules (Scheme 14). Phosphorus analogues have also been prepared (Scheme 15) the crystal structure of product (c) in Scheme 15 has been solved (59a). The d(W—P) of 2.181 A is shorter than the predicted rf(W=P) of 2.26 A, and the trigonal planar coordination of phosphorus indicates sp2 hybridization.308... 

Examples of neutral Lewis acids are halides of group 3A elements, such as BF3. Boron trifluoride, a colorless gas, is an excellent Lewis acid because the boron atom in the trigonal planar BF3 molecule is surrounded by only six valence electrons (Figure 15.12). The boron atom uses three sp2 hybrid orbitals to bond to the three F atoms and has a vacant 2p valence orbital that can accept a share in a pair of electrons from a Lewis base, such as NH3. The Lewis acid and base sites are evident in electrostatic potential maps, which show the electron poor B atom (blue) and the electron rich N atom (red). In the product, called an acid-base adduct, the boron atom has acquired a stable octet of electrons. 

Borane has the same structure as a carbocation. The boron is sjr hybridized, with trigonal planar geometry, and has an empty p orbital. Although neutral, it is electron deficient because there are only six electrons around the boron. It is a strong Lewis acid. An electron-deficient compound often employs unusual bonding to alleviate somewhat its instability. In the case of borane, two molecules combine to form one molecule of diborane ... 

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