Silicon, coordination compounds

The nature of the solvent as well as its aggregate state considerably affects intermolecular Interactions in silicon coordination compounds. Thus, in the IR spectra of crystalline complexes of OPPhj and OP(NMe2)3 with silicon tetrachloride, the P=0 bond stretching vibration frequency, v(PO), is by 45 cm lower than that of the free bases in the spectra of their diluted solutions (both in polar and nonpolar solvents), however, the frequency is the same In crystals of (aroyl-... 

Only recently, nitriles were shown to increase the silicon coordination number to up to five in cationic complexes (18) [95], even though nitriles had already been shown to form adducts with sihcenium ions, thus functioning as a donor moiety in a tetrahedral Si coordination sphere [96]. Amines are scarcely encountered in silicon coordination compounds, only few examples of crystallographically evidenced silicon complexes with monodentate amines have been reported so far, which include the adduct SiF4(NH3)2 [97-100], Imines, however, are well known to add to various halosilanes. Especially IV-heterocycles with imine functionahty, such as pyridines [101-103], imidazoles [104, 105], pyrazoles [106], and related compounds, can be found as ligands in various silicon complexes (e.g., in 19 and 20). 

To our best knowledge, there is no crystallographic evidence for telluroether silicon coordination compounds to date. 

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. 

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],... 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

Hexacoordinated silicon takes benefit from the favorable octahedral arrangement of ligands around a coordination center. As expected, negatively charged substituents are superior in stabilizing high silicon coordination numbers. Most recently however, compounds with chelate ligands, mono anionic derivatives and compounds with more electropositive donors have been added to the series of penta- and hexacoordinated silicon species. 

The most common oxidation states and corresponding electronic configurations of platinum are +2 (d8), which is square planar, and +4 (< ), which is octahedral. Compounds in oxidation states between 0 ( °) and +6 (ct) exist. Platinum hydro station catalysts are used in the manufacture of silicone polymers. Several platinum coordination compounds are important chemotherapeutic agents used for the treatment of cancer. 

Finally, a group of hypercoordinated silicon compounds, the decamethyl silicocenes, in which the formal silicon coordination number is ten, is worthy of mention in connection with this chapter. These analogs of ferrocene have been studied extensively232, and are described in detail in Jutzi s chapter in this volume. 

Many PSPs are composed of probe dyes, such as polycyclic aromatic hydrocarbons (e.g., pyrene) and coordination compounds (e.g., platinum por-phryins and ruthenium(II) polypyridyl complexes) immobilized in various gas permeable polymer films such as silicon polymer, organic glassy polymers (e.g., poly(methylmethacrylate), polystyrene), fluorinated polymers, or cellulose derivatives such as ethyl cellulose [9,10]. As probe molecules interact with polymer matrices directly, the properties of PSPs strongly depend on the properties of polymer matrices. The oxygen permeability of polymer matrix is an especially important factor for highly sensitive PSP. 

Because transition-metal anions can be prepared conveniently in tetrahydrofuran solution, this cyclic ether was often used in earlier attempts to prepare silicon-metal compounds. It is now generally realized, however, that tetrahydrofuran can frustrate these attempts in two ways. First, it promotes electrophilic attack by the silicon compound on oxygen atoms of coordinated carbonyl groups this leads to the formation of products with Si-0 bonds (54, 138, 262, 300, 306, 310, 336,337), e.g.,... 

It was recognized at an early stage in the development of silicon-transition-metal chemistry that silicon-oxygen compounds often appeared, either as by-products from preparations or as decomposition products on heating or even on storage. At first, adventitious hydrolysis or oxidation was blamed, but it soon became clear that attack on silicon by oxygen of coordinated carbonyl groups was responsible. Since metal carbonyls are known to form adducts with Lewis acids such as compound (XXXIII) (286),... 

Silicon molecular compounds with coordination numbers from three to six... 

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