Silylene complexes examples

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

Donor-bridged bis(silylene) complexes (Equation (4)) are now known for all the group 6 metals.72-74 For example, when CpCr(CO)3 is photolyzed with HSiMe2SiMe2OMe, the product is 26 (M = Cr X = OMe). Structural data for members of this family show that the four-membered ring is folded (internal dihedral angle for 26 with M=Cr, X = OMe is 162.4°), and this is attributed to steric repulsion between CO and silicon-bound Me groups. The fluxional... 

Many of the bridged silylene complexes show complex fluxional behavior or slow isomerization between different isomeric forms. An example is the iron-silylene complex shown in Scheme 11, in which the proposed mechanism is illustrated. A full discussion appears in Ogino and Tobita s review338. 

An early inorganic example of a silylene complex is HSiOH—OH2, obtained by the reaction of silicon atoms with water. See Z. K. Ismail, R. M. Hauge, L. Fredin, J. W. Kauffman and J. L. Margrave, J. Chem. Phys., 77, 1617 (1982). 

Gas phase ion chemistry has provided the means for obtaining labile intermediates that are thought to play a relevant role in solution chemistry. An example is given by cationic iron-silene and iron-silylene complexes that have been studied as model systems in... 

All cationic phosphenium complexes reported to date are base-free species. In contrast, silylene complexes tend to be stabilized by forming base adducts. Most silylene complexes are base-stabilized species54 59 and base-free silylene complexes are still rare.59-63 Typical examples are shown in Scheme 7. A phosphite-phosphenium complex exists in a solid state and even in solution, and does not take an OMe bridging form between the two P atoms,64 whereas a silyl-silylene complex is not detected, but exists as an OMe bridging form between the two Si atoms.65... 

Photolysis of bulky permethylated Fp complexes such as FpSi[Si(CH3)3]3 does not cause deoligomerization, possibly because stable intermediate iron-silyl(silylene) complexes are not formed (27). Other less bulky transi-tion-metal-oligosilane complexes are also unreactive under the photolysis conditions. For example, the ruthenium analogues of the iron complexes, [( ri -C5H5)Ru(CO)2-Si ], are essentially photostable (23). Whether this behavior is due to the strength of the Ru-CO bond or to the enhanced stability of the Si-Si bond is not clear, and this problem is currently under investigation. 

Silylene complexes have been prepared by coordination of free silylene, extrusion of dihydrogen from a dihydrosilane, a-hydrogen elimination from a hydrosilyl complex, extraction of hydride, halide, or pseudo-haUde from a metal-silyl complex, and photoextrusion of silylene from a disilanyl complex. Examples of these reactions are shown in Equations 13.39-13.45. 

The reactivity of metal-silylene complexes is more limited than the reactivity of carbene complexes. The cationic base-stabilized ruthenium-silylene complex in Equation 13.37 does not react with olefins or alkynes to undergo [2-1-2] addition reactions. However, a related complex did undergo [2-1-2] addition reactions with isocyanates, as shown in Equation 13.46. Other reactions of silylene complexes are distinct from those of carbene complexes or those of other conventional organometallic compounds. For example, the reaction of the silylene hydride with an acetylene generates a p-silylvinylarene complex... 

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. 

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