Protonated complexes

One molecule of acid first protonates TBHP to give a protonated complex (I), which is stabilized by the second molecule of H2SO4 involving hydrogen bonding in the following manner ... 

By Production of Azepinium Salts, Protonation, Complex Formation and Alkylation... 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

Ojamae, L., Shavitt, I., Singer, S. J., 1995, Potential Energy Surfaces and Vibrational Spectra of H502+ and Larger Hydrated Proton Complexes , Int. J. Quant. Chem. Symp., 29, 657. 

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

M = Eu(III), Y(III), Lu(III)] 8-16%. These protonated compounds were isolated as the sole product, in contrast to the similar reactions with TBPPH2, where the non-protonated analogues Lnnl(Pc)(TBPP) (TBPP, 5,10,15,20-tetrakis[(4- ert-butyl)phenyl]-porphyrin) were obtained instead [106]. These results clearly show that the inversion of a pyrrole ring in N-confused porphyrins can stabilize the protonated complexes. 

In these studies Ru(III) and Ru(EDTA) represent [RuCl2(H20)4]+ and a generalized EDTA complex of Ru(III), respectively. The coordination mode and composition, in particular with respect to possible protonated complexes, were not specified for the EDTA complex in the cited references. 

Furthermore, protonation results in a significant distortion of the coordination polyhedron, i.e., the metal ion is displaced from the plane formed by the four cyano ligand carbon atoms toward the oxo along the M = 0 axis by as much as 0.34 A, which represents about 20% of the total metal-oxo bond length. In spite of this distortion stronger metal-cyano bonds are observed crystallographically, suggesting a better n back-donation by the metal center to the cyano carbons since d-ff overlap is increased. This observation is in line with both the 13C and 15N chemical shift and kinetic data (Section V) for the protonated complexes (8). 

This study demonstrated the power of NMR spectroscopy whereby the use of carbon-13, nitrogen-15, and, more specifically, oxygen-17 in these oxo systems proved that the different species in the protonation, complex formation, ligand exchange, and condensation of the Re(V) system can be exceptionally well characterized in solution. The re-... 

Effect of protonation, complex formation with ligands and metal ions and reduction on dissolution rate. The structures given here are schematic short hand notations to illustrate the principal features (they do not reveal the structural properties nor the coordination numbers of the oxides under consideration charges given are relative). 

The total electro-osmotic coefficient = Whydr + mo includes a contribution of hydrodynamic coupling (Whydr) and a molecular contribution related to the diffusion of mobile protonated complexes—namely, H3O. The relative importance, n ydr and depends on the prevailing mode of proton transport in pores. If structural diffusion of protons prevails (see Section 6.7.1), is expected to be small and Whydr- If/ ori the other hand, proton mobility is mainly due to the diffusion of protonated water clusters via the so-called "vehicle mechanism," a significant molecular contribution to n can be expected. The value of is thus closely tied to the relative contributions to proton mobility of structural diffusion and vehicle mechanism. ... 

There are only a few cases where the dissolution of an iron oxide by all three types of processes under comparable conditions has been investigated. Banwart et al. (1989) found that at pH 3, the rate of dissolution of hematite increased in the order, protonation < complexation < reduction with a factor of 350 between the extremes. A similar factor (400) was found for goethite (Zinder et al, 1986) (Fig. 12.15). Hematite dissolution processes were also compared in the pH range similar to that found in neutral environments (Fig. 12.16). Again, dissolution by simple protonation was extremely slow, whereas reduction, especially when aided by Fe complexing ligands, was particularly effective (Banwart et al, 1989). It can, thus, be concluded that reduction, particularly when assisted by protonation and complexation will be the main mechanism for Fe transport in global ecosystems. 

Fig. 12.15 Di ssolution rate of goethite by protonation, complexation with oxalate and reduction by ascorbic acid as a function of pH (Stumm, Furrer, 1987, with permission).

In conclusion, it appears necessary to study more extensively those properties of the various oxides, which determine their specific dissolution behaviour. As pointed out by Postma (1993), the variation in reactivity, a solid phase parameter, may, in some cases, be twice as high as the effect of the type of dissolution (protonation, complexation, reduction). 

Once again, it is important to recall that a complexing agent which undergoes protonation (unlike Cl ) requires more consideration. This is because the concentration of the non-protonated complexing species varies as the pH changes. An example would be the acetate ion C2 H3 O2 which protonates... 

Sridharan and Mathai noticed that the transesterification of small esters under acid-catalyzed conditions was retarded by the presence of spectator polar compounds. " Thus, given that water can form water-rich clusters around protons (solvent-proton complexes) with less acid strength than methanol-only proton complexes, some catalyst deactivation may be expected with increased water concentrations. Also, water-rich methanol proton complexes should be less hydrophobic than methanol-only clusters, thus making it more difficult for the catalytic species (H" ) to approach the hydrophobic TG (and possibly DG) molecules and contributing to catalyst deactivation. Therefore, with water present in the feedstock or produced during the reaction in significant quantities, some catalyst deactivation can take place by hydration. 

Fig. 7. Optimized geometries of the most stable cyano-protonated complexes of [Fe(CN)sNO]3 and [Fe(CN)6HNO]3-.

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