Lewis base donors

Lewis acid sites Lewis-base donors Lewis bases Lewisite... 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

Figure 5.38 displays the optimized structure and primary npt— ooh donor-acceptor interaction of the complex (5.70a) in which PtH2 serves as the Lewis-base donor. The qualitative similarity to the water dimer structure (Fig. 5.5(b)) is immediately apparent, including the short Pt H distance (2.47 A, more than 0.5 A inside van der Waals contact), the roughly linear Pt H—O angle (161°), and the characteristic elongation of the H-bonded versus free O—H bond of the water monomer (by 0.01 A). The leading nPt a0H donor-acceptor interaction in Fig. 5.38(b) is... 

The effect of /rfive-coordinate aluminum alkyls have been discussed.94 Studies have also focused on the equilibrium between four-coordinate and five-coordinate isomers in compounds of the type [R2Al /i-0(GH2) ER1x ]2 (n = Z, 3 ER1x = OR1, SR1, NRX2) (shown in Figure 7(h) for n = Z). Factors that control this equilibrium and hence the coordination around the aluminum include the steric bulk of the substitutent at the aluminum (R), and the Lewis base donor R1, the basicity of the neutral donor group ER1, and the chelate ring size (as determined by n).95... 

A systemic study of compounds containing O, O N, O and S, O ligand sets has been conducted in which the effect of variables such as the steric bulk of the groups on the metal and on the ligand, strength of the Lewis base donor, and size of the chelate ring formed when the ligand becomes bidentate has... 

A further study on six-membered ylide formation examined the use of an aliphatic ester in place of a ketone as the Lewis base donor for carbonyl ylide formation. Although the same keto-substituted system underwent an intramolecular cyclization readily, the ester derivative gave no cycloaddition products. Padwa and co-workers (37,76) points to the major electronic differences between the two carbonyl groups to rationalize the disparity in carbonyl ylide formation. 

An important chemical measure of solvent polarity is the donor number, DN. It is a measure of the Lewis base donor power of the solvent. DN is defined as the negative enthalpy for the reaction of the solvent with the standard Lewis acid SbCls (eqn. 3.8). The enthalpy measurements are carried out in an inert solvent, 1,2-dichloroethane, which has a donor number of zero. The units are kcal/mol. 

Silylene 59 also behaves somewhat like a phosphine in its interactions with metal carbonyls98,149-376. Typical reactions involve substitution of silylene for CO, to give a silylene-metal complex. Three examples are shown in Scheme 20, and the structure of the nickel complex 75 is displayed in Figure 7149. This complex is both the first silylene-nickel complex, and the first example of a bis-silylene-metal complex free of stabilization by Lewis base donors. 

Complex formation can usually be viewed as a Lewis acid-Lewis base reaction. The Lewis acid (or acceptor) in the present context is an atom or ion M"+ (where M is an atom of a d or f block element and n is its oxidation number) and the Lewis base (donor) is a ligand. Lewis acids and bases can be classified as hard or soft . A hard base is one which donates via an atom (or atoms) of high electronegativity and low polarisability. The following are sequences of increasing hardness ... 

Group 2 metallocenes have the general formula M(Cp)2L , where Cp is a derivatized or underivatized cyclopentadienyl ring (see Table II) and L is a two- or four-electron Lewis base donor such as THF, PR3, Et20, or DME (n = 0-2). Several methods have been used to prepare alkaline-earth metallocene complexes. 

As mentioned earlier, many Lewis base metallocenes have been synthesized and structurally characterized. An examination of their structures shows that they are all predictably bent, more so than their base-free congeners, and have one two-electron, two two-electron or one four-electron Lewis base donor. Ca[l, 3-(SiMe3)2C5H3]2(THF),89 shown in Fig. 17, is an example of a Lewis base monosubstituted Ca metallocene complex, whereas Sr(t-BuC5H4)2(THF)2,88 shown in Fig. 18, is an example of a disubstituted metallocene. Even weak donor base such as alkynes, Me3SiCCCCSiMe3, will bind to the alkaline-earth metal centers (Fig. 19).104 In this case, the weak donor is easily displaced by stronger bases. 

With this type of shift reagent, the silver ion binds to soft Lewis base donors, such as olefins, aromatics, phosphenes, and halogenated compounds. At the same time, silver remains as an ion pair with Yb(fod)4 in a specific orientation such that a paramagnetic shift is induced in the resonances of the organic soft base. This method has proven to be useful for the separation of NMR resonance peaks of cis and trans geometric isomer mixtures of long-chain alkenes and complex mixtures of aromatic molecules [5]. 

The effect of solvation by Lewis base donor solvents is to reduce this complexity. THF, the most widely used solvent for these bases, forms bis-solvated dimeric complexes with most amidohthiums, since this smallest Li2N2 ring provides maximum room for the additional donor molecules. Chelating donors may even produce monomers, but ion separation is found in the sohd state only when the anion is stabihzed by electron-accepting heteroatoms. ... 

Reactions of [Sn( X3-NR)]4. An intriguing aspect of the chemistry of imidotin cubanes lies in their potential to act as multidentate Lewis-base donors to form coordination complexes. While this... 

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