Silane complexes stability

It is obvious from this table that the change of structure of the organosilicon compounds can have a more pronounced effect on the activation energy of chemical stage than on the heat of H complex formation. Moreover, steric effects of substituents at Si atom in these compounds have a more dominant role in the chemical interaction than in the complex stability. This is apparent from comparison of the Qajs change at adsorption of alkoxy-silanes with the observed chemisorption energies. The E., increases at enhancement (the absolute value) of a steric constant of the alkoxy substituent at Si atom. 

Mo(CO)(dppe)2 to W also leads to dihydride formation69 because W is a better backbonder than Mo (third-row metals have more diffuse d orbitals). Numerous examples of fine-tuning of H2 versus hydride coordination are known in group 5-10 systems (Table 4.7) and provide excellent probes of electronics such as BD capability at specific fragments and stereoelectronic ligand effects. An example of the latter is the stabilization of tj2-H2 and i/2-silane complexes over their dihydrido and hydrido(silyl) forms by Tp ligands relative to Cp analogues (see Sections 4.9 and 113.1). 

In terms of the influence of the electronic nature of L only, the stabilities follow the usual trends donors such as PR3 stabilize the complexes and CO destabilizes. Finally, as for silanes the stability of the a complexes depends on the substituents on B, in this case the substituents R, R, and R" on the catechol aromatic ring. The rates for borane dissociation decrease with increasingly electron-withdrawing R groups, just as such groups on R3SiH give more activated M(Si-H) bonds and more stable complexes. 

Weak adducts of alkanes and metal derivatives (the alkane molecules play the role of token ligands in these complexes) have been detected and even isolated using a number of methods [14], These complexes are unstable at room temperature. Matrix isolation is one of the best established methods for the stabilization and characterization of intermediates. Complexes of alkanes with metal atoms and ions have been detected in the gas phase. All these adducts belong to the larger class of cr-complexes, which has been defined as complexes where the donor is a o-bond [14c], Dihydrogen and silane complexes are also from this class. 

The properties and reactivity of alkane and silane - - complexes are closely related to those of dihydrogen complexes. However, the thermodynamic stabilities of the complexes are much different. The silane complexes have been studied in less detail than dihydrogen complexes, but, broadly speaking, they are similar in stability to the dihydrogen complexes or only slightly less stable. Alkane complexes have been studied intensively as reactive intermediates, " but detailed structural and reactivity studies have not been conducted because alkane complexes are unstable in solution. The most detailed data have been gained by NMR spectroscopic studies of CpRe(CO)2(RH) complexes. These data imply that the metal binds in an Tj -mode to a single C-H bond of the alkane. 

Trialkyltin substituents are also powerful ipso-directing groups. The overall electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of /S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipso-oriented c-complex. ... 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

Nowadays silenes are well-known intermediates. A number of studies have been carried out to obtain more complex molecules having Si=C double bonds. Thus, an attempt has been made to generate and stabilize in a matrix 1,1-dimethyl-l-silabuta-l,3-diene [125], which can be formed as a primary product of pyrolysis of diallyldimethylsilane [126] (Korolev et al., 1985). However, when thermolysis was carried out at 750-800°C the absorptions of only two stable molecules, propene and 1,1-dimethylsilacyclobut-2-ene [127], were observed in the matrix IR spectra of the reaction products. At temperatures above 800°C both silane [126] and silacyclobutene [127] gave low-molecular hydrocarbons, methane, acetylene, ethylene and methylacetylene. A comparison of relative intensities of the IR... 

The stability of the carbyne complexes is strongly dependent upon the steric requirements of the silane used, and only the /-butyl-substituted derivatives are stable for prolonged periods. 

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