Transition-metal silylene complexes

TABLE 8. Selected 29Si NMR chemical shifts for group-7 transition-metal silylene complexes... 

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. 

Somewhat related to transition-metal silylene complexes are transition-metal-sub-stituted silylenes (L M-Si-R). One report of such a species has appeared. Thermolysis of the 7-silanorbornadienyl iron derivative 24 in the presence of silylene trapping reagents provides evidence for the silylene Cp(CO)2Fe-Si-Me (Scheme 3)119. 

Figure 1. A mechanism for dehydrogenative coupling of silanes, based on a transition-metal silylene complex as an intamediate.

Common features and principal differences between caibene and silylene complexes of early and late transition metals are analyzed from a quantum chemical point of view. Cp2Ti=EH2 and (OC)4Fe=EH2 with E= C, Si were chosen as representative examples for that purpose. The nature of the transition metal carbon and silicon bond was analyzed with CDA, NBO and the EHT-method. The distinctive properties of transition metal silylene complexes are caused by two main reasons the silylene substituent acts mainly as CT-donor, and there is very weak 7t-bonding in silylene complexes. 

Transition metal carbene eomplexes were already discovered in the early years of organometallic chemistry [1]. Sinee then exciting reactions [2] with these complexes have been foimd and a number of useful applications were developed [3,4,5,6]. In contrast to that, transition metal silylene complexes have been explored only during the last deeade... 

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