Donative interaction

The electronic influence of the other ligands in the metal s coordination sphere is a major consideration when exploring the coordination chemistry of carbon dioxide. Since the metal-C02 bond is stabilized mainly by back-donative interactions, it would be expected that good donor ligands (e.g., the ubiquitous phosphine ligands) would enhance the binding ability of C02 to the metal center. This has been verified both experimentally (10) and theoretically (20), and has as well been demonstrated to be of importance in C02 insertion processes (see below). 

Fill ear insert a moist cotton plug repeat 1-2 h PRN Caution [C, ] Contra w/ perforated eardrum Disp Soln 5.4% andpyrine, 1.4% benzocaine SE Local iirita-don Interactions May t effects OF sulfonamides EMS None OD Unlikely, may be harmful if swallowed... 

Although the agostic interactions are mainly due to donative interaction from the C—H bond, Morokuma and Ziegler noticed that some donation can occur from the C—C a-bonding orbitals as well. The two molecular orbitals sketched in Figure 9 indicate such donation from both the Ca—and Cp— Cy bonds. ... 

Figure 9. Contour maps of occupied molecular orbitals (MO) showing donative interaction from the Cp—Cy and Cy—H bonds (MO 31) and from the C —Cp and Cp—Cy bonds (MO 32).

The changes in attractive interaction of the a symmetric donative interaction and the It symmetric backdonating interacting are thus seen to counteract... 

The a symmetric lone-pair orbital of ammonia is doubly occupied and hence as we discussed in Section 10.3.2 will experience a repulsive interaction with the metal free electrons. The attractive interaction to the chemisorption energy is mainly due to the interaction of the donative interaction of the ammonia a symmetric lone-pair molecular orbital with metal d-valence electrons. 

Extended networks of ions held together by ion-don interactions (NaCl, MgO)... 

In the chemisorbed state the adsorbate surface-orbital fragments are made up of a mix of adsorbate a and metal valence-electron molecular orbitals. The cr-electron occupation of the adsorbate, which was originally 2 electrons, decreases upon adsorption. This type of interaction is therefore referred to as an electron-donative interaction. 

The orbital interaction scheme in which the attractive contribution of the adsorbate surface bond is estimated from the donative and respective back-donative interactions is called the Blyholder model. It is the analogous to the Chatt-Dewar model which is used to describe chemical bonds in organometallic complexes. 

The preference for CO to be adsorbed atop is typical for Co, Rh, Ru, Ir and Pt. On Ni and Pd, however, CO prefers to adsorb at the higher coordination sites. We have already discussed that the preference for atop adsorption is the result of the minimization of the repulsive interaction between doubly occupied tt and valence electron orbitals on the metal atom. Back-donation into imoccupied 2-k molecular orbitals is maximum in high coordination sites. The back-donative interaction is enhanced by the additional interactions with antisymmetric combinations of smface s-atomic orbitals. The shift to higher coordination of CO on Ni and Pd indicates less involvement of the d-valence electrons in the smface chemical bond, consistent with the decrease in spatial extension of the d-atomic orbitals, when a metal changes position moving upward in a column or from left to right in a row of the periodic table. 

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