Electron pair acceptors, metals

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Quantitative Calculations The stoichiometry of complexation reactions is given by the conservation of electron pairs between the ligand, which is an electron-pair donor, and the metal, which is an electron-pair acceptor (see Section 2C) thus... 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

Electrophilic catalysis is catalysis by an electrophile (Lewis acid) acting as an electron-pair acceptor. For example, metal ions catalyze the decarboxylation of dimethyloxaloacetic acid. ... 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

Neutral transition-metal complexes that are not fully coordinatively saturated possess nucleophile metal centers capable of coordinating to electrophiles. On the other hand, group-IIIB halides serve as typical electron-pair acceptors and are, therefore, able to interact coordinatively with basic metal complexes. 

However, with soft electron pair acceptors such as Pt2+, Ag+, and Ir+, phosphines are stronger Lewis bases than are NH3 and amines, so phosphines and arsines interact better with class B metals than do amines. Generally, phosphines and arsines form stable complexes with second- and third-row transition metals in low oxidation states. 

Coordination compounds are also known as coordination complexes, complex compounds, or simply complexes. The essential feature of coordination compounds is that coordinate bonds form between electron pair donors, known as the ligands, and electron pair acceptors, the metal atoms or ions. The number of electron pairs donated to the metal is known as its coordination number. Although many complexes exist in which the coordination numbers are 3, 5, 7, or 8, the majority of complexes exhibit coordination numbers of 2, 4, or 6. 

In 1923 the American chemist G.N. Lewis provided a broad definition of acids and bases, which covered acid-base reactions not involving the traditional proton transfer an acid is an electron-pair acceptor (Lewis acid) and a base is an electron-pair donor (Lewis base). The concept was extended to metal-ligand interactions with the ligand acting as donor, or Lewis base, and the metal ion as acceptor, or Lewis acid. 

In reactions involving coordination compounds, the metal acts as the Lewis acid (electron-pair acceptor), while the ligand acts as a Lewis base (electron-pair donor). In the reaction above, the ammonia ligand displaced the water ligand from the chromium complex because nitrogen is a better electron-pair donor (less electronegative) than oxygen. 

Reactions of the ligand which are very slow in the absence of an electron pair acceptor or metallic coordination center. Basic hydrolysis of amino acid esters is an example of such a reaction. 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

The solubility product is the equilibrium constant for the dissolution of a solid salt into its constituent ions in aqueous solution. The common ion effect is the observation that, if one of the ions of that salt is already present in the solution, the solubility of a salt is decreased. Sometimes, we can selectively precipitate one ion from a solution containing other ions by adding a suitable counterion. At high concentration of ligand, a precipitated metal ion may redissolve by forming soluble complex ions. In a metal-ion complex, the metal is a Lewis acid (electron pair acceptor) and the ligand is a Lewis base (electron pair donor). 

Metal ions can act as electron-pair acceptors, reacting with electron donors to form coordination compounds or complexes. The electron donor species, called the ligand, must have at least one pair of unshared electrons with which to form the bond. Chelates are a special class of coordination compound which results from the reaction of the metal ion with a ligand that contains two or more donor groups. 

Many oxides of nonmetals are gaseous molecular compounds, such as C02, NO, and S03. Most can act as Lewis acids, because the electronegative oxygen atoms withdraw electrons from the central atom, enabling it to act as an electron-pair acceptor. For instance, carbon dioxide can react with the oxides of metals because the oxide ion in the metallic oxide is a strong Lewis base ... 

The molecules or ions that surround the central metal ion in a coordination compound are called ligands, and the atoms that are attached directly to the metal are called ligand donor atoms. In cisplatin, for example, the ligands are NH3 and Cl-, and the ligand donor atoms are N and Cl. The formation of a coordination compound is a Lewis acid-base interaction (Section 15.16) in which the ligands act as Lewis bases (electron-pair donors) and the central metal ion behaves as a Lewis acid (an electron-pair acceptor). 

In the last section we focused on the anions of the dissolved solutes, but in this section we will look at the cations. Metal ions can act as Lewis acids—which, as you recall, means they can be electron-pair acceptors when they are in the presence of Lewis bases (electron pair donors). One of the more common Lewis bases to interact with metal ions in this way is ammonia. These interactions are most common among the transition metals. One example of such an interaction is that of the silver ion and ammonia. Silver chloride is not very soluble in water (the Ks is 1.77 X 10-10) but is quite soluble if ammonia is added to the solution. The phenomenon can be understood by looking at the following equations ... 

In the first two of these reactions, BC13 behaves as a Lewis acid because the boron atom has only three pairs of electrons surrounding it in the BC13 molecule so it functions as an electron pair acceptor. Typical of most ions of transition metals, Cu2+ readily accepts electron pairs from NH3 molecules. 

Trihalides of the Group VA elements are pyramidal (C3v) with an unshared pair of electrons on the central atom. Typically, the molecules are Lewis bases, and they form acid-base adducts and metal complexes. In accord with the hard-soft interaction principle, these species are better electron pair donors toward soft electron pair acceptors. Therefore, most of the complexes of these EX3 molecules contain second and third row transition metals or first row metals in low oxidation states. 

In Chapter 5, Lewis bases (electron pair donors) were classified as nucleophiles and electron pair acceptors were designated as electrophiles or Lewis acids. These concepts will now be used to describe coordination complexes of metals. 

Figure 5.14 Molecular structure of the metal-phthalocyanine (M-Pc) complex. The central metal atom (M) can be a transition metal (i.e., Cu, Fe, Ni) or a heavy metal (Pb). The metal atom can act as an Lewis acid (electron pair acceptor) and interact with electron donors, whereas the extended aromatic ring structures on the periphery of the complex can interact with electronegative species (electron acceptors).

Triphenylstibine Phs Sb and, less freqnently, trialkylstibines or bi- and tridentate stibine ligands, for example, CH2(SbPh2)2 and CH3C(CH2SbPh2)3 are nsed as donor ligands in transition metal complexes. (Cp3)3Sb is exceptional becanse it does not act as a donor but as an electron pair acceptor in the reaction with a carbene (eqnation 5). ... 

When BFj is used as the electron pair acceptor acid trans-fluoro carbyne complexes are not formed, but a BF4 group enters at the trans position at the metal, instead . 

Finally, enzymes that bind metal cofactors such as Zn + and Mg + can use their properties as Lewis acids, for example, electron pair acceptors. An example is the enzyme thermolysin, whose mechanism is illustrated in Fig. 9. In this enzyme, glutamate-143 acts as an active site base to deprotonate water for attack on the amide carbonyl, which is at the same time polarized by coordination by an active site Zn + ion (6). The protonated glutamic acid then probably acts as an acidic group for the protonation of the departing amine. 

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