Complexes in aqueous solution

Martell, A. E. Hancock, R. D. Metal Complexes in Aqueous Solution, Plenum Press New York, 1996... 

Arsonium salts have found considerable use in analytical chemistry. One such use involves the extraction of a metal complex in aqueous solution with tetraphenyiarsonium chloride in an organic solvent. Titanium(IV) thiocyanate [35787-79-2] (157) and copper(II) thiocyanate [15192-76-4] (158) in hydrochloric acid solution have been extracted using tetraphenyiarsonium chloride in chloroform solution in this manner, and the Ti(IV) and Cu(II) thiocyanates deterrnined spectrophotometricaHy. Cobalt, palladium, tungsten, niobium, and molybdenum have been deterrnined in a similar manner. In addition to their use for the deterrnination of metals, anions such as perchlorate and perrhenate have been deterrnined as arsonium salts. Tetraphenyiarsonium permanganate is the only known insoluble salt of this anion. 

The properties of copper(Il) are quite different. Ligands that form strong coordinate bonds bind copper(Il) readily to form complexes in which the copper has coordination numbers of 4 or 6, such as tetraammine copper(Tl) [16828-95-8] [Cu(NH3)4], and hexaaquacopper(Il) [14946-74-8] [Cu(H,0),p+ ( see Coordination compounds). Formation of copper(Il) complexes in aqueous solution depends on the abiUty of the ligands to compete with water for coordination sites. Most copper(Il) complexes are colored and paramagnetic as a result of the unpaired electron in the 2d orbital (see Copper... 

Compounds of Tl have many similarities to those of the alkali metals TIOH is very soluble and is a strong base TI2CO3 is also soluble and resembles the corresponding Na and K compounds Tl forms colourless, well-crystallized salts of many oxoacids, and these tend to be anhydrous like those of the similarly sized Rb and Cs Tl salts of weak acids have a basic reaction in aqueous solution as a result of hydrolysis Tl forms polysulfldes (e.g. TI2S3) and polyiodides, etc. In other respects Tl resembles the more highly polarizing ion Ag+, e.g. in the colour and insolubility of its chromate, sulfide, arsenate and halides (except F), though it does not form ammine complexes in aqueous solution and its azide is not explosive. 

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). 

Dagnall and West8 have described the formation and extraction of a blue ternary complex, Ag(I)-l,10-phenanthroline-bromopyrogallol red (BPR), as the basis of a highly sensitive spectrophotometric procedure for the determination of traces of silver (Section 6.16). The reaction mechanism for the formation of the blue complex in aqueous solution was investigated by photometric and potentiometric methods and these studies led to the conclusion that the complex is an ion association system, (Ag(phen)2)2BPR2, i.e. involving a cationic chelate complex of a metal ion (Ag + ) associated with an anionic counter ion derived from the dyestuff (BPR). Ternary complexes have been reviewed by Babko.9... 

Some suggested calculation procedures and the variation in results obtained from different calculation methods for evaluation of concentration stability constants of metal ion complexes in aqueous solution. A. M. Bond, Coord. Chem. Rev., 1971,6, 377-405 (43),... 

Thermodynamics of the stepwise formation of metal ion complexes in aqueous solution. S. Ahrland, Struct. Bonding (Berlin), 1973,15,167-188 (86). 

Formation and stabilities of cobalt dioxygen complexes in aqueous solution. A. E. Martell, Acc. Chem. Res., 1982,15,155-162 (68). 

Trans activation and limiting N1 mechanisms for substitution reactions of cobalt(III) complexes in aqueous solution. J. E. Byrd and W. K. Wilmarth, Inorg. Chim. Acta, Rev., 1971, 5, 7-18 (42). 

Afanasyeva, V.A. and Mironov, I.V. (2001) Gold(III) macrocydic complexes in aqueous solution. Russian Journal of Coordination Chemistry, 27, 878 Afanasyeva, V.A., Glinskaya, L.A., Klevtsova, R.E. and Sheludyakova, L.A. (2005) Crystal and molecular structure of [Au(CuH24N4)][H30](C104)4. Joumal of Structural Chemistry, 46, 131. 

