Backdonation of Electrons

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... 

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm"1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

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. 

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. 

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. 

Backdonation of electrons from M to H2 o is crucial not only in stabilizing the bonding but also in splitting the H-H bond. If it is too strong, the H-H bond cleaves to form a dihydride because of overpopulation of the H2 a orbital. There is often a fine line between H2 and dihydride coordination, and in some cases equilibria exist in solution for W(CO)3(PR3)2(H2), showing that side-on coordination of H2 is the first step in H-H cleavage [2, 3]. 

Study carefully the pKgS for the haloform series, CHX3— they may not do what you think they should Chloroform is much more acidic than fluoroform even though fluorine is more electronegative (likewise with bromoform and chloroform). The anion CFamust be slightly destabilized because of some backdonation of electrons. The anion from chloroform and bromoform may also be stabilized by some interaction with the d orbitals (there aren t any on fluorine). The conjugate base anion of bromoform is relatively stable— you will meet this again in the bromoform/iodoform reaction (Chapter 21). 

The charge q is reduced due to backdonation of electrons from the doubly occupied orbital on atom (1) by an amount Ae. Backdonation reduces the electron density on atom 1, but enhances the electron density on atom 0. The corresponding energy contribution equals ... 

A second example involves the adsorption of ethylene on transition metal surfaces and offers an interesting challenge, in that it can bind via n or di-o adsorption modes. Complete structural optimizations were performed for ethylene in both coordination geometries (Fig. 3). In the 7t-mode, the TZ orbital on ethylene interacts with the dz orbital on the metal center. There is a backdonation of electron density into the antibonding 7C orbital of ethylene which leads to a small weakening of the C-C bond length. This is noted by the slight increase (0.05 A) in the C-C bond from the gas phase value 1.34A. There is considerably more backdonation of electron... 

Figure 8.4. Schematic of the C2H4-Ag interactions by jr-complexation, showing (A) donation of JT-electrons of efhylene to the 5s orbital of Ag, (B) backdonation of electrons from the 4dyz orbitals of Ag to the antibonding p orbitais of ethyiene, and (C) eiectron redistribution. (C) depicts the possibie eiectron redistribution from the 4dz2 orbitais to the 4dyz orbitals (Chen and Yang, 1996, with permission).

The difference derives because in the bridge site of the Niia cluster backdonation of electrons into the antibonding cr H2 level becomes possible by interaction with a populated antisymmetric combination of 4s Ni atom orbitals. In the bridgeing configuration, antisymmetric 4s Ni atomic orbital combinations become populated. 

NIR studies have been applied also to heteronuclear diatomics like CO and NO [95-97]. On alkali and alkaline earth cations the results are essentially similar to those obtained with homonuclear probes. In the case of CO and NO strengthening of the internal bonding is observed, caused by the withdrawal of electron density from slightly antibonding molecular orbitals in correspondence with quantum chemical calculations [70,72]. In spite of the uncertain Do determination on cobalt and copper ion-exchanged zeolites A the CO dissociation energy turns out to be decreased, which may be understood by backdonation of electronic charge into the carbonyl tt orbital [97,98]. 

Using carbon monoxide as the probe, its frequency turns out to be sensitive to the oxidation state of the cation, since the latter determines the withdrawal from, as well as the backdonation of electronic charge into, antibonding orbitals. The vibrational frequency of CO adsorbed to a metal surface or in front of a cation is classified as follows ... 

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