Lewis acid/base complex

Important general questions are raised by Lewis acid-base concepts (see W.B. Jensen, The Lewis Acid-Base Concepts An Overview WUey-Interscience, New York, 1980), such as  

In this section, we first address such questions in the framework of main-group chemistry, focusing on simple prototype species (A Cls, BH2NH2) that exhibit interesting intramolecular aspects of the covalent-coordinate dichotomy (Sections 8.1.1 and 8.1.2). We then briefly describe the analogous intermolecular aspects of coordinative bonding in the classic Bp3 NH3 adduct (Section 8.1.3). 

Let us first consider the simple example of aluminum chloride (AICI3), a prototype Lewis acid. The leading symmetry-unique valence NBOs of this trigonal species ( Aici = 2.0835 A) are shown in abridged form in 1/0-8.1. These include the ctaici (BD-type) polar covalent bond between Al(l) and Cl(2) (NBO 1), the three ci (LP-type) lone pairs on Cl(2) (NBOs 24—26), and the ai (LP -type) vacancy on Al(l) (NBO 33), as illustrated in Fig. 8.1. 

Each arrow represents a directed two-electron ai donor—acceptor interaction from the filled lone pair of Cl into the unfilled ai hole of Al. Such a Lewis acid-base interaction is called a coordinate covalent or dative bond, and is often symbolized by a double-dot (A1 C1) or directed arrow (Al —Cl) in the Lewis structure diagram, to distinguish it from an ordinary covalent (Al—Cl) bond-stoke. The distinctive coordinative A1 C1 bond will be written as cfAtcr to distinguish it from the covalent caici bond of Fig. 8.1a. 

More generally, we may envision an ordinary covalent ctab bond as forming from two singly occupied bonding hybrids h, hs  

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Neutral compounds such as boron trifluoride and aluminum chloride form Lewis acid-base complexes by accepting an electron pair from the donor molecule. The same functional groups that act as lone-pair donors to metal cations can form complexes with boron trifluoride, aluminum chloride, and related compounds. 

Closely related to, but distinct from, the anionic boron and aluminum hydrides are the neutral boron (borane, BH3) and aluminum (alane, A1H3) hydrides. These molecules also contain hydrogen that can be transferred as hydride. Borane and alane differ from the anionic hydrides in being electrophilic species by virtue of the vacant p orbital and are Lewis acids. Reduction by these molecules occurs by an intramolecular hydride transfer in a Lewis acid-base complex of the reactant and reductant. 

Reduction of ketones to triphenylsilyl ethers is effected by the unique Lewis acid perfluorotriphenylborane. Mechanistic and kinetic studies have provided considerable insight into the mechanism of this reaction.186 The salient conclusion is that the hydride is delivered from a borohydride ion, not directly from the silane. Although the borane forms a Lewis acid-base complex with the ketone, its key function is in delivery of the hydride. 

When silylenes are generated photochemically in hydrocarbon matrices in the presence of electron-pair donors, they may form Lewis acid-base complexes that act as intermediates in the silylene dimerization to disilenes.3233 In a typical example, Mes2Si(SiMe3)2 was photolyzed in 3-meth-ylpentane (3-MP) matrix containing 5% of 2-methyltetrahydrofuran. At 77 K, dimesitylsilylene (Amax 577 nm) was formed. When the matrix was... 

Ammonia reacts with boron trichloride to form a molecule called an adduct or Lewis acid base complex in which the lone pair on the ammonia molecule is shared with the boron atom to form a covalent bond and completing an octet on boron (Figure 1.16) ... 

We should note that the formation of this bond confers formal charges on the B and N atoms. In this bond and many similar Lewis acid-base complexes both the electrons forming the bond come from the same atom rather than from different atoms, as in the formation of a bond between two chlorine atoms. This type of bond is often called a donor-acceptor bond, a dative bond, or a coordinate bond, and is sometimes given a special symbol—an arrow denoting the direction in which the electron pair is donated ... 

Figure 1.16 The ammonia-boron trifluoride donor-acceptor complex (a) donor Lewis base, (b) acceptor Lewis acid, (c) the donor-acceptor or Lewis acid-base complex.

Neutral benzol,3,2-diazaphospholes or their tetrameric cycloaddition products react with hard Lewis acids to give N-coordinated Lewis acid-base complexes [13, 80, 81] this reaction can be used to disassemble the otherwise stable oligomers into monomeric units at ambient temperature. 

