Metal complexes—continued stability constants

L. G. SiLLfiN and A. E. Martell, Stability Constants of Metal-ion Complexes, The Chemical Society, London, Special Publications No. 17, 1964, 754 pp., and No. 25, 1971, 865 pp. Stability Constants of Metal-lon Complexes, Part A. Inorganic Ligands (E. Hcigfeldt, ed.), 1982, pp. 310, Part B. Organic Ligands (D. Perrin, ed.), 1979, pp. 1263. Pergamon Press, Oxford. A continually updated database is now provided by L. D. Pettit and K. J. Powell (eds.), IVPAC Stability Constants Database, lUPAC and Academic Software. 

The reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... 

Titration curves of HS fluorescence quenching versus concentration of added metal quencher have been used to obtain the CC values of HS ligands and the stability constants of HS-metal complexes (Saar and Weber, 1980, 1982 Underdown et al., 1981 Ryan et al., 1983 Weber, 1983 Dobbs et al., 1989 Grimm et al., 1991 Hernandez et al., 2006 Plaza et al., 2005, 2006). Two fluorescence techniques, lanthanide ion probe spectroscopy (LIPS) and fluorescence quenching of HSs by Cu-+, have been used in conjunction with a continuous distribution model to study metal-HS complexation (Susetyo et al., 1991). In the LIPS technique, the HS samples are titrated by Eu-+ ions, and the titration plot of the ratio of the intensities of two emission lines of Eu + is used to estimate the amount of bound and free species of the probe ion. In the other technique, titration curves of fluorescence intensity quenched by Cu versus the logarithm of total added Cu2+ are used. 

The complexation of the Group IIB metals by amino-acid and related derivatives continues to be a subject of interest stability constants reported include those for histidine and its derivatives, histamine, glycylhistamine, aspartic and glutamic acids, aspargine, glutamine, glycine, cysteine, and alanine. " ... 

Whilst the synthesis of new transition metal-olefin and -acetylene complexes continues unabated, only a relatively small amount of data has accumulated on the thermodynamic stability of these complexes and these are restricted almost exclusively to complexes of the unsatured species acting as monodentate ligands. Metals able to coordinate strongly with unsaturated ligands are restricted to those in a small triangle around the centre of the periodic table, and designated class (b) acceptors by Ahrland et al., 0>. Class (b) acceptors include Cu(I), Rh(II), Ag(I), Pt(II) and Hg(II). However the majority of such metals form inert complexes which are either very readily oxidised or involve solubility problems. If thermodynamic stability constants are to be measured reliably, the equilibrium should be reached reasonably quickly, the reaction should be clean and the stoichiometry should be known or easily deduced. Furthermore, the equilibrium must be followed by means of suitable electrodes or changes in some physical property of the reaction mixture. The solvent is therefore important. 

Data on the stability constants of metal complexes are continually reported in the scientific literature. A recent collection is presented in Martell, A. E and Smith, R. M. Critical Stability Constants (Plenum Press, New York, 1982). 

The development of routine and easy handling procedures for continuous and real-time speciation of trace metals in waters has led, in the last years, to the development of microsensors coupled to voltammetric techniques. Microelectrodes offer several advantages for speciation measurements in real-world samples, including their application in low ionic strength media (e.g., freshwaters), reproducibility, and sensitivity. Some Cd speciation studies carried out in river waters, heavily loaded with suspended material, using microelectrodes demonstrated that most of Cd was associated with colloidal material. In addition, this technique also enables the determination of the corresponding complexation stability constants for Cd and protons. 

Commission of the International Union of Pure and Applied Chemistry began to publish a series of critically evaluated stability constants (37). A great number of families of metal complexes were treated in this continuing series. (To date 12 reviews were published in either separate volumes or in Pure and Applied Chemistry.) Critically selected stability constants are offered in the volumes prepared by Martell and Smith (38). 

---