Silicon ligands

Very recently, the coordination chemistry of low valent silicon ligands has been established as an independent, rapidly expanding research area. With the discovery of stable coordination compounds of silylenes [35-38], a major breakthrough was achieved. Within a short time a variety of stable complexes with a surprising diversity of structural elements was realized. Besides neutral coordination compounds (A, B) [35, 36, 38], and cationic compounds (C) [37], also cyclic bissilylene complexes (D) [39,40] exist. A common feature of the above-mentioned compounds is the coordination of an additional stabilizing base (solvent) to the silicon. However, base-free silylene complexes (A) are also accessible as reactive intermediates at low temperatures. 

The coordination sphere of transition-metal complexes can furthermore be utilized for the fixation of silicon ligands in their lowest oxidation states. Even examples of compounds containing a formally zerovalent silicon (E) are now known [41]. 

Also n-complexes of silaethenes (F) and disilaethenes (G) have been realized recently, which in a wider sense can be classified as coordination compounds of low valent silicon ligands. Some examples are briefly discussed in the two final sections of this article [42-48],... 

Early-transition-metal complexes containing silicon ligands are becoming increasingly important in organometallic chemistry1. The first group-3 silyl complex... 

Compound 3 was obtained in toluene, THF, and CHCI3 from reaction of SiCU and 8-hydroxyquinoline in the stoichiometric ratio of 1 2. In contrast, the analogous reaction with SiBr4 (in toluene) yields the tris-chelate complex 4. Consequently, the spectrum of the products is predominantly influenced by the reactivity of the silicon source, but a significant dependence on the used solvents was not observed. However, this conclusion is contrary to the results for ligand L H (reactions with 2-[(iV-benzylimino)phenylmethyl]-5-methoxyphenol) mentioned above. Compounds 3 and 4 were isolated, but we do not know exactly the configuration of the silicon-ligand sphere. We were not able to isolate the dibromo complex 5 from the mixture with 4. 

FIGURE 18. Variations in silicon-ligand bond lengths in the series [R SiF5 ] , = 0-3 d(Si-Fax) , American Chemical Society... 

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