Silylenes bonding

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. 

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. 

Silylenes Bond angles Si-H Bond length, nm Si-X References... 

Does a Silylene-Complex exist This rhetorical question is the title of a theoretical paper published in 1983 [84], As a result of an ab-initio calculation, the authors came to the conclusion that a moderately positive answer can be given. However, silylene complexes are thermodynamically less stable than carbene complexes (the MSi bond energy for the hypothetical complex (OC)5Cr = Si(OH)H is 29.6 kcal/mol, the bond energy of the MC bond in (OC)5Cr = C(OH)H is 44.4 kcal/mol) [85], and therefore silylene complexes should be difficult to isolate. 

Basically the same methods known from the synthesis of classical metal-silyl complexes can also be applied to the preparation of low valent Si compounds. The procedures given here are summarized with the focus on silylene complexes These are a) reactions of appropriate metal anions with halosilanes, which are the most important methods for the formation of M-Si bonds. Alternatively, silyl... 

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]. 

Insertion of Silylenes into Metal-Hydrogen Bonds... 

Recently, a variety of silylenes were generated and characterized by matrix isolation techniques. The observed loose donor adducts between silylenes and the matrix molecules (THF, CO) are only stable at very low temperatures. Melting of the matrix induces polymerization of the silylenes which proceeds through disilenes. However, 0->Si transfer reactions do not occur only in the case of 1-methyl-THF has an insertion of the silylene into the C —O bond been observed [155-158],... 

The discussion about the mechanism of the dehydrogenative polymerization reaction has not yet been completed. However, the reaction mechanism seems to be strongly influenced by the specific random conditions that apply for each particular system. Presumably with late transition metals a silylene mechanism is more appropriate. It may be a matter of the steric constraints of the system to shift the reaction towards a-bond metathesis. 

Bimetallic silylene-bridged complexes have been known for a long time and numerous articles related to this subject have appeared. Several compounds have been characterized, some of them also by x-ray structure analysis [165-171], For instance, the complex Mn2(CO)8(Si(C6H5)2)2 shows a distorted (MnSi)2 four-membered ring with a Mn-Mn bond [169], In the following section selected examples which have been described recently or are of particular interest in the present context will be discussed. 

Starting from (OC)5MnSiR2H (R = Me, Ph, Cl), the p-silylene complex 70 is accessible via the oxidative addition of the Si —H bond to Pt(C2H4.)(PPh3)2 and Pt(PPh3)4, respectively. Structure 70 can be functionalized by displacement of the phosphine ligands alcoholysis and hydrolysis of the compound 70 leads to silicon-free complexes [175]. 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

Phosphinidenes (R-P) differ from other low-coordinate organophosphorus compounds, such as phosphaalkynes (R-C=P), phosphaalkenes (R2C=PR), and phosphaaromatics, in that the phosphorus atom carries only a single a-bonded substituent [7-9]. They relate to carbenes, nitrenes, and silylenes and likewise can exist as singlet and triplet species. The advances that led to stable carbenes [10, 11] and silylenes [12] stimulated an exploration of the chemistry of phosphinidenes. 

When alkynyldisilanes 13a and b were photolyzed in the presence of freshly generated dimesitylsilylene (Mes2Si ), the silylene added to the Si=C double bond of 1-silaallenes 14a and b to form disilacyclopropanes 15a and b (Scheme 5). Even without the independently generated silylene, photolysis of 13b produced 15b in 8% yield, but compound 13a gave only traces of 15a. In the case of 15b, the dimesitylsilylene most likely originated from silacyclopropene 16. 

Using CO-saturated hydrocarbon matrices, Pearsall and West" photolyzed sily-lene precursors at 77 K and monitored CO coordination to the silylenes by UV-vis spectroscopy (Scheme 13). Bis(trimethylsilyl)silanes 44a-c or SifiMcji were irradiated at 254 nm to create silylenes 45a-d, which reacted with CO, causing new peaks to ca. 290 and 350 nm, which were attributed to complex 46a-d, a resonance structure of silaketene 47a-d. Silylene adducts form fairly weak bonds, as seen by warming of the matrices. In the case of silylene adducts where one R = Mes, the CO dissociates and the corresponding disilene 48a-c peaks in the UV-vis spectra observed upon warming (R2 = Me most likely produced silane rings Si, Me6. etc.). 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

Other authors have observed similar behavior of silenes with silylene sources. Thus Ishikawa202 reported the addition of dimesitylsilylene to the silicon-carbon double bond of a silaallene, yielding the disilacyclopropene shown in Eq. (34). 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

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