The effects of Lewis acids on the stereoselectivities can also be understood in terms of orbital interactions. The variation in charge at the respective basic centre gives rise to a change in the magnitude of the orbital coefficients of the entire interacting molecular orbital. These effects are visualized by the HOMO and LUMO representations of the Lewis acid-base complex of acrolein and trifluoroborane (Figure 3), and in an even more extreme case by the HOMO and LUMO representations of one of the simplest dienophile-Lewis acid complexes protonated acrolein92,93. 

In summary, the mechanism of reduction of ketones with TIBA may be formulated as involving (a) fast Lewis acid-base complexation between the reactants, and (b) slow hydride transfer (Scheme 19). 

Scheme I. Examples of tetraphosphorus- or arsenic-based molecules with the adamantane-type core A4B6. Lewis acid base complexes are not shown. Key to superscripts (when no reference is given, consult Ref. 1-6) A unique non-ada-mantane P4(NR) c oso-structure has recently been obtained with R = i-CjH- (1) and has been shown to convert into the thermodynamically more stable adamantane-type isomer upon heating as in P SglS), P4Sg, and P4S10 Ref. 9 in P4S N- (101,- Hn[P404(CH2)2j04in) orAs40(CH2)2 (121,- 05 in (P406)04- S (13) and [P4(NMe)6l04. S (14).

The 1,4-dibenzylium dication (111) could not be formed in the superacidic media. The ionization of the 2,3,5,6-tetramethyl-l,4-bis(chloromethyl)benzene (109) in SbF5/S02ClF at -78°C gave only the monocation Lewis acid-base complex (110). Thus, the observed NMR chemical shifts of the positively charged benzylic carbon Cl and the CH2CI carbon are deshielded as expected for the monocation/ Lewis acid-base complex (5 C 170.2 and 82.9, respectively). 

It was also of interest to generate the related symmetric primary benzylic trication. However, the ionization of the 2,4,6-tris(chloromethyl)mesitylene (112) in excess SbF5/S02ClF at -78°C gave apparently only the dienylic allylic dication Lewis acid-base complex (113). The unionized chloromethyl carbon displayed a relatively deshielded NMR absorption, 35.3, indicative of a weak Lewis acid-base interaction. The terminal methylene carbons of the dienylic system showed a 8 C of 197.7 and the terminal carbons of the allylic system displayed a 8 C of 194.8, quite similar to that of the previously described dienylic allylic dications, 107 and 108. 

The Lewis acid/base complex is formed via an overlap between a doubly occupied orbital of the donor D and vacant orbital of the acceptor A. This acid/base approach was extended by Pearson who divided Lewis acids and bases into two groups, hard and soft, according to their electronegativity and polarizability (principle of hard and soft acids and bases (HSAB concept). Hard acids (e.g., H, Lf, Na, BF3, AICI3, hydrogen-bond donors HX) and hard bases (e.g., F", CL, HO, RO, H2O, ROH, R2O,... 

Acyl cations are now well-established chemical species. They can be prepared in quantity in solution, and several have been isolated as their crystalline salts. Recent papers have described their formation from carboxylic acids and esters under strongly acidic conditions81214, but they are most conveniently available from the reactions of acyl halides with L.ewis acids21 well-defined Lewis acid-base complexes are formed, which decompose in a second stage to the acyl cations21-23, viz. 

Maciel et al. (370,371,374) combined l3C and 15N CP/MAS NMR to study the adsorption of pyridine on silica-alumina. Hydrogen bonding was found to be the dominant interaction at high loading levels (0.5 to 1 monolayer). At lower coverages, a Lewis acid-base complex dominates and the pyridine is significantly less mobile. Bronsted complexes are found when the surface has been pretreated with HC1 gas. 

The BF3 molecule is the Lewis acid, the F ion is the Lewis base, and the BF4 ion is the Lewis acid-base complex. The bond formed in a Lewis acid-base reaction is a coordinate covalent bond. 

A Lewis acid is an electron pair acceptor a Lewis base is an electron pair donor they react to form a Lewis acid-base complex by forming a coordinate covalent bond. 

Ketones form more stable Lewis acid-base complexes with electrophilic metal salts than do aldehydes as a result of the increased basiciLy of the carbonyl oxygen atom thanks to the +1 effect of the ketone s alkyl groups. 

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