N-Acceptors

Group (3) o- and n-acceptor acids and donor bases with a-interactions predominating. In this group acceptors are capable of adding on electron pairs of donors in both types of interactions. Includes cations with stable closed electron shells AF , Mg , Ca and... 

Group (4) Strong a- and n-acceptor acids and donor bases. Bi , In " and Sn " are of some relevance to AB cements. 

Complexes of Halide Anions with Aromatic and Olefinic n-Acceptors. . . 142... 

The electronic configuration of tin after loosing two p electrons to form Sn2 + is 5s2. The <7-donor tendency of tin is relatively small but can be enhanced in the presence of a powerful acceptor, especially if the empty 5d orbitals of tin can act as n-acceptors [104]. This could be the situation for adsorbed Sn(II). Platinum may act as an acceptor of the tin a electrons while 5d orbitals of the latter accept 7t-electrons from H20 to build an adsorbed Sn(OH)+ complex ... 

Hence the quantity of EA can be simply calculated from the corrected sensitized emission image and the acceptor only image provided the ratio of the molar extinction coefficients of the donor and acceptor at the donor excitation wavelength is known (ct). This quantity can be determined from absorption spectra of purified labeled components or can be experimentally determined as follows. First, let us define a factor v that relates the signal of N acceptors in the S channel to the signal of the same number of donors in the D channel ... 

The UV-vis spectral analysis confirms the appearance of a new charge-transfer absorption band of the complexes of colorless a-donors (R3MH) and the n-acceptor (TCNE). In accord with Mulliken theory, the absorption maxima (Act) of the [R3MH, TCNE] complexes shift toward blue with increasing ionization potential of the metal hydrides (i.e., tin > germanium > silicon) as listed in Table 8. 

Heimbach et al.s conducted a careful experimental investigation of the influence of the ancillary ligand on the cyclodimer product distribution. Strong n-acceptors that are sufficiently space-demanding were found to catalyze the formation of cis,cis-COD almost exclusively. The VCH portion... 

For PR3/P(OR)3-stabilized nickel complexes, there are two borderline cases known from the experimental investigation of Heimbach et al. 1 which, unlike the usual behavior, redirect the cyclo-oligomerization reaction into the Ci2-cyclo-oligomer production channel. Catalysts bearing either strong a-donor ligands that must also introduce severe steric pressure (e.g., PBu Pr2) or sterically compact n-acceptors (like P(OMe)3) are known to yield CDT as the predominant product. From a statistical analysis it was concluded,8a,8c that the C8 Ci2-cyclo-oligomer product ratio is mainly determined by steric factors (75%) with electronic factors are less important. 

Charge-transfer complexes involving one-center n acceptor orbitals (n-n, a-n, and 7t-n Lewis-base-Lewis-acid adducts) have been discussed in Sections 3.2.10 and 3.6. Many CT complexes involving a acceptors have been illustrated for H-bonds (n-a ) and dihydrogen bonds (a-o ), as well as for the Cg H6 -B (7t-a ) example above. In the remainder of this section we shall therefore focus on CT... 

Although aliphatic alcohols are typically poor acceptors in the Mitsunobu-type glycosylation, Szarek and coworkers have highlighted one advance to this end [95]. For the triphenylphosphine and diethylazodicarboxylate promoted glycosylation of a monosaccharide acceptor, the addition of mercuric bromide is necessary to promote the reaction. For example, the (1,6)-disaccharide 44 was obtained in 80% yield using this modified Mitsunobu protocol. Unlike previous examples with phenol or N-acceptors, preactivation of the hemiacetal donor was performed for 10 min at room temperature prior to addition of the aliphatic alcohol nucleophile. 

Buono and co-workers (215) suggest that the increased activity of mixed P,N-ligands compared to bis(oxazohnes) is due to the ability of phosphorus to behave as a o donor and n acceptor. With this in mind, Buono and co-workers designed chiral quinoline-phosphine 302 as a ligand for copper-catalyzed Diels-Alder reactions. Acryloylimide and cyclopentadiene afford high selectivity of the cycloadduct in the presence of a catalytic amount of 302 Cu(OTf)2, Scheme 20. Buono invokes a square-planar metal center to account for the observed selectivity. 

Electrochemistry. Compounds 15a [Fen(pz)pzEts] 15b [Fen(l-MeIm)2Ets] exhibited two reversible oxidations at 0.21 and -0.06 V (vs Ag/AgCl), which were attributed to the 2 —> 3 and 3 —> 4 metal-centered oxidations. These values are 400 mV more positive than those reported for the analogous Fe TPP, suggesting that the pz stabilizes the lower oxidation state, Fe(II) better than does the porphyrin (63). This stabilization of a basic oxidation state for the pz may be attributable to the lower energy n orbitals of the pz making it a better n acceptor than the porphyrin. 

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