Pi complexation

Berghuis, A.M., et al. Structure of the GDP-Pi complex of Gly203-Ala Giai a mimic of the ternary product complex of Ga-catalyzed GTP hydrolysis. Structure 4 1277-1290, 1996. 

Finally, we briefly compare the NBO donor-acceptor picture of geminal delocalization with Dewar s sigma-aromaticity concept and the associated pi-complex... 

The importance of pi-complex resonance structures such as (3.181) can also be directly assessed with NRT analysis (Section 1.6). For the equilibrium C3H6 structure corresponding to the electron-density map in Fig. 3.81(a), the optimal NRT resonance weightings are found to be... 

Thus, NBO/NRT analysis generally supports the Coulson-Moffitt picture and indicates that pi-complex character (although present) is a relatively minor feature of the electronic structure of cyclopropane. 

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

The propylene forms a pi-complex with the vacant d-orbital of titanium as shown in the following structure. 

Dewar MJS, Rogers H (1962) Pi-complexes. 2. Charge transfer spectra of pi-complexes formed by tetracyanoethylene with polycyclic aromatic hydrocarbons and with heteroaromatic boron compounds. J Am Chem Soc 84 395... 

FIGURE 2. Left deprotonation of pVIg(OH2)6] lowers the coordination number of magnesium. Reprinted with permission from Reference 14. Copyright 2005 American Chemical Society. Right pentacoordinate geometry of Mg + in the solid-state structure of the Mg + —F —Pi complex of yeast enolase... 

More recently, a number of other pi-complexes have been shown to undergo reactions typical of aromatic systems. 

Iodine interacts with many organic compounds to form pi complexes that are colored. This method of detection is especially useful for lipids containing double bonds. A lipid with several double bonds will give a darker spot with iodine. The darker spots may also be due to a higher concentration of lipid. 

As early as 1964, Corey had suggested the intermediacy of an oriented pi complex in the cycloaddition of enones (43) and soon thereafter Hammond and coworkers, on the basis of arene fluorescence quenching by dienes, suggested the possible involvement of a polar excited state complex with substantial charge transfer character (44). Since then the possibility of cycloadditions occurring through the intervention of exciplex or excimer intermediates per se or as precursors for radical ions pairs, eq. 12,... 

The reorganization or collapse of the switch segments on GTP hydrolysis (signal termination) may occur before phosphate is released from the Gz GDP Mg2+ Pi complex, and may be triggered by dissociation of Mg2+ from the catalytic site or formation of a hypothetical catalytic intermediate (e.g., metaphosphate). GTP hydrolysis and switch reorganization can be mechanistically decoupled as a consequence of a point mutation that alters the dynamics of Gz and perturbs the Mg2+-binding site in the catalytically activated state. 

Hu, J. Barbour, L. J., Gokel, G. W., (2001a) Solid state evidence for pi-complexation of sodium cation by carbon carbon double bonds Chem. Commun. 1858-1859. 

---