Boron trifluoride trigonal planar structure

Boron trifluoride has a trigonal-planar structure. Formulate the bonding in terms of molecular orbitals for the Dsjj symmetry. In addition, construct wave functions for three equivalent sp2 hybrid orbitals, using the 2px, 2p, and 2s boron valence orbitals, which may be used to form three localized bonds with the three fluorines. Compare and contrast the molecular-orbital and the hybrid-orbital descriptions. 

Boron trifluoride has a trigonal-planar structure, with all F——F bond angles 120°. Boron has Is and 2 orbitals that bond with the fluorine Is and Ip orbitals. A convenient coordinate system for a discussion of bonding in BFj is shown in Fig. 4-1. 

Boron, a group IIIA element, has only three valence electrons. In the compound boron trifluoride (BF3) these three electrons are shared with three fluorine atoms. As a result, the boron atom in BF3 has only six electrons (three bonding pairs) around it. Maximum separation of three bonding pairs occurs when they occupy the comers of an equilateral triangle. Consequently, in the boron trifluoride molecule the three fluorine atoms lie in a plane at the comers of an equilateral triangle (Fig. 1.36). Boron trifluoride is said to have a trigonal planar structure. The bond angles are 120°. 

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°. 

A boron trifluoride molecule, BF3, has the Lewis structure shown in (5). There are three bonding pairs attached to the central atom and no lone pairs. According to the VSEPR model, as illustrated in Fig. 3.4, the three bonding pairs, and the fluorine atoms they link, lie at the corners of an equilateral triangle. Such a structure is trigonal planar, and all three F—B—F angles are 120° (6). 

Boron is an element in group 13 of the periodic table. Therefore, it has three valence electrons. The Lewis structure (Figure 4-45) of the boron trifluoride molecule, BF3, shows there are only three electron pairs (three electron domains) in the valence shell of the boron atom. These three bonding pairs repel each other equally, with the result that the boron trifluoride molecule is a trigonal planar (flat) molecule. The three boron-fluorine bonds point towards the three corners of an equilateral triangle. The bond angles are all equal at 120°. 

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