Donation of charge

When the charge on the electrode is made negative, the bond is weakened due to donation of charge from the metal into adsorbate x orbitals and the band frequency shifts to lower wavenumber. When the charge on the metal is made positive a shift to higher frequency occurs. At a mercury electrode, however, there are no p- or d-electrons available to participate in a back-bonding interaction. 

As for ethers and alcohols, the ease of hydrogen abstraction is determined by polar factors when operating with electrophilic radicals (X ). The polar character is influenced by the same factors as for ethers and alcohols, i.e., a back-donation of charge from nitrogen to the a-C radical accentuates the nucleophilic character. The influence of the abstracting species in the case of dimethylformamide is shown by the results given in Table V, where the attack of carbamoyl and a-amidomethyl radicals... 

An alternative explanation of negative pressnre dependencies can be found in the work of Li and co-workers [29], who have snggested that the dihydrogen bond involves donation of charge from the a-bond B-H to the N-H a -orbital. 

The description of the bonding of unsaturated hydrocarbons to metals was originally developed by Dewar, Chatt and Duncanson and is now known as the well-established DCD model based on a frontier-orbital concept [82]. In this model, the interaction is viewed in terms of a donation of charge from the highest occupied -orbital into the metal and a subsequent backdonation from filled metal-states into the lowest unoccupied -orbital, see Figure 2.33. Contrary to the case of the standard Blyholder model for CO and N2 the DCD frontier-orbital model is supported by experimental XES measurements [83]. In the present section, we will show how we can experimentally identify and quantify the contributions of the different -orbitals involved in the interaction with the surface. The DCD model will be shown to very well describe the chemical bonding of ethylene on Cu and Ni surfaces. Furthermore, the differences in bonding of benzene to Cu and Ni will be discussed. 

A simple view of the mode of bonding of the carbon monoxide molecule to the surfaces of the transition metals and in their carbonyl complexes is well known and is thoroughly described in the literature.4 These metals have unfilled d-orbitals (or holes in their d-band), and the molecule in the linear form is held by a push-pull bond in which charge is transferred from the 5a orbital of the molecule into the metal s d-band, while there is back-donation of charge from the top of the d-band into the molecule s vacant... 

If/ is the ionization potential of the reactant in the reduced form and S the change of its solvation energy on electron transfer, then the energetic condition for the process to occur in the direction of donation of charge is... 

The bis(propynyl)beryllium compound [Be(C=CMe)2NMe3]2 is unusual in crystallizing with two types of dimeric molecule in the lattice, one of which has a diamond-shaped (Be-C)2 ring very similar to that of 31. The other, structure 32, has a nearly rectangular (Be-C)2 ring, explicable in terms of donation of charge from the alkynyl triple bond into the available metal orbital. 

The bonding mechanism is now clear. It involves five orbitals (sd+), (sd ), 3d 2, n and n. When Ni ( D) approaches the ligand the electrons move from (sd ) to (sd+), which reduces the repulsion between nickel and the ligand n orbital. A much more effective interaction between the it system of C2H4 and the nickel 3d orbitals becomes possible, leading to the formation of a weak bond between and it and some delocalization of the it orbital onto the nickel atom. The total donation of charge to nickel is 0.42 electrons while 0.61 electrons are transferred in the opposite direction. 

Fig. 23.1 Components of metal-carbonyl bonding (a) the M—CO cr-bond, and (b) the M—CO tr-interaction which leads to back-donation of charge from metal to carbonyl. The orbital labels are examples, and assume that the M, C and O atoms lie on the z axis.

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