Affinity Towards Lewis Bases

Arhguie et al. [ 140] reported the synthesis of few uranium(III) thiolato complexes of the type Na[Cp 2U(SR)2] (where Cp = ri -CsMes R = Ph, Me, i-Pr) by the reductive reaction of their corresponding uranium(IV) bis thiolate complexes with sodiiun amalgam. It is worth mentioning here that 

The complexes [(C5H4-f-Bu)3UL] (L = pyridine, picoline) and [(C5H4SiMe3)3UL] (L = pyridine, lutidine, pyrimidine, and dimethylpyrazine) have been characterized by single crystal X-ray crystallography (Table 4). All the mononuclear complexes were found in the familiar pseudotetrahedral arrangement of the three ri -cyclopentadienyl ligands and the coordinated Lewis base. 

Complexes Bond distance Bond angle Refs. 

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Table 1.19 Orders of affinity of Lewis bases towards the reference acids CHCI3 and I2 given by the E and C plot. Comparison with experimental affinities (kj mol ).

Thionyl chloride reacts as both a Lewis acid and a Lewis base, and both the S and Se compounds are very reactive toward many other materials. The hydrolysis reactions take place readily, and SOCl2 has such an affinity for water that is used as a dehydrating agent. 

Nucleophilicity. A distinction is usually made between nucleophilicity and Lowry-Bronsted basicity [213]. The latter involves specifically reaction at a proton which is complexed to a Lewis base (usually H2O), while the former refers to reactivity at centers other than H. Linear correlations have been shown for gas-phase basicity (proton affinity) and nucleophilicity of nitrogen bases toward CH3I in solution [214] where the solvent is not strongly involved in charge dispersal. In each case, reaction of the base/nucleophile... 

In view of this affinity of S in S03 for electrons, it is not surprising that S03 functions as a fairly strong Lewis add toward the bases that it does not preferentially oxidize. Thus the trioxide gives crystalline complexes with pyridine, trimethylamine, or dioxane, which can be used, like S03 itself, as sulfonating agents for organic compounds. 

The Lewis acid BF3 is the most commonly employed reference to probe Lewis basicity. The affinity of BF3 towards many bases has been measured in dichloromethane solntion where the affinity is defined as the magnitude of the enthalpy change of adduct formation ... 

Ultimately, the affinity or the basicity scales themselves might be computed. This has been done successfully for the methanol affinity in the gas phase [41]. However, the MP2/aug-cc-pVTZ//B3LYP/6-3H-G(d,p) costly level required to obtain good agreement with experimental affinities has limited the scale to a few small Lewis bases. Proton and cation affinities and basicities in the gas phase are now computed on a routine basis (see Chapter 6). Nevertheless, the size of Lewis bases and cations and the number of bases studied are inversely proportional to the level of theory, that is, to the agreement with experimental data. As far as extended solution basicity scales towards usual Lewis acids... 

In the context of the BF3 affinity scale, a further interesting structural feature is the importance of steric effects in determining the structure of BF3 complexes. For example, the complexes of benzaldehydes with BF3 [27, 35] have the phenyl group anti to the BF3. In other words, the B—O bond points towards the less hindered sp lone pair of the carbonyl oxygen. In three BFs-ketimine complexes [36], the BF3 group orients towards the less sterically demanding side of the imine function. The steric strain between BF3 and Lewis bases has been explained by orbital interaction theory [37]. 

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