Transition-metal silylene compounds

Stable compounds containing simple three-coordinate silicon atoms other than the alkali metal salts were not isolated until a range of transition metal silylene complexes were prepared. However, instead of having simple structures of the type R2Si = ML , that is, analogous to their carbon analogs, the silicon atom is usually further coordinated by a donor ligand such as HMPA or THF. Some donor-free complexes have, however, been prepared in solution. 

Terminal transition-metal silylene complexes have proven to be elusive synthetic targets, but it seems likely that future investigations will uncover viable routes to stable M=Si double-bonded species. Such compounds are of interest as model systems for investigating the reactivity discussed in the previous section, and for providing new synthetic intermediates for silylene transfer reactions. Recent ab initio SCF MO calculations predict that (CO)5Cr=SiH(OH) should be a relatively stable molecule, but that it may be difficult to isolate due to its susceptibility to nucleophilic attack at the silicon atom. The Cr=Si bond dissociation energy was calculated to be 29.6 kcal mol-1, compared to the analogous Cr=C bond dissociation energy of 44.4 kcal mol-1107. 

With the differences in the electronic properties described above one can explain the different reactivity of these compounds. The high energy HOMO of (OC)4Fe=SiH2 (4) eauses stronger nucleophilicity. An electrophilic attack should occim preferably at the iron atom, whereas a nucleophilic attack should occim preferably at the silicon atom due to the imequal distribution of the coefficients of the m-orbitals. The munerous donor adduets of transition metal silylenes [32,33,34] are excellent examples for a nucleophilic attack at the silieon atom. 

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],... 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

A major problem in postulating silylenoid metal complexes as intermediates in the redistribution reactions is simply that good model compounds are lacking, and the decomposition mechanisms of silyl transition metal complexes have not been systematically investigated. While there is evidence for transient R2Si species produced by thermal or photochemical means (80-83), there are no known monomeric silylene metal complexes. Several monomeric stannylene and germylene complexes are... 

Treatment of tantalum hydride complexes with hexamethylsilacyclopropane at 65 °C in the presence of a Lewis base gave the corresponding silyl-tantalum compounds in excellent yield. It appears that this reaction takes place via direct insertion of dimethylsilylene into the tantalum-hydride bond, which would make it the first example of silylene trapping by a transition-metal substrate (Equation (15)) <87JA6210>. 

Coordinated silylene ligands have been invoked or suggested as intermediates in a number of chemical processes, including Rochow s Direct Process94, catalytic redistribution of silanes39 and various silylene-transfer reactions95-99. Evidence for such species is primarily circumstantial, and M=Si double bonds have never been detected in these systems. Presently there is no conclusive evidence for the involvement of silylene coordination compounds in any transition-metal-mediated silylene-transfer reactions. Some reactions that may involve silylene ligands are discussed below. 

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