Metal zwitterionic form

Amino acids and peptides in their compounds with alkali metal salts are in the zwitterion form. The anion is hydrogen bonded to the NHs+... 

Figure 9. A schematic representation of the loading of a metal(II) sulfate into the zwitterionic form of a ditopic reagent and sequential stripping of the cation and sulfate by pH-adjustment.[81]...

However, it is best to produce the compound in the absence of iron since this obviates the problem of subsequent removal of the metal. In the zwitterion form pyrimine may be extracted into benzyl alcohol and eventually crystallized (119). Alternatively, pyrimine may be isolated directly from the medium in crude form by taking advantage of the extreme attraction of substances of this type for Dowex 50H resin. 

Imidazolecarboxylic acids are stable, crystalline compounds which form salts with metals, and which may exist in zwitterionic forms such as (198 Scheme 106). Such zwitterionic forms have been implicated in the decarboxylation mechanisms of the compounds. The carboxyl functions exert a pronounced base-weakening effect on the parent molecules, but it is surprising that a 5-bromo substituent in imidazole-4-carboxylic acids is not subject to ready nucleophilic displacement by CN or SOs . 

X-ray diffraction data obtained for some of those ate complexes are consistent with a zwitterionic form incorporating a four coordinate cyclic phospho-nium center and a pentavalent 18-electron zirconium metal center. It is reasonable to postulate that the first step of the reaction is the nucleophilic attack of the phosphino group in 16a on an acetylenic carbon atom with formation of the corresponding transient zwitterion. In the second step, an intramolecular cyclization of the carbanionic center on the zirconium metal fragment occurs to form stable zirconate products 20 a -f. It was shown that the nature of the het-... 

The alkylzirconium(m) octaethylporphyrin complex, (OEP)ZrCH2SiMe3 1, was prepared from the dialkylzirconium(rv) complex by reduction with H2 (1 atm) in toluene at 20 °C (Scheme 1). This reaction therefore appears to be a rather rare example of the chemical reduction of Zr(rv) to Zr(m) by H2. The structure of 1 was elucidated by single crystal X-ray diffraction and has a Zr-C bond length of 2.216(8) A. Although this complex formally contains zirconium in oxidation state hi, careful consideration of the structural and spectroscopic data led the authors to conclude that this was an overly simplistic view. At 77 K, an EPR signal typical of a metal-centered radical was observed, while no signal was detected at 293 K. The UV/Vis spectrum of 1 contains bands typical of a porphyrin anion. The electronic structure of 1 is therefore better described as a combination of two resonance forms a Zr(m) metal-based radical, and a zwitterionic form with a positively charged Zr(iv) center and a porphyrin radical anion. 

In the solid state, amino acids exist in the zwitterionic form, E (Fig. 4.5). The primary ammonium group, NH3+, exhibits a broad band 3040 cm-1 and two bands common to amino acids at 2500 and 2100 cm-1. Strong bands at 1665 and 1590 cm-1 for the COO- asymmetric and symmetric stretches, respectively, and NH3+ deformations at 1550 cm-1 are also typical of amino acids. A full assignment for glycine is found in Nakamoto.10 The metal-bound COO- and NH2 groups will show characteristic shifts in these stretching frequencies. 

In all the Schiff s base complexes described above, no base was used to deprotcMiate the Schiff s base ligand. Therefore, the Schiff s base binds in a neutral, albeit an zwitterionic form to the central metal ion, resulting in complexes with stoichiometry [R(LH)3X3]. It was also mentioned that in some complexes, mily one of the ligands looses spontaneously a proton, giving complexes of stoichiometry [R(LH)2LX2]. With dimeric salen-type ligands (sa/c/i=2,2 -iVW -bis(salicylidene)ethylenediamine) is was possible to form... 

This chapter reviews the effect of alkali metal halogenides and zwitterionic form of glycine on the iodine-triiodide equilibrium in aqueous solution and the interaction of I2, and I3 with lithium halogenides (LiCl). It reveals and analyzes the results obtained by experimental studies using UV and IR spectroscopy, as well as the outcomes of theoretical investigations. Computational studies have been carried out using quantum-chemical methods of DFT-B3PW91/midi, explicit inclusion of solvent molecules, as well as by application of COSMO continuum model [30]. 

In combination with a membrane-based suppressor (see Section 4.3) a mixture of 2,3-diaminopropionic add (DAP) and hydrochloric acid is frequently used for the separation of alkaline-earth metals. Using this eluant provides the advantage of being able to adjust the elution power via the dissociation equilibrium of 2,3-diaminopropionic acid see Eq. (104) in Section 4.1.1. With pJC = 1.33 for the dissociation of the carboxyl group, DAP may exist in the mobile phase as a monovalent cation, a divalent cation, or a mixture of both. In any case, the product of the suppressor reaction is the zwitterionic form, which has no intrinsic conductance ... 

Scheme 13.6 Neutral and zwitterionic forms of the metal-only Lewis pair 54.

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