Other Trigonal-Planar Molecules

Molecule Bond Bond length, A Bond energy (BE), kcal/mole 

The bonding in diborane B2H6 is described in a number of other sources.  

The B(CHg)3 and AlCCHg) molecules have trigonal-planar parts. 

The structure around each carbon is tetrahedral, as will be described in Chapter V. 

In most cases it is convenient to have a normalized linear combination of orbitals to bond with a central atom. For example, the combination appropriate for 2s in a trigonal-planar molecule is 

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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... 

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. 

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... 

For example, let s look at the stereochemistry of SnI reactions. We already saw that Sn2 reactions proceed via inversion of configuration. But SnI reactions are very different. Recall that a carbocation is sp hybridized, so its geometry is trigonal planar. When the nucleophile attacks, there is no preference as to which side it can attack, and we get both possible configurations in equal amounts. Half of the molecules would have one configuration and the other half would have the other configuration. We learned before that this is called a racemic mixture. Notice that we can explain the stereochemical outcome of this reaction by understanding the nature of the carbocation intermediate that is formed. 

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 reaction of [Ni(ethene)3] with a hydride donor such as trialkyl(hydrido)-aluminate results in the formation of the dinuclear anionic complex [ Ni(eth-ene)2[2l 11 [22]. The nickel(O) centers in this complex are in a trigonal planar environment of two ethene molecules and a bridging hydride ion, with the ethene carbons in the plane of coordination. The two planes of coordination within the dinuclear complex are almost perpendicular to each other, and the Ni-H-Ni unit is significantly bent, with an angle of 125° and a Ni-Ni distance of 2.6 A [22],... 

The X-ray crystallographic analysis of 2 -Cu reveals a dimeric structure, where two copper ions are coordinated to two ligand molecules (Fig. 8). Each copper ion is situated in a trigonal planar carbene/alkenyl carbon ligand environment, coordinated to two carbene arms from one chelator and a third carbon from the pendant arm of a second chelator. The average Cu—C bond distance is 1.996 (1)A, consistent with that of other reported Cu(I) carbene complexes (29). 

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