Coordinate covalent bond, with acids

The sodium and calcium salts of EDTA (ethylenediaminetetraacetic acid, Fig. 9.3.1.) are common sequestrants in food products. A three-dimensional representation of EDTA is shown in color Fig. 9.3.2. The EDTA ion is an especially effective sequestrant, forming up to six coordinate covalent bonds with a metal ion. These bonds are so named because a lone pair of electrons on a single atom serves as the source of the shared electrons in the bond between the metal ion and EDTA. The two nitrogen atoms in the amino groups and the oxygen... 

In the same year that Bronsted and Lowry proposed their definition of acids and bases, an American chemist named Gilbert Lewis proposed an alternative definition that not only encompassed Bronsted-Lowry theory but also accounted for acid-base reactions in which a hydrogen ion isn t exchanged. Lewis s definition relies on tracking lone pairs of electrons. Under his theory, a base is any substance that donates a pair of electrons to form a coordinate covalent bond with another substance, while an acid is a substance that accepts that electron pair in such a reaction. As we explain in Chapter 5, a coordinate covalent bond is a covalent bond in which both of the bonding electrons are donated by one of the atoms forming the bond. 

That being the case, it is appealing to refer to anion receptors as ligands. However, traditionally the commonly understood definition of ligand has been a Lewis base capable of forming a coordinate-covalent bond with a metal ion. Since for anions, the ligand actually behaves as a Lewis acid, the definition needs to be expanded to include both Lewis acid and Lewis base behavior. 

Elements with very low electronegativity (alkali metals, alkaline earth metals, such as Na, Ca and Mg) and elements with high electronegativity (halogens such as Cl and I) occur mainly as free ions in biological materials, and are preferably involved in electrostatic interactions. However, even these elements can form less soluble compounds (calcium oxalate), covalent compounds (hormones thyroxine and triiodothyronine are iodinated aromatic amino acids, see Section 2.2.1.2.5) or complex compounds (chlorides as Hgands and some metal ions as central atoms). A ligand is an entity (atom, ion or molecule), which can act as an electron pair acceptor to create a coordinate covalent bond with the central ion. Cd and Hg also tend to form covalent compounds. 

Bases (NH3, C5H5N, C2H5OH, 1,4-dioxane). The basic characteristics of these compoimds can be demonstrated by the addition of a few drops of each to the four yellow solutions obtained in the above experiment. In each solution the color is changed back from yellow to violet on the addition of the base. These compounds are bases because they are electron-pair donors and can form coordinate covalent bonds with the protons of the above-mentioned acids. 

By these definitions, OH , a Bronsted-Lowry base, is also a Lewis base because lone-pair electrons are present on the O atom. So too is NH3 a Lewis base. HCl, conversely, is not a Lewis acid It is not an electron-pair acceptor. We can think of HCl as producing H, however, and H is a Lewis acid. H forms a coordinate covalent bond with an available electron pair. 

Species with an incomplete valence shell are Lewis acids. When the Lewis acid forms a coordinate covalent bond with a Lewis base, the octet is completed. A good example of octet completion is the reaction of BF3 and NH3. 

In the hydrated metal ion, the OH bond in a water molecule becomes weakened. This happens because, in forming the coordinate covalent bond with the O atom of the water, the metal ion causes electron density to be drawn toward it hence, electron density is drawn away from the OH bond. As a consequence, the coordinated H2O molecule can donate a H" to a solvent H2O molecule (Fig. 16-12). The H2O molecule that has ionized is converted to OH, which remains attached to the Al the charge on the complex ion is reduced from 3-F to 2-F. The extent of ionization of [A1(H20)6] , measured by its value and as pictured in Figure 16-13, is essentially the same as that of acetic acid (iCa = 1.8 X 10 ). Many other metal ions hydrolyze, especially the transition metal ions. These and other hydrated metal ions acting as acids are discussed in later chapters. 

HC1 is the acid, because it is donating an H+ and the H+ will accept an electron pair from ammonia. Ammonia is the base, accepting the H+ and furnishing an electron pair that the H+ will bond with via coordinate covalent bonding. Coordinate covalent bonds are covalent bonds in which one of the atoms furnishes both of the electrons for the bond. After the bond is formed, it is identical to a covalent bond formed by donation of one electron by both of the bonding atoms. 

Base In chemistry, the nonadd part of a salt a substance that combines with adds to form salts a substance that dissociates to give hydroxide ions in aqueous solutions a substance whose molecule or ion can combine with a proton (hydrogen ion) a substance capable of donating a pair of electrons (to an acid) for the formation of a coordinate covalent bond. [EU]... 

LEWIS BASK, a substance that forms a covalent hond by donating a pair of electrons, neutralization resulting from reaction between the base and the acid with lonnaliun of a coordinate covalent bond. It is also called a nucleophile. 

