Complex ions acid-base reactions

Cations often interfere with each other in the final tests designed to detect the presence of specific cations. Therefore, cations must first be separated before identification can be accomplished. In fact, as with many chemical mixtures, separation of cations may be considerably more difficult than identification. Careful work is again very important if the separations are not clean, results in identification tests may be masked by interfering cations. Separation of a complex mixture of cations is by no means simple and is generally broken down into several parts. Each part involves a fairly small group of cations which can be isolated from the mixture on the basis of some property which is common to the ions in the group and then studied as a separate set. After isolation, the cations within a group are further resolved by means of a series of chemical reactions into soluble and insoluble fractions which are sufficient to allow identification of each cation by one or more tests specific to that ion once interferences have been removed. Various types of chemical reactions will be used for separations and identifications in this experiment precipitation reactions, acid-base reactions, complex ion formations, and oxidation-reduction reactions. 

The formation of complex ions can also be looked at as Lewis acid-base reactions. Complex ions are formed when a metal ion bonds to electron pairs from molecules such as H2O or NH3 or from anions such as C=N . An example of a complex ion is A1(H20)6. Hydrated ions like A1(H20)6 are present in compounds (hydrates) and in aqueous solution. The formation of a hydrated metal ion, such as A1(H20)6 ", involves a Lewis acid-base reaction. 

The electric field-jump method is applicable to reactions of ions and dipoles. Application of a powerful electric field to a solution will favor the production of ions from a neutral species, and it will orient dipoles with the direction of the applied field. The method has been used to study metal ion complex formation, the binding of ions to macromolecules, and acid-base reactions. 

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—... 

Rust, which you can take to be Fe(OH)3, can be dissolved by treating it with oxalic acid. An acid-base reaction occurs, and a complex ion is formed. 

Scheme 5-14 may be called a two-dimensional system of reactions, in contrast to Scheme 5-1 which consists of a one-dimensional sequence of two acid-base equilibria. In Scheme 5-14 the (Z/E) configurational isomerism is added to the acid-base reactions as a second dimension . The real situation, however, is yet more complex, as the TV-nitrosoamines may be involved as constitutional isomers of the diazohydroxide. In order not to make Scheme 5-14 too complex the nitrosoamines are not included, but are shown instead in Scheme 5-15. The latter also includes the addition reactions of the (Z)- and ( )-diazoates (5.4 and 5.5) to the diazonium ion to form the (Z,Z)-, (Z,E)- and (2 2i)-diazoanhydrides (5.6, 5.7 and 5.8) as well as proto-de-nitrosation reactions (steps 10, 11 and 12). This pathway corresponds to the reverse reaction of diazotization, as amine and nitrosating reagent (nitrosyl ion) are formed in this reaction sequence.

In the practice of potentiometric titration there are two aspects to be dealt with first the shape of the titration curve, i.e., its qualitative aspect, and second the titration end-point, i.e., its quantitative aspect. In relation to these aspects, an answer should also be given to the questions of analogy and/or mutual differences between the potentiometric curves of the acid-base, precipitation, complex-formation and redox reactions during titration. Excellent guidance is given by the Nernst equation, while the acid-base titration may serve as a basic model. Further, for convenience we start from the following fairly approximate assumptions (1) as titrations usually take place in dilute (0.1 M) solutions we use ion concentrations in the Nernst equation, etc., instead of ion activities and (2) during titration the volume of the reaction solution is considered to remain constant. 

In general, complexation of an aquometal ion occurs when the ligand is a stronger base than H20, and analogously may be considered an acid-base reaction. The stability (or formation) constant, KMl, is used to describe the interaction of the metal ion (Mz+, shown here with the hydration sheath surrounding the metal ion omitted for reasons of clarity) with a complexant (L" ) ... 

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 ... 

This book was written to provide readers with some knowledge of electrochemistry in non-aqueous solutions, from its fundamentals to the latest developments, including the current situation concerning hazardous solvents. The book is divided into two parts. Part I (Chapters 1 to 4) contains a discussion of solvent properties and then deals with solvent effects on chemical processes such as ion solvation, ion complexation, electrolyte dissociation, acid-base reactions and redox reactions. Such solvent effects are of fundamental importance in understanding chem-... 

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. 

Lewis acids are thus electron-deficient molecules or ions such as BF-, or carbo-cations, whereasTewis bases are molecules or ions containing available electrons. such as amines, ethers, alkoxide ions, and so forth.,A Lewis acid-base reaction is the combination of an acid and a base to form a complex, or adduct. The stabilities of these adducts depend on the structures of the constituent acid and base and vary over a wide range. Some examples of Lewis acid—base reactions are given in Table 3.19. Lewis acid-base reactions abound in organic chemistry ... 

More recently, a quantitative scale for Lewis acidity based on fluoride ion affinities was calculated using ab initio calculations at the MP2/B2 level of theory.26 Due to its high basicity and small size, the fluoride ion reacts essentially with all Lewis acids thus the fluoride affinity (or reaction enthalpy) may be considered as a good measure for the strength of a Lewis acid. An abbreviated pF scale is given in Table 1.3. This scale was used recently by Christe and Dixon112 for estimating the stability of salts of complex fluoro anions and cations. The pF value represents the fluoride affinity in kcal mol 1 divided by 10. 

Sometimes the acid-base reaction involved is more complex than that indicated in scheme (19). The two most frequently observed complications are participation of proton-donors other than hydroxonium ion and dissociation of two or more protons. Participation of various proton-donors is demonstrated by the dependence of the height of the kinetically controlled wave on the nature and concentration of the buffer with the usual type of buffers, a pH-dependence of wave f in the shape of a deformed dissociation curve is obtained. For polybasic acids several po-larographic dissociation curves are observed at various pK -values under certain conditions the slopes of these curves may differ. 

The mechanism for both of these reactions is very similar to the mechanism for the reduction of acyl chlorides by LATB—H. The first step is an acid-base reaction between an unshared electron pair on oxygen or nitrogen with the aluminum atom of the DIBAL—H. The second step is the transfer of a hydride ion from the DIBAL—H to the carbon atom of the carbonyl or nitrile group. The last step is the hydrolysis of the aluminum complex to form the aldehyde. 

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

The bonding to form the imidazole ring would suggest that reduction with complex metal hydrides should be possible. Lithium aluminum hydride, however, would be expected to undergo an acid-base reaction with the N—H of the 1-position forming the anion of imidazole which would resist further attack by complex hydride ions. Such observations were made by Bohlmann97 with benzimidazole (131). Thus at room temperature or below only salt formation was observed on reaction of lithium aluminum hydride with benzimidazole... 

It has been recognized for many years that in a general way the basicity of the ligands has a great influence on the stability of complexes. After all, the formation of the coordinate bond is an acid-base reaction in the Lewis sense. However, as usually measured, basicity is toward the proton in aqueous solution. It sometimes provides a measure of the availability of electrons that might be expected when the ligands form coordinate bonds to metal ions. 

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