Pyramidal arrangement

Arylamines Aniline like alkylamines has a pyramidal arrangement of bonds around nitrogen but its pyramid is somewhat shallower One measure of the extent of this flat tenmg is given by the angle between the carbon-nitrogen bond and the bisector of the H—N—H angle... 

J. A. Le Bel, whose name is often also associated with this concept, did indeed independently suggest a 3-diitiensional modet for the 4-coordinate C atom, but vigorously opposed the tetrahedral stereochemistry of van t Hoff for many years and favoured an alternative square pyramidal arrangement of the bonds. 

Let s look at an example. In ammonia (NH3), the nitrogen atom is sp hybridized, so all four orbitals arrange in a tetrahedral structure, just as we would expect. But only three of the orbitals in this arrangement are responsible for bonds. So, if we look just at the atoms that are connected, we do not see a tetrahedron. Rather, we see a trigonal pyramidal arrangement ... 

Molecular geometrical differences between the amides (225) and (226), the latter having the more pyramidal arrangement around nitrogen, have been correlated with the known differences in their solvolytic behaviour. 

The square-pyramidal arrangement may be illustrated by two chemically very different compounds SnO (9)3) and phthalocyaninetin(II) (10)35) ... 

Figure 4.21 Geometry of the trigonal bipyramidal and the square pyramidal arrangements of five points on the surface of a sphere.

Compounds containing a pyramidally arranged (hence, chiral) sulfur to which are linked three alkyl or aryl groups, resulting in a net positive charge on the sulfur. A biologically important example is S-adenosyl-L-methi-onine chloride. Sulfonium salts can also be utilized as analogs or mimics of carbocation intermediates in enzyme-catalyzed reactions. For example, methyl-(4-meth-ylpent-3-en-l-yl)vinylsulfonium perchlorate proved to be an excellent inhibitor (Ki = 2.5 tM) of the enzyme that catalyzes the formation of the bicyclic (+)- -pinene ... 

Similar three-coordinate tripod" ligands are known of the type [C(R2PY)3]- (where Y can be 0, S, or a pair of electrons on the phosphor atoms).72 These tripod ligands strongly deftermine three coordination position in a pyramidal arrangement, leading to a fourth coordination by a halide. 

Amide resonance is a powerful stabilizing force and gives rise to a number of structural effects. Unlike the pyramidal arrangement of bonds in ammonia and amines, the bonds to nitrogen in amides lie in the same plane. The carbon-nitrogen bond has considerable double-bond character and, at 135 pm, is substantially shorter than the normal 147-pm carbon-nitrogen single-bond distance observed in amines. 

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