Electron backdonation

Figure 6.14. CO chemisorption on a transition metal. Molecular orbitals and density of states before (a,b) and after (c and d) adsorption. Effect of varying 0 and EF on electron backdonation (c) and donation (d). Based on Fig. 4 of ref. 98. See text for discussion. Reprinted with permission from Elsevier Science.

Figure 6.14c shows the electron backdonation interaction (electrons are transferred from the Fermi level of the metal to the hybridized 27t molecular orbital which was originally empty, thus this is, by definition, a backdonation interaction). 

Figure 6.14d shows the electron donation interaction (electrons are transferred from the initially fully occupied 5a molecular orbitals to the Fermi level of the metal, thus this is an electron donation interaction). Blyholder was first to discuss that CO chemisorption on transition metal involves both donation and backdonation of electrons.4 We now know both experimentally7 and theoretically96,98 that the electron backdonation mechanism is usually predominant, so that CO behaves on most transition metal surfaces as an overall electron acceptor. 

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. 

From an extended study on the sequence selectivity of UV-induced cleavage of dsODNs (Table 12.12) it has been concluded that an ET to neighboring bases must occur [reaction (40)] followed by a subsequent competition between electron backdonation [reaction (41)], decay of the 5BrUra radical anion [reaction (42)] and hole transfer [reaction (43) Chen et al. 2000. 

In order to avoid the influence of lateral interactions on the vibrational frequency of adsorbates, it is common to analyze the frequency of a single adsorbed molecule, i.e., the singleton frequency, which is obtained by extrapolating the adsorbate frequency to zero coverage. For the sake of comparison, the electronic backdonation must be the same for both UHV and electrochemical system. Since in UHV the potential is governed by the work function of the metal, an equivalent potential must be found for the electrochemical system. 

An important result recently obtained by Hammer and Norskov using extensive DFT/GGA calculation is that energies of adsorption of molecules, such as CO (Fig. 29), can be related in a linear fashion to the energy of the center of the d band, eFermi level of the metal (Fig. 30) [87]. The closer the center of the d band relative to the Fermi level of the metal, the stronger the binding propensity with molecules, such as CO, where enhanced electron backdonation leads to stronger binding to the surface. 

Electron backdonation occurs between the doubly occupied d y, dyz and d x orbitals and empty CO 2jr orbitals. 

These second-order perturbation expressions connect unoccupied with occupied orbitals. Orbitals Ej denote adsorbate orbitals, Ci surface metal orbitals. Relations such as Eq.(2.236) are commonly used in quantum chemistry, limiting the summation terms to the highest occupied and lowest unoccupied orbitals. The theory based on that approximation is called HOMO-LUMO or Frontier Orbital theory. The first term is the electron donation term (electrons are flowing from the adsorbate surface to the surface), the second is the electron backdonation term (electrons are flowing back from the surface to the adsorbate). We will show that Eq.(2.236) leads to ideal weak adsorption theory, but needs adaptation if one wishes to apply it to the quasi-surface molecule situation, which applies in most chemisorption processes. 

Figure 2.53. Quasi-surface molecule interaction between two doubly occupied orbitals, resulting in a bonding interaction due to electron backdonation from the antibonding fragment orbital to the Fermi level.

After holding the potential at +0.8 V for 20 min, the redox peaks disappear and a new set of redox peaks appear at -0.1 V as shown by the dashed trace in Fig. 8.13a. The corresponding SEIRAS spectrum (Fig. 8.13a, dashed trace) clearly shows the dissociation of the CO hgand. The electron backdonation from the Ru to CO weakens the CO-Ru bonding and results in the CO dissociation. The solvent molecule is believed to coordinate to the Ru center (solvent-SAM), and the redox peaks at -0.1 V are ascribed to the one-electron transfer between [(Ru -solvent) Ru "Ru"V° and [(Ru" -solvent) Ru" Ru"V -... 

Using TAfS° (68) rs 14 kJ mol, A,G (68) rs —49 kJ mol is finally obtained, that is, the energetics of reactions (66) and (68) are comparable. A possible reason for the different reactivity of arenes and alkanes is that an arene may have a kinetic advantage over an alkane by forming a strong T/ -bond with the metal. The electron backdonation from the metal to the antibonding tt orbitals of the arene weakens the C(sp )-H bond and favors the formation of the aryl hydride. While this kinetic explanation may account, by itself, for the preference of arene addition, it is observed in Table 1 that for late transition metal complexes the differences DH° (M-Ph) — DH°(M-Me) are substantially higher than Z)//°(Ph-H) — Z)/7 (Me-H). This trend will, of course, imply that benzene activation is thermodynamically favorable, relative to methane activation. 

Backdonation of metal electrons into the CO 2it orbital increases the charge on the parallel-oriented CO molecules and depletes the d-valence electron band of electrons. The former increases the direct repulsive interaction, the latter increases metal atom-metal atom interactions, but also decreases electron backdonation. It results in an overall increase of the repulsion of neighbor CO molecules with approximately 10 kJ mol T Shared bonding of CO adsorbed in bridge coordination and atop leads to additional repulsive interactions on the order 10-20 kJ mol . This interaction energy value is very similar to that found for the Rh4 cluster of Figure 10.22. 

The second term of Eq. (24) represents electron backdonation from adsorbate to surface, the third electron back donation from metal surface to adsorbate. 

The HOMO of CO is its Sa orbital. This orbital is also doubly occupied resulting in repulsive interaction with the Cu o orbital. This repulsion is not compensated by the electron backdonation interaction with the unoccupied CO 2tc level (curve I, Fig. 29). Promotion of electrons from the Cu (4s + 4s)(j to the empty CU2 (4s—4s) a molecular orbital, changes the repulsive interaction curve into an attractive one (curve II, Fig. 29 a). A HOMO of a symmetry becomes available for backdonation into the CO In orbital, and an unoccupied LUMO of a symmetry becomes available to bonding with the doubly occupied So CO orbital. 

Lowering the effective ionization potential enhances electron backdonation between metal and adsorbate. The authors have discussed this effect extensively elsewhere... 

The orbital occupancy of the hydrogen atoms is half. The work function decrease from Pt to Pd to Ni will favor bonding to the Ni surface if electron backdonation dominates chemical bonding. 

In general, trivalent phosphorus compounds, arsines, stibenes and several amines improve the thermal stability of hydrido metal-carbonyl complexes because of superior o-donor and weaker it-acceptor properties [18]. This feature enhances the electron density at the metal center, and hence the metal-CO bond is strengthened as a result of enhanced electron backdonation. However, the special effect of a ligand on the activity and selectivity may be entirely different from one metal to another, and therefore conclusions should be drawn only in close relation to the metal that is used. Only some selected observations will be detailed here, showing the uniqueness of each catalytic system. 

In addition, NO can also interact with the siuface cationic centres on metal oxides, giving rise to surface mono-nitrosyl, gem-dinitrosyl, and tri-nitrosyl species. When the cation or the metal atom contains not only empty orbitals, but also full or partly filled d-type orbitals, they can interact with the 7r -type orbitals of the NO molecule, via a TT-type electron backdonation. This gives rise to bent nitrosyls, where N is likely to be a sp hybrid and the NO stretching frequency is decreased. 

Liu, D., Hagelberg, R, 8c Park, S. S. (2006). Charge transfer and electron backdonation in metallofullerenes encapsulating NSC3. Chemical Physics, 330(3), 380-386. 

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