Electron back-donation

After the discovery by Fischer and Maasbol of the first stable carbene complexes in 1964, i.e., [(CO)5W =C(OMe)R ] [21], generation of related metaUacumulene derivatives [M]=C(=C) =CR2 (n > 0) was obviously envisaged. Thus, it is presently well-established that stabilization of these neutral unsaturated carbenes by coordination to a transition metal center is possible by the use of the lone pair of electrons on the carbenic carbon atom, via formation of a metal-carbon a-bond (electron back-donation from the metal fragment to the carbon ligand may strengthen this bond). This has allowed the development of a rich chemistry of current intense interest due to the potential applications of the resulting metallacumulenic species in organic synthesis, as well as in the construction of molecular wires and other nanoelectronic devices [22]. 

The value of KM decreases with increasing electronwithdrawing capability of the aluminum component, i.e. with decreasing electron density at the vanadium induced by the aluminum component bonded to the vanadium in the bimetallic structure of the active center. This result seems to suggest that electron back-donation from a filled vanadium d orbital to the empty propylene jc obital (it-bonding) is the main factor in determining the vanadium-propylene interaction. 

In the presence of an excess of potassium ethoxide, the ethoxide anion joins the less negative end of the double bond. Strong electron back-donating power of fluorine increases negativity at the carbon linked to chlorine and causes the addition of the ethoxide ion to the carbon linked to fluorine. Subsequent ejection of fluoride anion restores the double bond, and the final product is a diether, compound P [7/J. 

Addition of dimethylamine to chlorotrifluoroethylene occurs in such a way that the nucleophile, dimethylamine, joins the carbon having two fluorine atoms, because this carbon has lower electron density owing to strong electron back-donation by fluorine. 

However, NO can also interact with surface metal centres on metal oxides, giving rise to surface mononitrosyl, gem-dinitrosyl and trinitrosyl species. When the cation or the metal atom contains, besides empty orbitals, full or partly filled d-type orbitals, they can interact with the 7r -type orbitals of NO molecules, via a TT-type electron back-donation. This gives rise to bent nitrosyls, where N is Hkely sp hybrid and the NO stretching frequency is decreased. 

The lone pairs on the halogen atoms occur in p-orbitals, and readily interact with acceptor orbitals on metals. By the aforementioned analogy to Groups VA and VIA elements, one might expect to encounter electron back-donation from the metal onto the halogen atom (24,25). In methyl iodide (1), the LUMO is a a orbital localized primarily on iodine and is... 

One more band was found in TS-la sample at 2126 cm below that of free CO in the gas phase. Frequency lowering is characteristic of d-electron back donation to the antibonding orbitals of CO, that is why this band can be considered as an evidence for the reduced transition metal sites on the surface. 

For analogous reasons to the monomer requirements that favor cationic initiators, vinylic monomers with electron-withdrawing substituents on the carbon-carbon double bond are amenable to polymerization by anionic catalysts, since under these conditions the electron-withdrawing substituent assists in stabilization of the propagating carbon ion as it forms. However, this class of monomer is usually still sensitive to free radical-initiated polymerization because of electronic back donation from the electron-withdrawing group to the carbon-carbon double bond (Table 22.4 [11, 12]). 

Our own quantum mechanical calculations point to a transition state for oxygen transfer to the olefin being partly stabilized by electron back-donation from an oxygen lone pair fo a n orbital on the C=C double bond. Of the two oxygen lone pairs, the one in the TiOO plane is already involved in O to Ti a donation and is therefore less available for C-O bond formation than the one perpendicular to the TiOO plane (see Fig. 13.5). 

E.s.r. parameters for (40) suggest that the bonding is similar to that in ferrocene, with 0.7 of an electron back-donated to each borabenzene ring. ... 

CoOx may affect the adsorption of CO or O2 on R. Since at low temperatures the reaction rate on R is determined by the slow adsorption of oxygen due to CO inhibition, it is most likely that CoO serves as 0-supplier for die reaction. No influence of cobalt oxide on the CO adsorption on R was detected by IR measurements. If we assume that R-Co alloy formation does not play an important role in the CO/O2 reaction over Pt/CoOx/SiQj, several models may account for flie observed effects. According to our first model, cobalt cations enhance the adsorption of O2 on R by an increased electron back-donation into the anti-bonding orbitds of O2, which facilitates O2 dissociation. The increased back donation may be induced by the electrical field of the cobalt cations. The second model is shown schematically in figure 4. CO is adsorbed on R. O2 dissociates on CoO and the dissociation may be assisted by the presence of O-vacancies present on cobalt oxide. COa on R will react with Oa on cobalt. This reaction will then take place at the interface between R and CoOx. It is also possible that Oa migrates fi"om tiie CoO to R, in which case the reaction proceeds on the R surface (third model). The authors are in favour of the last two models since R itself is already able to dissociate O2 around 100 K if fi ee R sites are available (no CO inhibition) [33]. 

When heteroatoms are attached to the positive carbon, the cation gains added stability via donation of electrons (back donation) to the electron deficient center, which leads to greater stability, often by resonance delocalization of the charge on the heteroatom. Two common examples of this are oxo-stabilized cations such as 106 and sulfur-stabilized cations such as 107. In both cases, back donation from the heteroatom leads to resonance stabilized cations. Both 106 and 107 are more stable than a simple tertiary cation bearing three... 

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