Nitrogen Lewis bases

Bis(l-methylboratabenzene)lead 42 is synthesized in 63% yield by the reaction of LifCsHsBMe) with lead(n) chloride. The X-ray structure analysis of 42 reveals some remarkably short intermolecular contacts from the lead atoms to the ortho-carbon atoms of neighboring molecules which create a tetrameric packing unit. Compound 42 forms weak adducts such as 43 with nitrogen Lewis bases like TMEDA or 4,4 -dimethyl-2,2 -bipyridine.71... 

Tetracarbonyinickel undergoes disproportionation with nitrogen Lewis bases". Relatively weak bases such as pyridine initially give unstable substitution products, which then disproportionate to Ni(II) and Ni( -1) ... 

As a conclusion to this section it should he mentioned that reactions between diborane and several other nitrogen Lewis bases besides those mentioned above have been investigated. These nitrogen donors include hydrazine and its methyl derivatives (130), hydroxylamine and its. V-metliyl derivatives (45), and O-methylhydroxylamine and its A-methyl derivatives (11). In general, borane adducts are formed (Table II) which are thermally unstable with respect, to release of hydrogen. 

Figure 2.1 Main nitrogenated Lewis bases shown in this chapter.

The pentacoordinated triel atoms are also observed in numerous crystal structures. Figure 9.9 presents two examples, two species of the tris(hydrido)-bis (trimethylamine)-aluminium and of the trichloro-bis(trimethylamine)-aluminium (III) crystal structures. In both cases the trigonal bipyramid arrangement for the aluminium center is observed. In one case the species observed in the crystal structure may be treated as the interaction between AIH3 and nitrogen Lewis base centers, in the second case that is the interaction of AICI3 with the same ligands. 

The distribution of the electrostatic potential for two bromine species has further consequences particularly the location of the maxima of EP shows the most probable nucleophilic attacks here. This is why the liner halogen bonds are formed with BrFa while bent ones for the BrFs molecule. Figure 15.8 shows the molecular graph of the BrFa-NCH complex with the reactive surface corresponding to the laplacian of the electron density equal to zero. The straight bond path connecting the bromine atom with the nitrogen Lewis base center of HCN molecule is observed... 

Figure 15.6 Nitrogen Lewis bases used in the enantioselective reduction of ketimines.

Fig. 4. The calculated total electron density at the 2.0 (top) and 1.7 e A level (bottom) shows considerable Lewis base abilities at the central nitrogen atom in [Me2AlPy2N] (left) while at the central phosphorus atom of [Me2AlPy2P] 13 hardly any electron density is left...

Ammonia is a prime example of a Lewis base. In addition to its three N—H bonds, this molecule has a lone pair of electrons on its nitrogen atom, as Figure 21-1 shows. Although all of the valence orbitals of the nitrogen atom in NH3 are occupied, the nonbonding pair can form a fourth covalent bond with a bonding partner that has a vacant valence orbital available. 

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 ... 

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