Forward, J.M., Assefa, Z. and Fackler, J.P. Jr (1995) Photoluminescence of gold(I) phosphine complexes in aqueous solution. Journal of the American Chemical Society, 117, 9103-9104. 

Stability of the bidentate and multidentate complexes in aqueous solution [16] compared with monodentate complexes. Kinetic studies of gold(III) reactions with ethylenediamine and related ligands show that the initial displacement of one end of the chelate is most often followed by rapid reclosure of the ring, rather than displacement of the second bond to the metal ion [15]. 

Like the gold(I) thiolates discussed earlier, the position of the equilibrium depends upon the affinity of the thiols for gold(I) with the established sequence being AtgSH >TgSH > GSH >TmSH. This sequence corresponds to increasing pKsn [12] or by decreasing P NMR chemical shifts for the EtsPAuSR complex in aqueous solution [55]. 

Ahrland, S. Thermodynamics of the Stepwise Formation of Metal-Ion Complexes in Aqueous Solution. Vol. 15, pp. 167-188. 

Fig. 8. Correlation between Pearson s hardness parameter (7P) derived from gas-phase enthalpies of formation of halide compounds of Lewis acids (19), and the hardness parameter in aqueous solution (/A), derived from formation constants of fluoride and hydroxide complexes in aqueous solution (17). The Lewis acids are segregated by charge into separate correlations for monopositive ( ), dipositive (O), and tripositive ( ) cations, with a single tetrapositive ion (Zr4+, ). The /P value for Tl3+ was not reported, but the point is included in parentheses to show the relative ionicity of Tl(III) to ligand bonds. 

PnAO was found to form a neutral-lipophilic "mTc complex in aqueous solutions and to exhibit a high first-pass cerebral extraction efficiency [13]. All... 

Some recent interest in the technetium chemistry has been focused on complexes possessing a Tc=N3+ core. Tetrachloronitridotechnetate(VI) complexes can easily be synthesized by the reaction of pertechnetate with sodium azide in concentrated hydrochloric acid [34], Although its square-pyramidal structure resembles that of tetrachlorooxotechnetate(V) complexes, stable character of the nitrido complexes in aqueous solution shows a remarkable contrast to the oxo complexes. However, when a strong acid and a coordinating ligand are absent, the interconversion of di(p-oxo)nitridotechnetium(VI) complexes to the monomeric form occurs in the following complicated manner [35]... 

Holum, J.R. (1995). Elements of General Organic and Biological Chemistry (4th ed.). Wiley, New York Hornby, M. and Peach, J. (1993). Foundations of Organic Chemistry. Oxford University Press, Oxford Martell, A.E. and Hancock, R.D. (1996). Metal Complexes in Aqueous Solution. Plenum Publishing, New York... 

Yonemura et al. [72], Saito et al. [73], and Watanabe et al. [74] reported complex formation of a-CD with some charged derivatives of oligomethylene, but observed only the formation of one-to-one complexes in aqueous solutions, not of any crystalline one. 

Lopez-Nicolas JM, Bru R, Sanchez-Ferrer A and Garcia-Carmona F. 1995. Use of soluble lipids for biochemical processes linoleic acid cyclodextrin inclusion complexes in aqueous solutions. Biochem J 308 151-154. 

Fig. 2. Overview of stability constants (logio-Ka, on the molar scale) for formation of calcium complexes in aqueous solution, at (or close to) 298 K and in ionic strengths in the region of 0.1-0.15 M.

Schneider and coworkers have reported on the hydrolysis of BNPP by Pr(III) in the presence of the potentially dinucleating ligand 45 [66]. An aqueous solution of a 2 1 Pr(III)/45 complex which was prepared in organic solvent is 70 times more reactive toward BNPP than the metal salt alone at pH 7.0 and 323 K. The rate enhancement over spontaneous hydrolysis is 5 x 106-fold. The authors suggest cooperation of two metal ions, but there is no direct evidence for the presence of a dinuclear-Pr complex in aqueous solution. Catalytic turnover was not demonstrated. 

Halides other than fluoride form very weak complexes in aqueous solution there are no reliable equilibrium constants to be found in the literature. The solution chemistry of aqueous solutions of beryllium chloride, bromide, and iodide have been reviewed previously (9). Some evidence for the formation of thiocyanate complexes was obtained in solvent extraction studies (134). 

---