Electron Lewis base

Lewis add—any substance that can accept a pair of nonbonding electrons. Lewis base—any substance that can donate a pair of nonbonding electrons. 1 point for correct answer. 

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. 

However, the reaction of a protic acid with transition-metal complexes bearing non-bonding valence electrons (Lewis bases) usually leads to transition-metal hy-... 

Substitution reactions, which occur in all areas of chemistry, are those in which an atom or group of atoms is substituted for another. A Lewis base is an electron pair donor, and a Lewis acid is an electron pair acceptor. Some common Lewis bases are H2O, NH3, OH , F , etc., while some common Lewis acids are AICI3, BCI3, carbocations (RsC ), etc. In a Lewis acid-base reaction, a coordinate bond is formed between the acid and base with the base donating the pair of electrons. Lewis bases are known as nucleophiles and Lewis acids are known as electrophiles. In fact, when A is a... 

A basic site can be described in terms of its ability to accept a proton (Bronsted base) or to donate electrons (Lewis base). In order for base-catalysed reactions to occur, the basicity must be sufficient to stabilise an anionic or polarised species with a significant negative charge that forms part of the catalytic cycle. For a typical base-catalysed C-C bond formation reaction (such as a Michael addition or the Knoevenagel condensation (Scheme 9.18)), the basic site stabilises the intermediate carbanion, which can then act as a nucleophile in the C-C bond formation. Surface sites on MgO are typical strong basic sites. 

We have seen that the first successful conceptualization of acid-base behavior was proposed by Arrhenius. This useful but limited model was replaced by the more general Br0nsted-Lowry model. An even more general model for acid-base behavior was suggested by G. N. Lewis in the early 1920s. A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor. Another way of saying this is that a Lewis acid has an empty atomic orbital that it can use to accept (share) an electron pair from a molecule that has a lone pair of electrons (Lewis base). The three models for acids and bases are summarized in Table 14.10. 

Strategy Determine which species in each reaction accepts a pair of electrons (Lewis acid) and which species donates a pair of electrons (Lewis base). 

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