Monomeric compound electronic stabilization

The heterocycles can be cleaved by reaction with 4-(dimethylamino)pyri-dine, yielding Lewis base-stabilized monomeric compounds of the type dmap—M(R2)E(Tms)2 (M = Al, Ga E = P, As, Sb, Bi). This general reaction now offers the possibility to synthesize electronically rather than kinetically stabilized monomeric group 13/15 compounds. These can be used for further complexation reactions with transition metal complexes, leading to bimetallic complexes of the type dmap—M(Me2)E(Tms)2—M (CO) (M = Al, Ga E = P, As, Sb M = Ni, Gr, Ee). 

The structures of some stable stannylenes, such as several amino-, " alkoxy-, and arylthio-substituted intermediates, have been revealed by X-ray crystallography. They are monomeric crystals and the tin atom has the coordination number 2. The divalent tin in such compounds is stabilized by the effects of electronegativity of the ligand atoms and by the donation of the lone-pair electrons to the vacant 5p orbital of the tin. Although the first monomeric dialkyl- and diaryl- stannylenes in... 

Before studying some examples more closely, let us consider some cases which are not listed in Table 13. There are numerous compounds SnX2 which are definitely monomeric but are nevertheless no carbene analogs since their valence electron number at the tin atom is at least eight. These compounds contain chelating ligands which can stabilize the carbenoid tin atom due to intramolecular Lewis acid-base interactions as shown by structure A and B (see also Chapter 3). 

The two zinc atoms in EtZnNBu ZnEt are in a di-coordinate state because of the monomeric structure of this compound. Since a zinc atom is usually stabilized by assuming a tetra-coordinate state, this compound is expected to form a complex with an electron donor substance. In fact, several complexes were obtained in the pure crystalline state. The stability of these complexes differed rather widely. 

This compound, which is monomeric in the solid state, exists in toluene solution in equilibrium with its dimer (Scheme 2)18 as evident from analyses of variable temperature H, 7Li and 13C NMR. Whereas the monomer is favored at higher temperatures in terms of the entropy effect, the dimer is favored at lower temperatures in terms of electrostatic interactions through which electrons of the germanium negative centers are efficiently stabilized by the two lithium cations. The thermodynamic parameters for this equilibrium were estimated. 

In covalent organolithium compounds and covalent Grignard reagents neither the lithium nor the magnesium possesses a valence electron octet. This is energetically disadvantageous. In principle, the same mechanism can be used to stabilize these metals that monomeric boranes BH3 n Rb use to attain a valence electron octet at the boron atom (Section 3.3.3) the formation either of oligomers or, with suitable electron pair donors, of Lewis acid/Lewis base complexes. 

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