Hybrid orbitals of boron

The molecular geometry and three-center bonding scheme 4S, 18) for B2H6 are shown in Fig. 11. If we assume that two of the sp hybrid orbitals of boron have maxima directed toward the singly bonded hydro-... 

FIGURE 10.12 The sp hybrid orbitals of boron overlap with the 2p orbitals of fluorine. The BF3 molecule is planar, and all the FBF angles are 120°. 

To explain the planar trigonal structure of boron trichloride with a bond angle 120 , the central boron atom is said to be sp hybridized. The sp hybrid orbitals are obtained by the hybridization of s and two p orbitals. The three sp hybrids lie in a plane and the angle between any two of them is 120°. Each B-Cl bond is formed by the overlap of an sp- hybrid orbital of boron with a p orbital of chlorine atom. 

Boron in the boron trifluoride molecule, BF3, provides an example of sp hybridization (Figure 14.29). The boron atom has the electron configuration ls 2s 2pb After hybridization the electrons in the outer shell of the boron atom are in three spr hybrid orbitals. Single bonds between the boron and fluorine atoms involve the sharing of the unpaired electrons in the sp hybrid orbitals of boron and the sp orbitals of the fluorine atoms (Figure 14.30). 

Fig. 2.29. Hybrid orbitals of boron useful in bonding description of boranes and carboranes...

The boronhalides (BX3) are volatile, highly reactive, monomeric compounds, which show no detectable tendency to dimerize. In this they resemble organoboranes, BR3, but differ sharply from diborane, B2H6. The molecules have a planar trigonal structure with the boron atom (sp2 - hybridization) in the centre. This class of compounds also exhibits partial double bond character due to pT - px interactions of the empty pz-orbital of boron and the filled pz-orbital of the halogen. Because of the incomplete octetstructure of the central boron atom, borontrihalides behave as strong Lewis acids. 

It can be understood by formation of sp hybrid orbitals by Boron atom, one of these sp hybrid orbitals combines with H, Is, forming two M.O. (one bonding and one antibonding) and the second sp hybrid orbitals, which does not overlap much is non-bonding. 

The usual hybrid orbitals for boron in its better-known chemistry are tetrahedral (e.g., BH4 ) or trigonal (e.g., BF3). We shall start our description with these hybrids and with the Is orbital of hydrogen (Fig. 

The molecule has two three centre-two electron bonds. The exact state of hybridization of boron is possibly indecisive. Let us take the view that each boron is sp3 hybridized. Two of the hybrid orbitals of one boron atom overlap with the terminal hydrogen orbitals. The remaining two hybrid orbitals have only one electron and these overlap with two similar orbitals of the other boron atom, through the Is orbitals of the bridging hydrogens. 

Overall, the orbitals used in the hybridization are the 2s, 2p and 2py orbitals of boron, comprising the familiar sp hybrids. The difference between this approach and the molecular orbital approach is that these orbitals are combined to form the hybrids before considering their interactions with the fluorine orbitals. Because the overall symmetry is trigonal planar, the resulting hybrids must have that same symmetry, so the three sp orbitals point at the three comers of a triangle, and each interacts with a fluorine p orbital to form the three a- bonds. The 2p orbital is not involved in the bonding and, according to the hybrid approach, is empty this orbital serves as an acceptor in acid-base reactions. 

Each boron or carbon atom has sp hybridization. One of the hybridized orbitals of each atom forms a bond with the terminal hydrogen atom, and the remaining three orbitals (one sp and two p orbitals) give multicenter orbitals of the cluster grouping. The molecular orbital theory shows that borane polyhedra having n vertices have such symmetry that 3n atomic orbitals of the cluster possessing n skeletal atoms form +1 bonding molecular orbitals. ... 

Boron, atomic number 5, has three electrons in its valence shell. To bond with three other atoms, boron uses sp hybrid orbitals. The unoccupied 2p orbital of boron is perpendicular to the plane created by boron and the three other atoms to which it is bonded. An example of a stable, trivalent boron compound is boron ttifluotide, BF3, a planar molecule with F—B—F bond angles of 120° (Section 1.2E). Because of the vacant 2p orbital in the valence shell of boron, BH3, BF3, and aU other trivalent compounds of boron are electrophiles. These compounds of boron resemble carbocations, except that, unlike carbocations, they are electrically neutral. BH3 is a planar molecule with H— B—H bond angles of 120° (see margin). 

PROBLEM 2.5 The 2p, orbital of boron was not used in our construction of the three sp hybrid atomic orbitals for the atom. Sketch it in, using the drawing of Rgure 2.6. 

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... 

Borazine, B3N3Hft, a compound that has been called inorganic benzene because of its similar hexagonal structure (but with alternating B and N atoms in place of C atoms), is the basis of a large class of boron—nitrogen compounds. Write its Lewis structure and predict the composition of the hybrid orbitals used by each B and N 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°. 

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