Thermodynamic stability, coordination complexes

The reaction control should be emphasized amongst the conditions of reactions of competitive complex formation [19,23], It is necessary to take into account that it is possible to determine, and frequently predict, the direction of the electrophilic attack to the donor center of di- and polyfunctional donors (ligands) only in the case when the thermodynamically stable products are formed under conditions of kinetic control. Thus, the thermodynamic stability of complexes is discussed, when the bond between the metal and di- and polydentate ligands is localized in the place of primary attack on one of any of the donor centers by the electrophilic reagent, without further change of coordination mode in the reaction of complex formation. 

In tetraazamacrocyclic Co(II) complexes, the formation of /<-peroxo-Co(III) complexes following coordination of O2 to Co(II) complexes can be suppressed if the macrocycle is functionalized to inhibit face-to-face approach of the Co(II) centers. The thermodynamic stabilities of complexes [(X)LCo] (X =SCN or Q", L = C-meso-5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraaza-cyclotetradecane) have been determined and the effects of the anion X on the rate of dioxygen binding have been studied by laser flash photolysis after the flash, there is an immediate bleaching of the solution at the absorbance wavelength of the complex, followed by a slower return of absorbance. Reestablishment of the equilibrium for SCN can be analyzed in terms of Eq. (4) to (6) an analogous sequence applies when the anion is Cl . 

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. 

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. 

The above account of selectivity of inorganic plus organic chemistry in synthesis is given rather extensively to stress three points. All the four (Mg, Fe, Co and Ni) porphyrin products came from one source, the synthesis of uroporphyrin. The basis of selection is very different from that in primitive centres which use thermodynamic stability constant selectivity based on different donor atoms for different metal ions. Here, all ion complexes have the same donor atoms, nitrogen, the most constrained being the coordination of Mg2+ by five nitrogens exactly as is seen for Fe in haemoglobin. Hence, there also has to be a new control feedback to ensure that the appropriate quantities of each metal cofactor is produced in a balanced way, that is synthesis from uroporphyrin has to be divided based upon... 

Several forms are imaginable for the [Ni°(butadiene)2L] and [Ni°(butadiene)J active catalysts, depending on the monodentate (p2) or the bidentate (p4) coordination mode of butadiene from either its s-cis or its s-trans configuration. The two butadienes can be coordinated in bis(p2), p4, p2, and bis(p4) modes for the PR3/P(OR)3-stabilized catalyst complex, giving rise to formal 16e, 18e, and 20e species. On the other hand, bis(p4)- and p4,p2-butadiene species and also tris(p2)- and p4,bis(p2)-butadiene compounds are possible species for the [Ni°(butadiene)2] and [Ni°(butadiene)3] forms for the [Ni°(butadiene)J active catalyst. In general, for butadiene to coordinate in a bidentate fashion, the p4-cis mode is thermodynamically favorable relative to the p4-trans mode, while the p2-trans mode prevails for monodentate coordination. 

Under these conditions, the formation rate constant, k, can be estimated from the product of the outer sphere stability constant, Kos, and the water loss rate constant, h2o, (equation (28) Table 2). The outer sphere stability constant can be estimated from the free energy of electrostatic interaction between M(H20)q+ and L and the ionic strength of the medium [5,164,172,173]. Consequently, Kos does not depend on the chemical nature of the ligand. A similar mechanism will also apply to a coordination complex with polydentate ligands, if the rate-limiting step is the formation of the first metal-ligand bond [5]. Values for the dissociation rate constants, k, are usually estimated from the thermodynamic equilibrium constant, using calculated values of kf ... 

As mentioned above, the calculations performed for styrene as a substrate suggests that the enantioselectivity can be directly correlated with the relative thermodynamic stabilities of the r 3-allylic complexes. Indeed, the exo stereoisomer, precursor of the enantiomeric product found in excess experimentally, becomes favoured with respect to the endo one upon t 3-coordination, and remains thermodynamically more stable until product release. However, the observed energy differences in the relative stabilities of the different allylic forms (1-2 kcal/mol) are certainly at the limit of accuracy of density functional calculations. 

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