Kinetically labile complexes

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

The dimer complex 13 showed, for the first time for kinetically labile complexes, an enhanced activity toward the hydrolysis of the activated... 

Ir(IV), Pt(IV), with the states from Rh(III) being termed inert. Thus, kinetic factors tend to be more important, and reactions that should be possible from thermodynamic considerations are less successful as a result. On the other hand, the presence of small amounts of a kinetically labile complex in the solution can completely alter the situation. This is made even more confusing in that the basic chemistry of some of the elements has not been fully investigated under the conditions in the leach solutions. Consequently, a solvent extraction process to separate the precious metals must cope with a wide range of complexes in different oxidation states, which vary, often in a poorly known fashion, both in kinetic and thermodynamic stability. Therefore, different approaches have been tried and different flow sheets produced. 

In such cases slow rearrangements to the thermodynamically favoured form occurred. With kinetically labile complexes, the bridging mode adopted by the dithiooxalate ligand was a reflection of the thermodynamic stability of M—S2C2O2 vs. (R3P)2—Ag—S2C2O2 interactions. 

A new type of octaazamacrobicyclic Schiff bases was synthesized in high yields via a template condensation on Group 2 metal ions in ethanol at 40-50°C (Scheme 92) [196], The resultant kinetically labile complexes readily transmetallize when reacted with transition metal (cobalt, nickel, iron, copper(II)) salts to form the corresponding mononuclear [M(imBT)]X2 complexes (where X is BF4 and C104 ). 

While the natural porphyrin derivatives are exclusively hydrophobic, some artificial porphyrins having ionic substituents made it possible to prepare water-soluble metaUoporphyrins of both regular and SAT type. Kinetically labile complexes are mostly examined in the excess of the ligand. 

This restricts possibilities of obtaining complexes of the corresponding ligand with metals unaWe to act as templates. One solution is transmetallation, namely, the treatment of a kinetically labile complex with a metal ion to form inert complexes. Thus it is possible to synthesise complexes of nickel(II), copper(II), iron(II), cobalt(II) and cobalt(in) with the unstable ligand LI 6 from the corresponding [Pb(L16)]X2 or [Ag(L16)]X, themselves assembled with the use of lead(II) or silver(I) as templates [46] (Eq. 1.8). 

For kinetically labile complexes to exist in aqueous solutions the ligand needs to be in considerable excess [1, 2, 3, 4, 5]. Species with higher ligand numbers are generally formed in aprotic solvents of low coordination ability [6, 7, 8, 9, 10]. Application of reverse micelles, however, gives us the possibility to examine aqueous-phase complexes with different ligand numbers which do not exist in homogeneous aqueous solutions. In nonpolar solvents cationic surfactants, for example, cetyltrimethylammonium bromide (CTAB), form micelles, the outer sides of which consist of hydrophobic hydrocarbon... 

Main-group metal ions of and electronic configurations form kinetically labile complexes with halo ligands both in homogeneous aqueous solutions and in water droplets inside reverse micelles. Complexes of the 5 metal ions are exclusively featured by LMCT reactions as with Pb(IV), Sb(V), and Hg(II) [8, 22, 23, 24]. The Hg(II) cation possesses a filled d subshell thus, its chemical features are rather similar to main-group metal ions. In the case of the configuration, however, electron ejection (with oxidation of the metal center) can also occur, demonstrated by the example of hydroxo complexes of Sn(II), T1(I), and Pb(II) [25, 26]. 

A further complication in the identification of target sites and chemical forms of metals is the kinetic lability of coordinate covalent bonds. Metal ligands exchange rapidly in and out of the coordination sphere, in particular for first-row transition metals. This kinetic lability varies between metals, and, as indicated above, is influenced by the nature of the ligand, whether mono- or multidentate, and by the pH and ionic strength of its immediate environment. Copper, for example, forms relatively low affinity complexes with albumin or amino acids, but is tightly bound to ceruloplasmin. Similarly, mercury and cadmium form kinetically labile complexes with amino acids, glutathione, or albumin, but more stable chelates with metallothionein. 

The complex [BF3(N(SiH3)3)] can be prepared and stored at low temperatures ( — 80 °C) since the decomposition then proceeds very slowly—at this temperature the complex is kinetically fairly stable. At room temperature the complex is kinetically unstable and the rate of decomposition is much greater. This is the key distinction made in Chapter 4 between kinetically inert and kinetically labile complexes. There it was pointed out that the species which crystallizes from a solution of a mixture of related labile complexes depends not only on the cation and ligand concentration but also on the solvent and crystallization temperature. Although it may be a relatively minor component in the solution, the least soluble complex is probably the one which crystallizes. In the solution there is a series of equilibria such that. 

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