Metal silylene

Metal complexes involving silylene 1 have been described in previous sections for a range of d-block metals. Silylene 1 also reacts with CuI(PPh3)3 and forms CuI(PPh3)2(l). Structural characterization of the latter confirms a distorted tetrahedral coordination environment for the Cu(l) center (Cu-Si = 2.289(4) A).285... 

They found that the dissociation energy for the silylene Fe—Si(Me)H+ lies between 56 and 78 kcalmol-1, and that of the silene isomer Fe—H2Si=CH2+ between 55 and 70 kcalmol-1. The similarity of the two energy ranges once again demonstrates the relative lability of the metal-silylene bond and suggests a potential stabilization of molecules which contain silicon, jr-bonded to transition metals. 

TABLE 5. Selected M-Si distances in group-6 transition-metal silylene and silyl complexes... 

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

The structure and bonding of metal silylene complexes varied from those of their carbon analogs. While Fischer-type metal carbene complexes without solvent adducts have been extensively characterized,48 most metal silylenoid complexes contain a bound solvent molecule or counterion on the silicon atom. The bond energy for donor silicon complex 22 was determined to be between 15 and 20kcal/mol 49,50... 

The reaction illustrated in equation 24 involves a series of 1,3-alkyl, silyl and germyl migrations between the metal-silylene and germylene transients formed subsequent to an a-elimination reaction initiated by the elimination of CO109. 

Another important class of bridged silylene complexes that should be handled separately is complexes that have hydrogen atoms bridging the metal-silylene bonds C. This bonding can be regarded as a metal-hydrogen-silicon 3c-2e bond, or the r -coordination of Si-H tr-bonds to metals that has been well established for mononuclear complexes.1011 These C-type complexes are now considered to play important roles in the formation of A- or B-type complexes. 

Corey proposed a scheme that incorporated both o--bond metathesis and oxidative addition/reductive elimination without resort to a metal-silylene intermediate (Scheme 6).49 51-68 The authors did not provide detailed kinetic... 

Thus, the metal-catalyzed reactions of several silicon species with acetylenes clearly show evidence for the generation of metal silylene species as well as Si—Si and Si—H cleavage. The nature of the metal catalyst, its ancillary ligands, and the nature of the substrate determine the ultimate product distribution. It is probable that certain silicon species are com-... 

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. 

Keywords metal silyl, metal silylene, hydrosilation... 

Harrod s preliminary suggestions for the dehydrocoupling of hydrosilanes by early transition metal catalysts involved the formation of metal-silylene intermediates, Cp2M=SiRR [138a]. The most plausible mechanism for the condensation reaction, however, has been presented elegantly by Tilley and co-work-ers [144] and involves ff-bond metathesis from M-H species generated from the catalyst precursor. 

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. 

---