Iron compounds silylenes

IV, E, while the silylene-iron compound is considered again in Section... 

The coordinated silylenes in both the iron and the chromium compounds can be photolytically activated Photolysis of the complexes in the presence of triphenylphosphine gives the trans-silylene-phosphine complex, which in a second step is transformed into the trnns-bisphosphine compound by excess phosphine. If the silylenes are not trapped, polysilanes are isolated in almost quantitative... 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

A formally pentavalent cationic silicenium complex, with intramolecular base-stabilization, is the silylene-iron 167, with a Fe=Si double bond. The crystal structure of 167 shows that silicon is essentially tetrahedral in this compound, and the solution 29 Si chemical shift is 8 = 118.3 ppm, compatible with the double bond character rather than with pentacoordination at silicon189. 

It is stable only below -20°C. Attempts to produce an unsupported silylene-iron complex by the removal of HC1 from compound (LXXXV) were unsuccessful (394). 

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. 

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