Low-coordinate species

In this progress report we have reviewed the latest developments in the large area of cationic low-coordinated species and their coordination with Lewis donors. It is clear that these species are of a broad interest, in particular for catalysis. In some cases, e. g. the methylene phosphenium cation, the donor adducts also open new routes for synthesis. Regarding the mechanism for the diverse donor-addition reactions, the structural details are only poorly understood and need a better classification. In particular the variation of the Lewis-donor has to be established. Hitherto in most cases iV-donation is studied. It includes amines or pyridines. Obviously the effect of other donors, such as phosphines, thioethers needs to be studied as well. The siliconium cation for which these effects are better known could provide an understanding for further investigations within this field. 

Low-coordinate species of the main group elements of the second row such as carbenes, olefins, carbonyl compounds (ketones, aldehydes, esters, amides, etc.), aromatic compounds, and azo compounds play very important roles in organic chemistry. Although extensive studies have been devoted to these species not only from the physical organic point of view but also from the standpoints of synthetic chemistry and materials science, the heavier element homologues of these low-coordinate species have been postulated in many reactions only as reactive intermediates, and their chemistry has been undeveloped most probably due to... 

The ligand l,3-bis(2,2", 6,6"-tetramethyl-m-terphenyl-5 -yl)imidazole-2-ylidene) ITmt was designed to facihtate the formation of low-coordinate species. It has a much higher overall degree of bulkiness than the well-known l,3-bis(2,4,6-trimethylphenyl)imidazoline-2-ylidene (IMes), but in the vicinity of the carbene center it is less severe because of the absence of substituents in the ortho-position. 

This chapter outlines the recent progress in the chemistry of multiply bonded species of Ge, Sn, and Pb. As can be concluded from this chapter and related recent reviews which deal with the chemistry of this family of compounds which are stable at room temperature, the field of low-coordinated species of heavier group 14 elements has matured considerably in recent years. 

For several years, our laboratory has been interested in the chemistry of main group elements in low coordination number environments (7). Some low coordinate species from Group 15 are shown below. They include two-coordinate cations (phosphenium ions), two-coordinate radicals (phosphinyl radicals), as well as stable double-bonded and even triple-bonded neutral molecules. For many years it was thought that... 

Thionophosphenous Acid Esters and Amides. Another ring system that we have found (75) to serve as a precursor of low-coordination species is the 7-phosphabicyclo[2.2.1]heptene system (also known as 7-phosphanorbomene). We have used this sytem for the thermal generation of derivatives of two previously unknown 2-coordinate species, the thionophosphenous amides 12 and the thionophosphenites 13 (Scheme 3). These species are trapped by a cycloaddition reaction with a diene to form the 5-membered 3-phospholene ring, as shown for 12. 

B(C6Fs)3, showing that such low-coordinate species can behave as effective Lewis bases.Alternatively, the A1(I) and Ga(I) complexes HC(CMeNAr)2 M (M = Al, Ga) behave like Lewis acids when reacted with the bulky azide compound... 

There has been significant activity in the synthesis of very sterically crowded organometallic halides, as precursors to new low-coordinate species potentially containing M-M multiple bonds. Some advances in this area are described separately... 

Iridium, rhodium, and ruthenium complexes of N-heterocyclic carbenes are particularly sensitive to decomposition by C-H bond activation of the substituents on nitrogen. As described in Section 2.4.1, this reaction can be used productively if reversible, and may afford some degree of stabilisation to otherwise very reactive and low-coordinate species. However, it often leads to degradation and loss of catalytic activity. 

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