Replacement reactions, anion

Reduction of the pH of solutions of carbonylate anions yields a variety of protonated species and, from acid solutions, carbonyl hydrides such as the unstable, gaseous H2Fe(CO)4 and the polymeric liquids H2Fe2(CO)g and H2Fe3(CO)n are liberated. The use of ligand-replacement reactions to yield hydrides of higher nuclearity has already been noted. 

Double-substitution or double-replacement reactions, also called double-decomposition reactions or metathesis reactions, involve two ionic compounds, most often in aqueous solution. In this type of reaction, the cations simply swap anions. The reaction proceeds if a solid or a covalent compound is formed from ions in solutions. All gases at room temperature are covalent. Some reactions of ionic solids plus ions in solution also occur. Otherwise, no reaction takes place. For example,... 

Of course, ions dissolved in solution move about freely, not as pcirt of cation-anion complexes. So to allow double replacement reactions to progress, one of several things must occur ... 

Replacement of the Au(PR3)C1 molecule by the AuC12 anion in reactions with transition metal anions gives rise to linear trinuclear Au-M-Au heterometallic complexes, as in the reactions (234)... 

As summarized in Scheme II, PLP enzymes can catalyze replacements at the y-carbon of amino acids and eliminations of HY between C-fi and C-y. In mechanistic similarity to the aspartate-/3-decarboxylase reaction, in these processes the quinoid intermediate 1 loses a proton from C-/3, followed by elimination of an anionic group (Y ) from C-y, to generate the central intermediate PLP-vinylglycine, 4 (Scheme II). This species, the vinylogue of 1, can undergo a number of reactions. Addition of a new anionic group (Y ) and reversal of the reaction sequence constitutes the y-replacement reaction, as in cystathionine-y-synthase. On the other hand, in analogy to the protonation of 1 at C-a, 4 can be protonated at C-y,... 

The rapid chemical transformation of the initial radical anion precludes the use of ETC-substitution chemistry to make substituted derivatives, because of the short lifetime and hence inactivity of the isomerized radical anion [299] - towards the CO/PR3 replacement reaction.484... 

As with the AFm phases, anion exchange reactions occur (H35), and meixnerite readily takes up CO2, giving a material similar to hydrotalcite. Among the natural minerals of the group, ones with interlayer COj"" are relatively common, and ones with interlayer OH very rare. This may be a further indication that replacement of OH by COj " occurs easily. Differing schemes for the packing of H2O molecules and ions in the... 

In this generic equation, A and B represent positively charged ions (cations), and X and Y represent negatively charged ions (anions). You can see that the anions have switched places and are now bonded to the other cations in the reaction. In other words, X replaces Y and Y replaces X—a double replacement. More simply, you might say that the positive and negative ions of two compounds switch places. The reaction between calcium hydroxide and hydrochloric acid is a double-replacement reaction. 

When magnesium hydroxide and hydrochloric acid react, the resulting solution has properties characterishc of neither an acid nor a base. This type of reaction is called a neutralizahon reaction. A neutralization reaction is a reaction in which an acid and a base react in aqueous solution to produce a salt and water. A salt is an ionic compound made up of a cation from a base and an anion from an acid. Neutralizahon is a double-replacement reaction. In the reachon between magnesium hydroxide and hydrochloric acid, magnesium replaces hydrogen in HCl and hydrogen replaces magnesium in Mg(OH)2. The reachon may be described by this balanced formula equahon. 

Stille coupling reactions of organic triflates are often facilitated by added LiCl when conducted in THF solvent. Presumably the chloride anion replaces the triflate ligand on palladium following oxidative addition, which leads to increased stability of the Pd(Ar)(X) species [35] this additive may also assist in the transmetalation step. However, LiCl is not needed when the StiUe reactions are carried out in polar solvents like NMP [36]. 

Where the product of a reaction is neutral, as occurs when red anionic IPICI4I2 has two chloride anions replaced by two neutral ammonia ligands to form yellow neutral [PtCl2(NH3)2], the solubility of the product may be inherently lower than for an ionic product, leading to its ready and selective precipitation from aqueous solution (6.5). 

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