The solubility of an ionic compound increases dramatically if the solution contains a Lewis base that can form a coordinate covalent bond (Section 7.5) to the metal cation. Silver chloride, for example, is insoluble in water and in acid, but it dissolves in an excess of aqueous ammonia, forming the complex ion Ag(NH3)2 + (Figure 16.13). A complex ion is an ion that contains a metal cation bonded to one or more small molecules or ions, such as NH3, CN-, or OH-. In accord with Le Chatelier s principle, ammonia shifts the solubility equilibrium to the right by tying up the Ag+ ion in the form of the complex ion ... 

When atoms possess an incomplete outer shell (e.g., nonpaired electrons), yet their net charge is zero, attraction between such atoms takes place because of their strong tendency to complete their outer electron orbital shell by sharing their unpaired electrons. This gives rise to a covalent bond. One example of a covalent bond is the bimolecular chlorine gas (Cl2) (Fig. 1.1). Covalent bonding is a characteristic of some nonmetals or metalloids (bimolecular molecules), but may also arise between any two atoms when one of the atoms shares its outer-shell electron pair (Lewis base) with a second atom that has an empty outer shell (Lewis acid). Such bonds are known as coordinated covalent bonds or polar covalent bonds. They are commonly weaker than the covalent bond of two atoms which share each other s unpaired outer-shell electrons (e.g., F2 and 02). Coordinated covalent bonds often involve organometallic complexes. 

The use of transition metals or transition metal clusters to act as nodes for the modular self-assembly of diamondoid networks that are sustained by coordinate covalent bonds is also well established. Such architectures are of more than aesthetic appeal. Indeed, such structures have resulted in a class of compound with very interesting bulk and functional properties. Metal-organic diamondoid structures in which the spacer moiety has no center of inversion are predisposed to generate polar networks since there would not be any inherent center of inversion. Pyridine-4-carboxylic acid is such a ligand and bis(isonicotinato)zinc exists as a three-fold diamondoid structure that is thermally stable and inherently polar.33... 

The interaction between a metal ion and a ligand can be viewed as a Lewis acid-base reaction, with the ligand donating a lone pair of electrons to an empty orbital on the metal ion to form a coordinate covalent bond ... 

The Lewis concept of acids and bases (G. N. Lewis, 1923) interprets the combination of acids with bases in terms of the formation of a coordinate covalent bond. A Lewis acid can accept and share a lone pair of electPDns donated by a Lewis base. Because protons readily attach themselves to lone electron pairs, Lewis bases are also Biyinsted bases. Lewis acids, however, include a large number of substances in addition to proton donors for examjjle, metal ions, acidic oxides, or atoms. 

Here, the Lewis base CaO donates an electron pair (one of the lone pairs of the oxygen atom) to the Lewis acid (CO2) to form a coordinate covalent bond in the CO ion. Similar Lewis acid-base reactions can be written for other acid-base anhydride pairs. Sulfur trioxide, for example, reacts with metal oxides to form sulfates ... 

B is correct. In a coordinate covalent bond, one atom donates an electron pair to share with another atom. In this case, ammonia has the unbonded pair to donate to boron, so ammonia is the Lewis base and boron is the Lewis acid. 

Since Cu ions on the zeolite surface exist in an isolated environment, they may interact with the sulfate species on the catalysts deactivated by SOj. The sulfate groups might partially surround the copper ions, as previously supested by Choi et al. [37] and Hamada et al. [38]. The sulfate may also have some characteristics of a coordinate covdent bond, where Cu ions and sulfate species may act as a Lewis acid and base, respectively [39], Ligands such as HjO, NHj, (C2Hj)3P, CO molecules and Cf, CN, OH , NOj, and C204 ions should at least contain a lone pair of electron to form a coordinate covalent bond between metal ions [40]. It should be noted that the sulfate catalyst species formed on the catalysts deactivated by SO2 contains lone electron pairs on O atoms which surround S atom of SO4 groups. Therefore, it is expected that the electrostatic interaction between Cu ions and sulfate species probably influences the local structure of Cu ions on the zeolite catalyst surface. 

He determined the number of moles of AgCl produced. This told him the number of CH ions precipitated per formula unit. The results are in the second column. Werner reasoned that the precipitated Cl ions must be free (uncoordinated), whereas the unprecipitated CH ions must be bonded to Pt so they could not be precipitated by Ag ions. He also measured the conductances of solutions of these compounds of known concentrations. By comparing these with data on solutions of simple electrolytes, he found the number of ions per formula unit. The results are shown in the third column. Piecing the evidence together, he concluded that the correct formulas are the ones listed in the last two columns. The NH3 and Cl within the brackets are bonded by coordinate covalent bonds to the Lewis acid, Pt(TV) ion. 

Coordinate covalent bonds involve the unequal sharing of an electron pair by two atoms, with both electrons (originally) coming from the same atom. The electron pair donor is the ligand, or Lewis base, whereas the acceptor is the central atom (because it frequently can accept more than one pair of electrons), or Lewis acid. These bonds are important in all interactions between transition metals and organic ligands (e.g., Fe + in hemoglobin and the cytochromes). 

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