Back-donation of electronic charge

An examination of Table XII shows that in all cases the M-C interaction is a dative bond, i.e., donation of electron charges from the n orbital of olefin to the vacant s orbital of metal and, simultaneously, back-donation of electron charges from the d orbitals of M to the n orbital of olefin (Fig. 12). This can be interpreted in more detail as follows. When the olefin molecule approaches M+, some electronic charge is transferred from the C=C it orbital to the valence s orbital of M+ at the same time, electrons in the filled d orbitals of metal are transferred to the symmetry-matched 7r orbital of olefin. It can be seen from Table XII that upon adsorption, the electron occupancies of the valence s orbitals of Cu and Ag always increase, whereas the total occupancy of their Ad or 5d orbitals always... 

From the analysis of the charge populations on the different atoms of the metal and the adsorbate, we can conclude that the interaction of benzoate with an iron cluster involves the donation and back donation of electronic charge from the >C=C< and >C=0 functional groups of the molecule to the metal and vice versa. The donation takes place via an electron transfer from the tt orbitals of the adsorbate to the metal unoccupied d orbitals, whereas the back donation populates the tt orbitals of the adsorbate with the electrons from the occupied metal orbitals. Both interactions and electron transfers are attractive processes and the contribution of the repulsion is only the result of the interaction between the occupied orbitals of the adsorbate and the metal. 

The interplay of donation and back-donation of electronic charge between a metal and 7r-acceptor ligand is an example of a synergic ejfect. 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

In principle, the effects of oxygen substituents are consistent with CF2 being a relatively stable carbene despite the corresponding carbocation being quite unstable. This is understandable if the dipolar structure produced by resonance interaction in the carbene (52) is compensated by an inductive back donation of electrons in the cr-bonds. In the cation (53), back donation accentuates rather than compensates charge separation arising from resonance... 

The surface composition and availability of certain adsorption sites are not the only factors that determine how CO binds to the surface rather, interactions between CO and co-adsorbed molecules also play an important part. The RAIRS study conducted by Raval et al. [35] showed how NO forces CO to leave its favored binding site on palladium (see Fig. 8.10). When only CO is present, it occupies the twofold bridge site, as the infrared frequency of about 1930 cm-1 indicates. However, if NO is co-adsorbed, then CO leaves the twofold site and ultimately appears in a linear mode with a frequency of approximately 2070 cm-1. Raval and colleagues [35] attributed the move of adsorbed CO to the top sites to the electrostatic repulsion between negatively charged NO and CO, which decreases the back-donation of electrons from the substrate into the In orbitals of CO. In this interpretation, NO has the opposite effect that a potassium promoter would have (see Chapter 9 and the Appendix). 

The conclusion that palladium particles in zeolites may carry a partial positive charge follows from the IR study of CO adsorption. This adsorbate can be considered to be a probe of the electronic state of palladium. Namely, the shift toward higher frequencies of the CO linear band (for Pd°-CO it appears at <2100 cm ) reflects a decrease in the back donation of electrons from Pd to CO. Along with such an interpretation, Figueras et al. (138) detected the presence of electron-deficient Pd species in Pd/ HY but not in Pd/Si02. More recently, Lokhov and Davydov (139) confirmed the presence of positively charged Pd species apart from Pd° in reduced (at 300°C) Pd/Y samples and ascribed a 2120- to 2140-cm"1 band to Pd+-CO complexes (Fig. 7). Similarly, Romannikov et al. (140) report that adsorption of CO on Pd/Y samples reduced at 300°C produces IR bands at >2100 cm 1 ascribed to Pd+-CO and Pdzeolite protons, because the IR band of the zeolite O-H group decreases when CO is released and increases when CO is added to the cluster (141, 142). 

The double bond in chlorotrifluoroethylene is polarized by back-donation of electrons of fluorine in such a way that the negative charge is on the carbon linked to chlorine and fluorine. Consequently, the difluorome-thylene end of the double bond is more electrophilic and is attacked by the ethoxide anion. Subsequent ejection of fluoride anion gives an unsaturated intermediate, l-chloro-l,2-difluoro-2-ethoxyethylene, compound V. This compound reacts with another ethoxide anion in a similar way and yields l-chloro-2,2-diethoxy-1-fluoroethylene. Nucleophilic addition of a third molecule of ethanol gives the final product, the orthoester of chlorofluoroacetic acid, compound W [75]. 

The M-CO bond, according to Blyholder [47, 48], is formed by charge transfer from the 5 o molecular orbital of CO to the metal and back-donation of electrons from the metal d-orbitals to the 2 7t orbital of adsorbed CO. Since the 2 n is strongly antibonding and the 5 a only weakly bonding, the C - O bond strength is lowered in the adsorbed state. Consequently the C - O stretching frequency is shifted to lower wavenumbers. 

N = 1.0976 for free N2), consistent with back-donation of electron density from Ru" to N2." A molecular orbital description for (58) has been discussed. (58) shows a metal ligand charge transfer band at 263 nm ( = 4.8 x 10 M cm Replacement ofNHj by HjO in the dimer decreases hH of binding of N2 by 25.1 kJmol . " The "N NMR of (58) has been reported recently."" ... 

There is at present considerable controversy " 9 -i03 on the occurrence and the extent of stabilization that is achieved by the back-donation of electron density into vacant d-orbitals in phosphorus. The CNDO/2 calculations show some interesting trends in this respect, in particular concerning the effect on the relative stability of the various isomers of a given molecule. The method of the calculation is first illustrated for PFs. Table 10 is an extract from the density matrix of the program. The diagonal elements correspond to electron densities of the described orbital. The sum of these elements, 3 8890, is equal to the total valence shell electron density for the phosphorus. Of this, 1 3671 units of electron charge, or about 35%, reside in the five d-orbitals of the phosphorus. 

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