Silylene-metal complexes

The chemistry of silylene-metal complexes has developed in quite another direction, however, from reactions of disilyl-metal complexes, leading to complexes of otherwise unstable disilenes such as Me2Si=SiMe2. Molybdenum and tungsten complexes have been particularly well investigated by Berry and co-workers,103 and platinum complexes have also been isolated.104 Readers interested in this field are directed to a 1992 review of silylene, silene, and disilene-metal complexes.105... 

The most general method for synthesis of silylene-metal complexes is the reaction of a transition metal dianion with a dichlorosilane, in the presence of a donor molecule D to stabilize the resulting silylene complex (equation 97). 

Silylene-metal complexes, with tricoordinate silicon, free of coordinating donors, have been difficult to synthesize. Only a few examples were known until recently one example is the osmium complex Cp (Me3P)2RuSi=Os(CO)4 synthesized by Tilley... 

The nature of the bonding in silylene-metal complexes, as compared with the better known metal-carbene complexes, is a question of considerable interest. MO calculations on H2Si=Mo(CO)5 indicate that the Si—Mo bond consists of a cr-donor and --backbond component, like the carbon-metal complexes. The -component is, however, weaker than for metal carbenes251. Infrared C=0 frequencies for the base-free silylene metal complexes support this model. Theoretical considerations of the bonding in silylene-metal complexes are treated more fully in Section IV.E. 

Silylene 59 also behaves somewhat like a phosphine in its interactions with metal carbonyls98,149-376. Typical reactions involve substitution of silylene for CO, to give a silylene-metal complex. Three examples are shown in Scheme 20, and the structure of the nickel complex 75 is displayed in Figure 7149. This complex is both the first silylene-nickel complex, and the first example of a bis-silylene-metal complex free of stabilization by Lewis base donors. 

In all three complexes the Si-metal bond is short (Si—Fe = 219, Si—Ni = 221, Si—Cr = 228 pm). These distances are about 10 pm less than for typical Si-metal single bonds, consistent with partial back-bonding from the metal d-orbitals to the p-orbital on silicon. Reactions of 59 to form silylene-metal complexes appear to be general, and many more examples are likely to be prepared in the future, for this and other stable silylenes. 

The first silylene-metal complex with no base stabilization was prepared by extrusion of SiMe2 from a substituted disilane (384), as in Eq. (165) where R is H or Me. 

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... 

An X-ray crystal structure has been determined for 7 and is shown in Fig. 3. This compoimd is the first silylene complex of nickel, as well as the first bis-silylene metal complex of any kind without base stabilization [20]. 

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],... 

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... 

Linear polycarbosilanes and polycarbosiloxanes-especially those containing arylene units in the chain-have specific physico-chemical properties which can be applicable in heat-resistant materials [29-31]. Phenylene-silylene-ethylene-polymers, which may serve as potential substrates for applications as membrane materials are usually obtained in the presence of platinum catalysts [32], although other transihon-metal complexes have also been tested in this process. 

Like silylenes and carbenes germylenes are isolobal with phosphines and may function as ligands to transition metal complexes. (Adapted from Bazlnet et ah, 2001)... 

The chapter table of contents contains subjects that were either unknown or merely distant hopes a decade ago, such as persistent silylenes, the dissociation of disilenes to silylenes and terminal silylene-transition metal complexes. The kinetics and spectroscopy of silylenes and theoretical treatments of silylene structure and reactivity have made such gigantic strides in the intervening years that they represent new vistas in our understanding. 

Addition of a persistent silylene (see Section V.III.C) to nickel carbonyl resulted in the first bis-silylene transition metal complex without Lewis base stabilization... 

The past ten years have seen extensive development of the chemistry of silylene-transition metal complexes. Since this topic has been thoroughly treated in several recent reviews by Zybill and coworkers330-332 and by Tilley333,334, who have carried out much of the fundamental work, this active area will only be summarized here. 

The metal, M, can be Fe, Ru, Os (with n = 4) or Cr (n = 5). Substituents on silicon can vary widely, including aryl, alkyl, alkoxy and Et2N groups. With a slight modification, the same reaction is useful for silylene-tungsten complexes (equation 98). 

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