Trigonal-Planar Molecules

Boron trifluoride is a trigonal planar molecule There are six electrons two for each B—F bond associated with the valence shell of boron These three bonded pairs are farthest apart when they are coplanar with F—B—F bond angles of 120°... 

Multiple bonds are treated as a single unit m the VSEPR model Formaldehyde is a trigonal planar molecule m which the electrons of the double bond and those of the two single bonds are maximally separated A linear arrangement of atoms m carbon diox ide allows the electrons m one double bond to be as far away as possible from the elec Irons m the other double bond... 

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). 

The example of COj discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses a center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0, 0, 0), for every atom positioned at (au, yi, z ) there will be an identical atom at (-a ,-, —y%, —z,). A square planar molecule XY4 has a center of symmetry at atom X, whereas a trigonal planar molecule XYS does not possess a center of symmetry. 

The B atom has an incomplete octet in all its trihalides. The compounds consist of trigonal planar molecules with an empty 2p-orbital perpendicular to the molecular plane. The empty orbital allows the molecules to act as Lewis acids, which accounts for the catalytic action of BF3 and BC13. 

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. 

A proper axis of symmetry, denoted by Cn, is an axis around which a molecule is rotated by 360°ln to produce an equivalent configuration. The trigonally planar molecule BF3 may be set up so that the molecular plane is contained by the xy Cartesian plane (that containing the x and y axes) and so that the z Cartesian axis passes through the centre of the boron nucleus, as is shown in Figure 2.1. 

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. 

BC13 has a hexagonal bimolecular cell, Cj , P63, with a = 6.08 and c = 6.55 A. The trigonal planar molecules are in A and B positions as for SbCls and Sb(CH3)3Br2 (Figures 4.26 and 4.27). All other boron halides are also planar. Phosphorus trihalides, like NH3 and nitrogen halides, are pyramidal because of the lone electron pair on P and N. 

Neutral monoboron species are invariably trigonal planar molecules of the type BX3 involving trivalent boron, a consequence of the easy promotion of the s2p... 

For a trigonal planar molecule such as BF3, the orbitals likely to be involved in bonding are the 2s, and 2py orbitals. This can be confirmed by finding the reducible representation in the point group of vectors pointing at the three fluorines and reducing it to the irreducible representations. The procedure for doing this is outlined below. 

Trigonal planar molecules, such as BF3, are flat structures in which three atoms are evenly spaced around the central atom. 

COF2. COF2 and BF, are both trigonal planar molecules, but it would be... 

A trigonal planar molecule is a flat molecule in which all three bond angles are 120°. 

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... 

Trigonal planar molecule such as BFS cannot have degenerate orbitals ... 

Sketch the shape of a linear triatomic molecule, a trigonal planar molecule containing four atoms, a tetrahedral molecule, a trigonal bipyramidal molecule, and an octahedral molecule. Give the bond angles in each case. 

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