Backdonation bonding

The net increase in occupancy indicates a net electron transfer from the adsorbate to the Ag-containing adsorbent. The net increase in the occupancy of the 5 orbital of Ag indicates the strength of the forward cr-donation bond. The net decrease in the occupancies of the 4d orbitals of Ag indicates the strength of the d-Tt backdonation bond. The ratios of AOc of 5s over the AOc of 4d are approximately 2 1 to 3 1, indicating the relative contributions of the ct-donation over ht d-n backdonation to the overall bond. 

The fundamental differences in interactions with different anions and cations can be further understood from the changes in orbital occupancies that give rise to the <7-donation bond and the d-ir backdonation bond. The magnitudes of... 

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

The coadsorption of oxygen as well as of other electronegative additives on metal surfaces favors in general the 7t-bonded molecular state of ethylene, as the latter exhibits, compared to the di-o bonded state, a more pronounced electron donor character and a negligible backdonation of electron density from the metal surface. 

It is also clear that in the present case oxygen is the electron acceptor (A) while CO is the electron donor (D). It has been already discussed that CO is an amphoteric adsorbent, i.e., its chemisorptive bond involves both electron donation and backdonation and that, in most cases, its electron acceptor character dominates. However, in presence of the coadsorbed strong electron acceptor O (see section 2.5.2.1) it always behaves as an electron donor. 

The chemisorptive bond is a chemical bond. The nature of this bond can be covalent or can have a strong ionic character. The formation of the chemisorptive bond in general involves either donation of electrons from the adsorbate to the metal (donation) or donation of electrons from the metal to the adsorbate (backdonation).2 In the former case the adsorbate is termed electron donor, in the latter case it is termed electron acceptor.3 In many cases both donation and backdonation of electrons is involved in chemisorptive bond formation and the adsorbate behaves both as an electron acceptor and as an electron donor. A typical example is the chemisorption of CO on transition metals where, according to the model first described by Blyholder,4 the chemisorptive bond formation involves both donation of electrons from the 7t orbitals of CO to the metal and backdonation of electrons from the metal to the antibonding n orbitals of CO. 

In the presence of Bi or Te, the C=0 bond is weakened, as concluded from the displacement of the CO stretching band to lower wavenumbers. There is also a change in the dependence of the band frequency on electrode potential, with the slope dv/dE increasing for the adatom-modified surfaces. These changes indicate that the adatom alters the electronic properties of the surface, increasing the amount of electronic backdonation and stabilizing the adsorbed CO molecule. No catalytic enhancement is expected from this effect. 

It did not prove possible to synthesize a substituent-free Ga complex with formula Cp (CO)2Fe Fe(CO)4 Ga (Scheme 13).43 Addition of bipy to 30 resulted in halide elimation, but the main group element in the product 31 was coordinated by the bipy ligand. Upon addition of dppe, however, substitution of the carbonyl ligands occurred instead along with halide ion elimination to produce the substituent-free Ga complex 32. It has a linear coordination environment (Fe-Ga-Fe angle = 176.01(4)°), and the Ga-Fe bond distances are much shorter than in those related adducts where donor ligands are also bound to the Ga atom.43 The authors attributed the non-observation of the carbonyl derivative to a need for an electron-rich metal center to stabilize the Fe-Ga bond via 7r-backdonation. 

The bonding between these two fragments can be understood using the Dewar-Chatt-Duncanson model of donation and backdonation [79, 80], The frontier orbitals responsible for these interactions between 15 and 16-R are drawn to scale in Figure 15. 

The DCD scheme allows us to understand the conditions required to stabilize a CT-complex. Since backdonation from metal is the crucial factor controlling the Si-H bond interaction with a metal, any factor that reduces this component will lead to the strengthening of the residual Si-H c bonding. These factors are ... 

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