Oxygen lone-pair orbitals, back-donation

IG basis for methanol, fluoromethanol, and methanediol. In methanol itself, the t-type lone-pair electrons are subjected to back-donation into antibonding-type orbitals of suitable symmetry on the methyl group. This leads to a decrease of the n-orbital population from the value of 2.00, appropriate to water, 1.97. A comparable electron displacement occurs in fluoromethanol when the C-F bond is in the nodal plane of the oxygen lone-pair orbital. If, however, the C-F bond and lone-pair orbital are in the same plane (in the og orientation), the back-donation lowers the population to... 

The 5cr level is primarily located on the carbon atom and can be identified with the carbonyl lone pair orbital, but in addition it is somewhat antibonding between carbon and oxygen. The population of this orbital has a significant influence on the carbonyl stretching force constant and, therefore, changes in the carbonyl force constant cannot be assigned solely to back donation effects. Force constants k in the M(CO)5L molecules can be described by the following equation ... 

Our own quantum mechanical calculations point to a transition state for oxygen transfer to the olefin being partly stabilized by electron back-donation from an oxygen lone pair fo a n orbital on the C=C double bond. Of the two oxygen lone pairs, the one in the TiOO plane is already involved in O to Ti a donation and is therefore less available for C-O bond formation than the one perpendicular to the TiOO plane (see Fig. 13.5). 

Concerning CO/O2 reaction on perovskite oxide, the "suprafacial" mechanism is assumed, as well as for the carbon black/oxygen reaction. Carbon adsorption on the catalyst surface could be made tlirough the C-C bond or the C-0 surface complexes, assuming that C is bonded to the Mn ion with donation of carbon lone pair into the empty 3dz orbital to form s bond accompanied by back donation of the t2g electrons of Mn ion to anti-bonding i orbital of C-0 or C-C. Moreover, the mechanism begins by simultaneous adsorption of carbon and oxygen, the interaction between adsorbed species causes the CO2 formation, the desorption of which releases the catalytic active sites. 

Dialkyl H-phosphonates exhibit a number of complexation modes with transition metals depending on their type. In complexes of dialkyl H-phosphonates with some early transition metals (Zr, H) in their higher oxidation states, the phosphonate ligands act as lone-pair a-donors and are coordinated to the metal center through the phosphoryl oxygen atom. In contrast to this, the late transition metals (Ni, Pd, Pt, Co, Ir, Rh, and Ru) exhibit a well-expressed preference to the more soft phosphorus donor of the phosphite tautomeric form. The late transition metal complexes of dialkyl H-phosphonates are additionally stabilized by the electron back donation from the electron-rich metal center to the empty and 3a orbitals at the phosphorus donor. 

Fischer Carbenes Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (Eq. 11.1). Going back to the bonding picture mentioned above, we saw that the methoxy substituent will also help stabilize the empty p orbital on the carbene carbon by ir donation from one of the lone pairs on oxygen. Resonance form 11.3, which is probably the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this bond. This effect is responsible for the restricted rotation often observed for the heteroatom-carbene carbon bond in NMR studies. For example cis and trans isomers 11.8 and 11.9 of methoxymethyl carbenes are rapidly interconverting at room temperature (Eq. 11.2), but can be frozen out in the proton NMR at -40 C. ... 

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