Trigonal-planar

Both these molecules exist in the gaseous state and both are trigonal planar as indicated by reference to Table 2.8. However, in each, a further covalent bond can be formed, in which both electrons of the shared pair are provided by one atom, not one from each as in normal covalent bonding. For example, monomeric aluminium chloride and ammonia form a stable compound ... 

The formation of a fourth covalent bond by the aluminium atom results in spatial rearrangement from the trigonal planar, for three bonding electron pairs, to tetrahedral, for four bonding electron pairs. 

As in the case of NH4 the charge is distributed over the whole ion. By considering each multiple bond to behave spatially as a single bond we are again able to use Table 2.8 to correctly deduce that the carbonate ion has a trigonal planar symmetry. Structures for other covalently-bonded ions can readily be deduced. 

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,... 

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... 

These molecules exist in the solid halides, explaining the low melting points of these halides, and also in the vapour phase at temperatures not too far above the boiling point. At higher temperatures, however, dissociation into trigonal planar monomers, analogous to the boron halides, occurs. 

In each case the configuration around the boron changes from trigonal planar to tetrahedral on adduct formation. Because of this ability to form additional compounds, boron trifluoride is an important catalyst and is used in many organic reactions, notably polymerisation, esterification, and Friedel-Crafts acylation and alkylations. 

When heated above 673 K the dimer, AljCl, begins to dissociate into the monomer in which the aluminium has a regular trigonal planar configuration. 

Carbon symmetry -letraheJnil isp ) C -C bond length 15.4 nm. trigonal planar sp ) C C bond length 14.2 nm interplanar distance 3J.5 nm... 

The carbon atoms of the double bond have a trigonal planar configuration and free rotation about the C—C bond is prevented by the n bond. The inability to rotate means that geometrical isomers can be produced, with substituents a and b, thus ... 

To ensure that the arrangement of four atoms in a trigonal planar environment (e.g., a sp -hybridized carbon atom) remains essentially planar, a quadratic term like V(0) = (fe/2) is used to achieve the desired geometry. By calculating the angle 9 between a bond from the central atom and the plane defined by the central... 

Boron trifluoride (BF3) F 20. B- F F Boron has three bonded pairs Trigonal planar Trigonal planar ... 

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... 

Section 1 10 The shapes of molecules can often be predicted on the basis of valence shell electron pair repulsions A tetrahedral arrangement gives the max imum separation of four electron pairs (left) a trigonal planar arrange ment is best for three electron pairs (center) and a linear arrangement for two electron pairs (right)... 

The geometry at carbon changes from tetrahedral trigonal planar linear... 

The chemistry of propylene is characterized both by the double bond and by the aHyUc hydrogen atoms. Propylene is the smallest stable unsaturated hydrocarbon molecule that exhibits low order symmetry, ie, only reflection along the main plane. This loss of symmetry, which implies the possibiUty of different types of chemical reactions, is also responsible for the existence of the propylene dipole moment of 0.35 D. Carbon atoms 1 and 2 have trigonal planar geometry identical to that of ethylene. Generally, these carbons are not free to rotate, because of the double bond. Carbon atom 3 is tetrahedral, like methane, and is free to rotate. The hydrogen atoms attached to this carbon are aUyflc. 

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 copper(I) ion, electronic stmcture [Ar]3t/ , is diamagnetic and colorless. Certain compounds such as cuprous oxide [1317-39-1] or cuprous sulfide [22205-45 ] are iatensely colored, however, because of metal-to-ligand charge-transfer bands. Copper(I) is isoelectronic with ziac(II) and has similar stereochemistry. The preferred configuration is tetrahedral. Liaear and trigonal planar stmctures are not uncommon, ia part because the stereochemistry about the metal is determined by steric as well as electronic requirements of the ligands (see Coordination compounds). 

Valence shell electron-pair repulsion (VSEPR) model (Section 1.10) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another. Four electron pairs will arrange themselves in a tetrahedral geometry, three will assume a trigonal planar geometry, and two electron pairs will adopt a linear arrangement. 

The Be ring has a chair conformation (dihedral angle 57.6°) with mean B-B distances of 172pm. All 6 B and all 6 N are trigonal planar and the 6-exocyclic NMe2 groups are each twisted at an angle of 65° from the adjacent B3 plane, with... 

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