Complexes thermodynamic stability

TOXICITY OF Gd111 COMPLEXES THERMODYNAMIC STABILITY AND KINETIC INERTNESS... 

Marin, D. Mendicuti, F. Polarographic Determination of Composition and Thermodynamic Stability Constant of a Complex Metal Ion, /. Chem. Educ. 1988, 65, 916-918. 

The factors leading to the high resistance of the noble metals to chemical attack, i.e. their thermodynamic stability over a wide range of conditions and the possibility of the formation of very thin protective films under oxidising conditions, have already been mentioned. A factor tending to reduce corrosion resistance in aqueous solutions is the tendency of these metals to form complexes with some anions. 

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

Matsui and Mochida24) have determined the thermodynamic stabilities (log 1 /Kd) for a- and P-cyclodextrin complexes with a variety of alcohols (Table 2) and analyzed the results in connection with the physicochemical properties of the guest molecules by the multivariate technique. The log 1/Kd values were plotted against log Pe, where Pe is the partition coefficient of alcohol in a diethyl ether-water system. The plots for the a- and P-cyclodextrin complexes with eight 1-alkanols gave approximately straight lines with slopes of around one. 

Silylene complexes are not only stable with donor substituents but also with simple alkyl residues at silicon. These alkyl complexes still have a sufficient thermodynamic stability, but otherwise are reactive enough to allow a rich and diverse chemistry. Particularly the chlorocompounds 7 and 11 are valuable starting materials for further functionalization reactions the details of these reactions will be discussed in the forthcoming sections. The data for the known compounds are summarized in Table 1. 

Compared to the sum of covalent radii, metal-silicon single bonds are significantly shortened. This phenomenon is explained by a partial multiple bonding between the metal and silicon [62]. A comparison of several metal complexes throughout the periodic table shows that the largest effects occur with the heaviest metals. However, conclusions drawn concerning the thermodynamic stability of the respective M —Si bonds should be considered with some reservation [146], since in most cases the compared metals show neither the same coordination geometries nor the same oxidation states. 

The first thing to point out is that the use of a pentafluorophenyl CgFs group with late transition metal confers on the complexes great stability, both thermodynamic and kinetic. This general fact, that is also true in gold chemistry, can be explained by different factors ... 

In general, thermodynamic stability of a mixed-valence dinuclear complex, which is denoted as a combination of reduced (Red) and oxidized (Ox) sites, Red-Ox, is exhibited as a difference in redox potentials AE° = E°(Ox-Ox/Red-Ox) - °(Red-Ox/Red -Red). This difference is related to the comproportionation constant, Kc, defined in Eq. (1) ... 

A review11 with 123 references is given on amino acid bonding preferences in complexes of platinum and palladium. The preferences for a particular donor atom for Pd11 depends primarily on relative thermodynamic stabilities of the complexes formed. Sizes of potential chelate rings often play a crucial role in determining donor atom preferences. 

Complexation of Cd with a series of polyamine macrocycles, but also related open-chain polyamines, comprising or attached to the 2,2 -bipyridine (bipy) and 1,10-phenanthroline (phen) moieties, has been studied by combined UV/vis spectrometry and potentiometry.24 Formation constants and distribution diagrams of the species present have been evaluated. As a result the thermodynamic stabilities, i.e., the formation constants, are lower for the bipy- and phen-contain-ing ligands than those for Cd complexes with aliphatic oligoaza macrocycles containing the same number of N donors. The probable reason is loss of flexibility of the ligands caused by the size and stiffness of the inserted heteroaromatic moieties. 

The rate at which displaceable ligands leave or enter a complex is obviously important and is a quite separate consideration from the thermodynamic stability of the complexes. The reaction... 

From this sort of data it can be concluded that the cobalt(III) ion in this complex is a soft, or class b, Lewis acid. It has been suggested previously that the cobalt(III) ion in vitamin B12 was also a class b acid (138). The kinetic order, which presumably corresponds closely to the order of thermodynamic stability, suggests that for at least some ligands jr-bonding may be important. In this connection it is noteworthy that methylcobaloxime forms a complex with CO (139). 

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

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