Lewis acid-base adduct, formation

The unique reactivity of the above system with H2 appears to arise from the unquenched Lewis basicity and acidity of the respective donor P and the acceptor B centers. This inference prompted questions about the nature and reactivity of other phosphine-borane systems and, more broadly, of Lewis acid/base combinations. Is it necessary to have a link between the donor and acceptor sites Could similar H2 activation arise from combinations of donors and acceptors in which steric encumbrance frustrates Lewis acid-base adduct formation If indeed such frustrated Lewis pairs could be uncovered, could one exploit them for the activation of small molecules and applications in catalysis ... 

The impressive sulfur-based reactivity of square planar nickel complexes containing tetradentate N2S2 ligands has been known for many years. Interest has recently resurfaced because of the discovery of similar donor sites in metalloproteins that bind nickel, iron, and cobalt.The iV,iV -bis(mercaptoethyl)-l,5-diazacyclooc-tane ligand H2(BME-D ACO) and its nickel complex have been particularly useful in establishing the scope of S-based reactivity with electrophiles as displayed in the reaction summary shown in Scheme 1." The fundamental features of this reactivity include templated macrocycle production, S-oxygenation as contrasted to oxidation, Lewis acid/base adduct formation, metal-ion capture, and the synthesis of heterodi- and polymetallic complexes. ... 

Although surface organometallic chemistry is still in its infancy, there are already several examples of surface reactions leading to well-defined surface complexes (Table l-I). It appears that these reactions obey the same principles as those encountered in molecular chemistry nucleophilic attack at the ligands, electrophilic attack of the metal-carbon bond, oxidative addition, Lewis acid-base adduct formation, redox reactions, disproportionation, and the cooperative effect of dual acid-base sites in an insertion reaction. 

The reaction between a Lewis acid R3M and a Lewis base ER3, usually resulting in the formation of a Lewis acid-base adduct R3M—ER3, is of fundamental interest in main group chemistry. Numerous experiments, in particular reactions of alane and gallane MH3 with amines and phosphines ER3, have been performed [14]. Several general coordination modes, as summarized in Fig. 2, have been identified by X-ray diffraction. 

These particular properties of chloroalanes favor the formation of simple Lewis acid-base adducts, as was observed for the reaction of R2AICI with Sb(Tms)3 (R = Et, f-Bu). In contrast, reactions of the analogous gallanes and indanes yielded the desired heterocycles. The same tendencies were observed in reactions of R2MCI (M = Al, Ga, In R = Et, i-Bu) with P(Tms)3 and As(Tms)3. The gallane and indane react under formation of the expected M—E heterocycles [71], while the corresponding alanes yield the simple adducts... 

A general equation summarizes the formation of Lewis acid-base adducts ... 

C21-0030. The reaction between CO2 and H2 O to form carbonic acid (H2 CO3) can be described in two steps formation of a Lewis acid-base adduct followed by Brcjmsted proton transfer. Draw Lewis structures illustrating these two steps, showing electron and proton movement by curved arrows. 

The coordination of the phosphine P(ft-Pr)2Ph to the Lewis acidic Ga center is essential for the synthesis of both compounds. In the absence of any Lewis base, the most likely reaction product would be the heterocubane [ClGaSbSi(/-Pr)3]4. However, in analogy to the results observed for reactions of heterocycles [R MER with Lewis bases, leading to base-stabilized monomeric compounds, both the formation of 84 and 85 can be explained by reaction of such a heterocubane intermediate with the phosphine base. According to the description of heterocycles as head-to-tail adducts, heterocubanes may be described as Lewis acid-base adducts between two four-membered rings as shown in Fig. 45. 

Nebenvalenz) to describe the chemical forces underlying formation of inner complexes, and G. N. Lewis s general concept of the Lewis-acid-base adduct allowed many types of coordinate bonding to be recognized as simple extensions of Lewis-like covalent concepts. 

At present, the correlation contains one transition metal complex, Cu(Hfacac)2. The results on this complex are very interesting and somewhat unusual for a transition metal system in that enthalpies have been obtained in a poorly solvating solvent with nonionic donors (52), instead of the t5 ical stability constant study on a metal cation in some highly polar solvent. Data from this latter type of investigation have many practical uses, but are impossible to interpret and understand. The transition metal ion complex we have studied can be incorporated into the E and C scheme using the same base parameters that are used to correlate the enthalpies of formation of all the other Lewis acid-base adducts in the scheme. 

The problem is that a substance HA undergoing a reaction in which it behaves as a Bransted acid is not behaving as an acid in the Lewis sense it is, however, behaving as a Lewis acid-base adduct. The issue is further clouded by the fact that HA can enter into reactions in which it does behave as a Lewis acid, as for example in the formation of a hydrogen bond (Equation 3.61) in this case it is not, however, behaving as a Bransted acid, since the proton remains... 

When iodosylbenzene is treated with boron trifluoride etherate, it is both depolymer-ized and rendered more electrophilic, presumably because of the formation of a Lewis acid-base adduct (equation 4). 

Upon reaction with basic transition metal complexes the general tendency of electronically unsaturated boranes to form Lewis acid-base adducts should result in the formation of borane complexes (I). [CpzWILfBFs)] (1), which is known from many textbooks represents the best known example for such a compound. Recently, however, the proposed constitution of 1 was disproved, and it was shown that reactions of [Cp2WH2] with boranes yield salt like compounds 2 and zwitterionic species 3, respectively. For the formation of 2 one equivalent of HF is required, and in this reaction the borane acts as a fluoride source, while the WH2 moiety supplies a proton (Figure 3). Until now no structurally authentic borane complex was reported. 

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