Olefin complexes electronic spectra

SbCla is a relatively weak Lewis acid, since in the presence of a mixture of mesity-lene and benzoyl chloride it gives a complex with mesitylene Antimony penta-chloride is stronger and gives a 1 1 complex with perylene whose electronic spectrum is similar to that of the perylene cation Complexation has been claimed between antimony pentafluoride and aromatic olefins such as 1,1-diphenylethylene and stilbene ... 

Class 1—Photoexcitation of a preformed copper-olefin complex. Complexation plays a key role by shifting the absorption spectrum of the system into a wavelength region accessible to the irradiating light and/or by providing a sterically or electronically favored pathway to the photoproduct. 

Infrared spectroscopy of olefin complexes is a less useful probe of n-bonding than infrared spectroscopy of CO complexes. Binding of an olefin to an electron-rich metal center does reduce the C-C stretching frequency, as one would expect from the reduction of the C-C bond order due to Ti-backbonding. However, the C-C stretch of a coordinated olefin is weaker than that of coordinated carbon monoxide because the vibration of the olefin creates a smaller change in the dipole moment. (Recall that symmetric vibrations are not observed in the infrared spectrum because of a lack of change in the dipole moment.) Thus, the olefin stretch is weak and lies at a frequency that overlaps with other bands. 

The order of reactivity of these three catalysts towards alkenes (but also towards oxygen) is 1 > 3 > 2. As illustrated by the examples in Table 3.18, these catalysts tolerate a broad spectrum of functional groups. Highly substituted and donor- or acceptor-substituted olefins can also be suitable substrates for RCM. It is indeed surprising that acceptor-substituted alkenes can be metathesized. As discussed in Section 3.2.2.3 such electron-poor alkenes can also be cyclopropanated by nucleophilic carbene complexes [34,678] or even quench metathesis reactions [34]. This seems, however, not to be true for catalysts 1 or 2. 

The wave function of the complex will be a linear combination of the four structures in Figs. 3.5b and c. In a series of olefins, we may expect to see a spectrum of cases. For example, in a series of olefins where the singlet-to-triplet tttt excitation is gradually lowered we may see an increasing metallacyclic character up to complexes, where the C—C distance is that of a single bond. With olefins that are good electron donors, we may see a wave function dominated by a mixture of ionn, and d>nb, while for powerful electron acceptors, we may expect a wave function dominated by d>ion(2) and [Pg.63]

The transition-metal allyl complexes are air- and temperature-sensitive solids Cr(allyl)3, m.p. ca. 70° Ni(allyl)2, m.p. ca. 0°. The infrared spectrum of both compounds indicates that the bonding of the allyl group to the metal involves r electrons (the olefinic bond appearing at 1520 and 1493 cm.-1, respectively) they can be identified by their mass spectra. 

An electron transfer-sensitive oln coordination alternative of substituted olefins to metal centers has been demonstrated [81] in the system [(NC)2C=C(CN)2W(CO)5]" (20) where the n complex with n = 0 (olefin coordination) changes reversibly to an ESR detectable a complex (nitrile coordination) after one-electron reduction ( = -1). Even in the absence of a detailed spin population analysis the low symmetry of the anionic complex is evident from at least three distinctly different N coupling constants in the ESR spectrum [81]. 

The carbon-boron heterocycle, 3-phenyl-3-benzoborepin, exhibits oxidative stability upon exposure to air, an unusual feature for a trivalent boron compound. In Table XVI are recorded the chemical shift data for the vinyl protons for the benzometallepins of B, Sn, and Si. The PMR spectrum of 3-phenyl-3-benzoborepin exhibits vinyl proton resonances at lower fields than would be expected for an olefinic boron compound (compared to trivinylboron or 4,5-dihydroborepin see Table XV), and also at lower field than the benzostannepin derivative (217). The shift to lower field of 0.4 to 0.8 ppm may be consistent with the presence of a ring current, which would require the participation of the Bp orbital in the 7r-electron system. Support for increased electron density at boron might be provided from B NMR measurements, but such data have not yet been reported. Complexation of boron, which converts the... 

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