Lewis acid strengths

Mechanistically the rate-determining step is nucleophilic attack involving the hydroxide ion and the more positive siUcon atom in the Si—H bond. This attack has been related to the Lewis acid strength of the corresponding silane, ie, to the abiUty to act as an acceptor for a given attacking base. Similar inductive and steric effects apply for acid hydrolysis of organosilanes (106). 

At RT, NH3 adsorbed on Lewis acid sites, Zr and V. Accordingly, the intensity of bands from NH3 decreased little with the V-content, by 15% at most, as expected on account of the similar Lewis acid strengths of Zr V and VV. The symmetric bending... 

Lewis acid strength of B(C6F5)3 is between BF3 and BCI3 which means that the electron-withdrawing effect of the C Fs group should be comparable with that of these two halogens [20]. 

Lewis acid strength and hardness of the lithium cation. Both LiBH4 and Ca(BH4)2 are more reactive than sodium borohydride. This enhanced reactivity is due to the greater Lewis acid strength of Li+ and Ca2+, compared with Na+. Both of these reagents can reduce esters and lactones efficiently. 

MR donor bonds, is o-bond metathesis. Alternatively, as the Lewis-acid strength of M increases, the tendency toward agostic or bridging interactions can finally result in H—H bond scission and formal migration of hydride to the metal atom,... 

The H-bonded species in Fig. 5.5 exhibit smooth variations of H-bond length and strength with respect to position in the periodic table, in a manner consistent with the expected ordering (5.36b) of Lewis-acid strengths. In each case the inter-molecular interaction is dominated by the no->aAH donor-acceptor stabilization shown in the right-hand panel. Energetic, geometrical, and vibrational properties of... 

Martra and coworkers studied low-temperature CO and room-temperature CO2 adsorphon on Na- and Ba-exchanged X and Y zeolites [138]. They were able to determine from the CO adsorphon IR data that Ba ions located in the Sn sites in both X and Y zeolites exhibit higher Lewis acid strength compared to Na ions in the same sites. Furthermore, the Lewis acid shength of both Ba and Na ions was dependent on the framework aluminum content of the zeolite. Using CO2 as a probe, they found that subshtuhng Na with Ba caused a reduchon in the basicity of the framework oxygens in proximity to the cahons. 

Coordination causes electron-spin density redistribution in the N-O fragment the contribution of resonance structure II increase. The redistribution of spin density results in changes in the parallel component of the nitrogen hyperfine tensor. TEMPO and anthraquinone (AQ) have been used in this way to probe the Lewis acidity of alumina and Li and Mg doped alumina matrices.176 The differences in the Lewis acidic strength towards TEMPO and anthraquinone are discussed. An interesting study has appeared aimed to study the guest-host interaction between poly(amidoamine) dendrimers labelled with nitroxides and several porous solids including alumina.177... 

Metal complexes with fluoro-/3-diketones have been comprehensively reviewed.1585 The introduction of electron-withdrawing groups in the chelate ring increases the Lewis acidity strength of the ML2 complexes, and consequently the bis adducts of the fluoro-/3-diketonato complexes are more stable than the corresponding complexes with /3-diketones. As an example of a nickel complex with 1,1,1-trifluoroacetylacetone which does not have a counterpart in the nickel acetylacetonate complexes we can mention the hexanuclear complex Ni6Lio(OH)2(H20)2.1586... 

Regardless, it is important to keep in mind that superacidity encompasses both Brpnsted and Lewis acid systems and their conjugate acids. The qualitative picture of Lewis acid strengths will be discussed in Section 1.4.7. 

The Lewis base chemistry of the B(ArF)3 derivatives III-VI is not nearly as diverse as that of the parent B(C6F5)3, but some chemistry with simple donors has been reported, mainly in the context of comparative Lewis acid strength experiments. Competition... 

From the above results, the surface structure appears to be S04 combined with Zr elements in the bridging bidentated state, as Okazaki et al. proposed in the case of titanium oxide with sulfate ion (155, 156). The double-bond nature of the complex is much stronger compared with that of a simple metal sulfate thus, the Lewis acid strength of Zr4+ becomes remarkably stronger by the inductive effect of S = O in the complex, as illustrated by arrows in the previous scheme. If water molecules are present, the Lewis acid sites are converted to Bronsted acid sites (129, 151, 157). 

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