Trigonal planar figure

Trigonal Planar Geometry A trigonal planar species has bond angles of 120°. AX3 species, such as urea, are trigonal planar (Figure 7.8). Note there are no lone pairs of electrons on the central carbon atom. 

The three electron pairs on boron have a trigonal planar arrangement, and the molecular geometry, which is determined by the directions of the bonding pairs, is also trigonal planar (Figures 10.1 and 10.4). 

In ethene the situation is rather different here, each carbon atom has one 2s and two 2p orbitals hybridised to form three sp single-pear orbitals which are trigonal planar (shown shaded in each half of Figure 2. JO). The remaining 2p orbital is not hybridised,... 

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 ... 

Figure 1.35 The triangular (trigonal planar) shape of boron trifluoride maximally separates the three bonding pairs.

The solid-state structure of 68c, Figure 36, shows that the zinc atom is surrounded in a trigonal-planar fashion by one methyl group and two 2,4,6-trisilacyclohexyl ligands, of which one has a chair conformation, while the other one has a boat conformation. 

Carbene adducts of dimethylcadmium 188-190 have been obtained by simple addition of CdMe2 to the corresponding carbene at room temperature.2 9 All the three adducts have similar structures. In the case of 190, the Ccarbene-Cd bond of 2.33 A is 0.15 A longer than the Cmethyi-Cd bonds of 2.18 A (Figure 28). As a result, the cadmium atom is in a distorted trigonal-planar environment with a Cmethy -Cd-Cmethyl angle of 136°. 

Figure 4.1 shows the common structural types that describe a very large number of inorganic molecules. Linear, trigonal planar, tetrahedral, trigonal bipyramid, and octahedral structures result when... 

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