Complex ions Lewis bonding

Valence bond (VB) theory, which helps explain bonding and structure in main-group compounds (Section 11.1), is also used to describe bonding in complex ions. In the formation of a complex ion, the filled ligand orbital overlaps the empty metal-ion orbital. The ligand (Lewis base) donates the electron pair, and the metal ion (Lewis acid) accepts it to form one of the covalent bonds of the complex ion (Lewis adduct) (Section 18.8). Such a bond, in which one atom in the bond contributes both electrons, is called a coordinate covalent bond, although, once formed, it is identical to any covalent single bond. Recall that the... 

When a complex ion is formed from a simple cation, the electron pairs required for bond formation come solely from the ligands. Reactions such as these, in which one species donates an electron pair to another, are referred to as Lewis acid-base reactions. In particular—... 

When a salt is dissolved in water, the metal ions, especially transition metal ions, form a complex ion with water molecules and/or other species. A complex ion is composed of a metal ion bonded to two or more molecules or ions called ligands. These are Lewis acid-base reactions. For example, suppose Cr(N03)3 is dissolved in water. The Cr3+ cation attracts water molecules to form the complex ion Cr(H20)63+. In this complex ion, water acts as the ligand. If ammonia is added to this solution, the ammonia can displace the water molecules from the complex ... 

A complex ion is composed of a metal ion (Lewis acid) covalently bonded to two or more molecules or anions called ligands (Lewis base). 

The preceding historical survey is actually underlying a fundamental - but simple -aspect of the Lewis theory. Given an [(TVO L] complex ion (or complex HQ- 0), where Q represents the sum of metal-ccaijugafed charges, the bond is dissected... 

Complex ions. Ligands are generally electron pair donors (Lewis bases). Important ligands are NH3, CN", and OH". Ligands bond to a central atom that is usually the positive ion of a transition metal, forming complex ions and coordination compounds. 

Lewis hi his It tG paper and in his book on valence emphasized the fact that there exist only a few stable molecules and complex ions (other than those containing atoms of the transition elements) for which the total number of electrons is odd. He pointed out that in general an odd molecule, such as nitric oxide or nitrogen dioxide, would be expected to use its unpaired electron to form a bond with another such molecule, and that the monomeric substance should accordingly be very much less stable than its dimer and he stated that the method by winch the unpaired electron is firmly held in the stable odd molecule v/as not at that time understood. Since then the explanation of the phenomenon has been found, as the result of the... 

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 ... 

The solubility product, Ksp, for an ionic compound is the equilibrium constant for dissolution of the compound in water. The solubility of the compound and Ksp are related by the equilibrium equation for the dissolution reaction. The solubility of an ionic compound is (1) suppressed by the presence of a common ion in the solution (2) increased by decreasing the pH if the compound contains a basic anion, such as OH-, S2-, or CO32- and (3) increased by the presence of a Lewis base, such as NH3, CN-, or OH-, that can bond to the metal cation to form a complex ion. The stability of a complex ion is measured by its formation constant, Kf. 

The cyanide ion is called a pseudohalide ion because it behaves like Cl- in forming an insoluble, white silver salt, AgCN. In complex ions such as Fe(CN)63-, CN - acts as a Lewis base (Section 15.16), bonding to transition metals through the lone pair of electrons on carbon. In fact, the toxicity of HCN and other cyanides is due to the strong bonding of CN- to iron(III) in cytochrome oxidase, an important enzyme involved in the oxidation of food molecules. With CN attached to the iron, the enzyme is unable to function. Cellular energy production thus comes to a halt, and rapid death follows. 

Quantum theory sheds more light on the character of such silicenium ion-Lewis base complexes, e.g. on the nature of the bonds formed. Are they covalent Do d-orbitals contribute Flow large are the complexation energies and how do they change upon substitution Can free tricoordinate silicenium ions be formed at all in 7r-donor or aromatic solvents ... 

The a-chiral ketone from Figure 10.18—the a-substituent is a benzyloxy group—is reduced to the Cram chelate product by Zn(BH4)2, a Lewis acidic reducing agent. The Zn2 ion first bonds the benzyl and the carbonyl oxygen to a chelate. Only this species is subsequently reduced by the BH 4 ion because a Zn2 -complexed C=0 group is a better elec-... 

The classical rules of valency do not apply for complex ions. To explain the particularities of chemical bonding in complex ions, various theories have been developed. As early as 1893, A. Werner suggested that, apart from normal valencies, elements possess secondary valencies which are used when complex ions are formed. He attributed directions to these secondary valencies, and thereby could explain the existence of stereoisomers, which were prepared in great numbers at that time. Later G. N. Lewis (1916), when describing his theory of chemical bonds based on the formation of electron pairs, explained the formation of complexes by the donation of a whole electron pair by an atom of the ligand to the central atom. This so-called dative bond is sometimes denoted by an arrow, showing the direction of donation of electrons. In the structural formula of the tetramminecuprate(II) ion... 

The difference in the chemistry of the light and heavy actinides may be rationalized in this way. The early members beyond thorium have unpaired d and / electrons available for forming covalent bonds and hence, for example, they readily form many complex ions and intermetallic compounds. Such ions are soft acids. Beyond americium, the 5/ electrons are not competitive and the closed shell of six 5/5/2 electrons will not be readily available for bonding, so that only those / electrons with /=7/2 are available. These tend to become buried radially as the atomic number increases and hence their divalent ions become relatively hard Lewis acids. These considerations are especially helpful in the region of superactinides because these elements do not have analogs in the known periodic table, where we have deeply buried but loosely bound 5g electrons. 

With regard to the formation of ionic compounds, it is not too relevant whether the 8p or 7d shell is occupied in the neutral atom, as studied in extenso by Mann and Wdber (50). Instead, the significant question for more ionic compounds is whether in the ions, after all outer s, p and d electrons are removed, some g or f electrons will be in frontier orbitals or whether they might be easily excited to an outer electron shell so that they can be removed as well. Prince and Waber (103) showed that even in the divalent state of element 126 one g electron has changed to an / electronic state. However, the 8s electrons are not the first to be removed. Thus, the divalent ions will be expected to act as soft Lewis acids and possibly form covalent complex ions readily. Crystal or ligand fields influence the nature of the hybridization. Details such as directionality of bonds... 

Ligand is the name given to an atom or a group of atoms bonded to the central element in complex ions. Ligands are Lewis bases. 

Electron configurations of the elements of the three li-transition series are given in Table 25-1 and in Appendix B. Most li-transition metal ions have vacant d orbitals that can accept shares in electron pairs. Many act as Lewis acids by forming coordinate covalent bonds in coordination compounds (coordination complexes, or complex ions). Complexes of transition metal ions or molecules include cationic species (e.g., [Cr(OH2)( ]5+, [Co(NH3)g]3 +, [Ag(NH3)2]+), anionic species (e.g., [Ni(CN4)]2-, [MnCl ] ), and neutral species (e.g., [Fe(CO)5], [Pt(NH3)2Cl2]). Many complexes are very stable, as indicated by their low dissociation constants, (Section 20-6 and Appendix 1). 

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