Amines complexation constants

Most reactions involve reactants and products that are dispersed in a solvent. If the amount of solvent is changed, either by diluting or concentrating the solution, the concentrations of ah reactants and products either decrease or increase. The effect of these changes in concentration is not as intuitively obvious as when the concentration of a single reactant or product is changed. As an example, let s consider how dilution affects the equilibrium position for the formation of the aqueous silver-amine complex (reaction 6.28). The equilibrium constant for this reaction is... 

Table 3.24 NMR coupling constants for platinum amine complexes [156]...

Siderophore-ionophore supramolecular assembly formation via host-guest complexation of the pendant protonated amine arm of ferrioxamine B has been confirmed by X-ray crystallography (Fig. 28) (203). The stability and selectivity of this interaction as a function of ionophore structure, metal ion identity, and counter anion identity were determined by liquid-liquid extraction, isothermal calorimetry, and MS (204 -211). Second-sphere host-guest complexation constants fall in the range 103— 106M-1 in CHC13 and methanol depending on ionophore structure. 

All three methods lead to consistent surface complexation constants in clay minerals (27, 28) and zeolites (82). The surface protonation constants of the organic compounds such as amines can be defined and determined similarly (41, 83) and are also found to be enhanced in the clay interface. 

The Force Constants of Ion-Ligand Bonds of Hexaaquo and Amine Complexes... 

As can be seen, the alcohol autocomplexes possess the highest strength the bond strength in the series of amine complexes with alcohol falls from the primary to the tertiary ones, which proves the decisive role of steric factors in the formation of H-complexes with amines. At low temperatures, the equilibrium constant of the A H complexes falls in the series Keql > Keq 2 > Keq3 however, at temperatures over 6Q-70 °C the series becomes reversed. It means that during the reaction at elevated temperatures not only the absolute but also the relative concentrations of the A2H... 

The [Cr(en)3]2+ and [Cr(pn)3]2+ salts have reflectance spectra (Table 11) resembling those of the hexaammines, and the six N donor atoms are assumed to complete tetragonally distorted octahedra around the metal. Stability constant measurements (Table 39) have shown that the ions [Cr(en)(aq)]2+ (vmax= 18 300 cm-1, e = 25 dm3 mol-1 cm-1) and [Cr(en)2(aq)]2+ (vma = 17 500 cm-1, e = 17 dm3 mol-1 cm-1) exist in aqueous solution, but that, as in the copper(II) system, the third ethylenediamine molecule is only weakly bound, and care is needed to prevent loss of en from tris(amine) complexes in the preparations. Several bis(amine) complexes, e.g. [CrBr2(en)2], have been isolated, and these are assigned trans structures because of IR spectral resemblances to the corresponding oopper(II) complexes. Since the spectrum of [Cr(S04)(en)2] also shows the presence of bidentate sulfate, this is assigned a trans octahedral structure with bridging anions. 

One of the first differences to be noted about the Rhm acido-amine complexes is that the chloro complexes are frequently much more stable with respect to solvolysis than their Co111 and Cr111 analogues and solvolytic equilibrium is reached when very little of the chloro complex has aquated, even in dilute solution and in the absence of added chloride ions. This is, to some extent, the consequence of the move away from class a character already mentioned above. As a result, the rate constants for aquation are obtained from ligand substitution reactions (including chloride exchange) which first have to be shown to be mediated by a rate determining aquation. More recently, data have been obtained from a study of the solvolysis in basic solution. This serves to... 

Apart from a comparison of the cis and trans dichloro complexes and a comparison of dichloro with chloroaquo species, little has been done to study the effect of the non-amine, monodentate non-participating ligands upon the lability of Rum complexes. Where studied, cis-dichlorotetra-(amine) complexes are considerably more labile than their trans isomers but there are insufficient data at this time to say whether there is any great sensitivity of lability to the nature of the tetraamine ligand in such systems. The extremely low reactivity of irans-[Ru(cyclam)Cl2]+ (the rate constant for Cl- exchange is reported293 to be 3 x 10 7s 1 at 102 °C) seems to be an exception that is, as yet, unexplained. Some relevant data are collected in Table 11. 

Complexes of (( Ir(III) are kinetically inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of [IrBr(NH3)5]2+ [35884-02-7] at 298 K has been measured at -2 x 10-10 s-1 (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid greatly accelerates the rate of substitution via a transient, labile Ir(H) species (169). Optical isomers can frequently be resolved, as is the case of ot-[IrCl2(en)2]+ [15444-47-0] (170). Ir(III) amine complexes are photoactive and undeigo rapid photosubstitution reactions (171). Other iridium complexes... 

Com corroles form five- and six-coordinate complexes in the presence of ligands but the second association constant is much lower than the first. A five-coordinate triphenylphosphine complex was isolated and shown to have a distorted square pyramidal structure with the Co atom displaced out of the N4 plane by 0.28 A.245 In pyridine, it is converted to a diamagnetic bis-pyridine complex.239 Isocyanides displace pyridine ligands to form five-coordinate complexes, from which amine complexes may be obtained (Scheme 78). 

Formation constants for the ammonia and ethylenedi-amine complexes of nickel(II) indicate that Ni(en)32+ is much more stable than Ni(NH3)g2+ ... 

Another early effort in this direction [51] considered the competition for the bridging proton within the H-bond connecting a formate HCOO anion and an imine base of the sort HN=CH2. In the gas phase, it was found that the neutral pair HCOOH NHCH2 was preferred over an ion pair. However, again as in the aforementioned XH-amine complexes, the ion pair is progressively more favored as the dielectric constant of the medium rises. More specifically, in an in vacuo situation, the neutral pair in which the proton resides on the carboxylate rather than the imine... 

In addition, the stability constants of amine complexes were determined with the dyed-host 40 using the Benesi-Hildebrand method in acetonitrile and chloroform at 25 °C [23], The data show the following order of complex stability ... 

Note that all the physical states are specified here for the sake of clarity of the demonstration that follows). However, when it forms the ethylenedi-amine complex, its stability increases considerably, as shown by the corresponding constant ... 

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