Sigma bonds electron acceptors

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

When a halogen is in close contact with another atom approximately 180° from the halogen-halogen sigma bond, it is acting as an electron acceptor. When the contact is approximately 90°, it is acting as an electron donor. 

Simple 2c/2e bonds to the transition metals commonly are weaker than the corresponding sigma bonds from the p-block elements, resulting in lower-lying acceptor ctml antibonds and increased electronic delocalization. 

Figure 2 shows the structures of the bifunctional model systems l(n), (n=4,6,8,10,12), investigatedThe synthesis of these systems and a discussion of their electrochemical properties as well as of their electronic absorption and emission spectra have been given elsewhere (Oevering et al., 1987). These molecules contain a 1, 4-dimethoxynaphthalene unit as the photoexcitable electron donor (E./2 =+1.1 V vs. see in acetonitrile) and a 1,1-dicyano-vinyl moiety as a moderately powerful electron acceptor (Ev=-. l V) separated by an array of at least n C-C sigma-bonds. The centre-to-centre separation (Rc) and the edge-to-... 

The 7i-stacked bases of ds DNA might be expected to provide a better medium for bridge-mediated electron transfer than the sigma bonds of proteins or hydrocarbons. It has in fact been proposed by Turro and Barton [18d] that ultrafast photoinduced electron transfer processes involving intercalated donors and acceptors can occur with little or no distance dependence. According to this paradigm, duplex DNA can function as a molecular wire or r-way . 

Examination of the chemical, electrochemical, and spectroscopic behaviour of R3EM(C0)3L (R = Ph or Me, E = Ge or Sn, M === Mn or Re, L = phen or bpy) reveals that the lowest state is one in which an electron has been transferred from the HOMO, which has sigma E—M bonding character to the LUMO which is mainly localized on The rhenium complexes emit in fluid solution at room temperature, and the luminescence may be quenched by both electron acceptors and electron donors, as in reactions (5) and (6). No net photochemistry is observed... 

Finally, replacement of TFE by tetracyanoethylene (TONE) will adversely affect the metal-olefin bonding because TONE does not have the abnormally weak pi bond of TFE. TFE will tend to utilize carbon 2s AO s more than TONE because the sigma electronegativity of fluorine is greater than that of CN. Thus, only the greater pi electron acceptor ability of TONE will favor M-TCNE over M-TFE. 

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