Bonding 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]. 

Many metals bind to the sulfur of sulfhydryl groups in proteins. Metals that bind sulfur in preference to oxygen not only form strong cr bonds with the readily polarizable ligands, but also tt bonds by back-donation of electrons from metal drr to ligand dir orpir orbitals. The electronegativity... 

In their metal complexes, bonding of either species to the metal atom is via a ligand - > metal a donor bond and a metal - >ligand n bond, enabling back donation of electron density to the n orbitals of the C-C multiple bond system to take place. Vinylidene is one of the best 7t-acceptors known, and is exceeded only by S02 and CS in this respect the relationship between phenylvinylidene and other common ligands has been determined (18) from the CO force constants exhibited by a series of Mn(CO)2(q-C5H5) complexes, which increase in this order ... 

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 case (ii), it is now the bonding level t2g which is the MO counterpart of the CF t2g orbitals. The number of electrons to be placed in it will depend on the number of d electrons possessed by M in the appropriate oxidation state. But since electrons in t2g will have some L character, and will be to some extent delocalised over the ligands, such occupancy leads to electron transfer from M to L. Thus ligands in this category are known as jt acceptors, and we say that there is some degree of jt back-bonding or back-donation. Note that the stabilisation of the CF t2g level as a consequence of such interaction will increase the splitting A0. 

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. 

The close similarity between transition metal PF3 complexes and their CO analogues is indicated by a variety of studies (72,174,272), and has been rationalized in terms of a bonding scheme for PF3 involving (1) a u-donor component from donation of the phosphorus lone pair electrons to the metal, and (2) a symmetry-allowed n bond involving back donation of metal d electrons into empty 3d orbitals on phosphorus. 

Schwartz s reasoning for optimizing these thermodynamic considerations led to the development of hydrozirconation. Hydride complexes of the late transition metals do not in general exhibit the hydrometallation reaction, probably because the alkene complexes are too stable. This may be understood from the Dewar-Chatt-Duncanson model for alkene bonding, wherein back donation of metal d-elec-trons to the alkene Tr -orbital is a major contributor. For metal centers with d -electron configurations, there should be substantial stabilization of (3) with respect to (2). Such metals are only found towards the left end of the Periodic Table, particularly Groups III A to VA. 

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]. 

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