Stability Constants Of Coordination Compounds Research Articles

MAIN TRENDS AND DIFFERENCES IN THE COMPLEX FORMATION OF AMINO ACIDS WITH CALCIUM(II) AND MAGNESIUM(II) IONS ACCORDING TO OPTICAL SPECTROSCOPY DATA

The thermodynamic stability constants of coordination compounds of calcium(II) and magnesium(II) ions with amino acids in aqueous solutions at 298 K were calculated by spectrophotometry using the mathematical processing of experimental data. The main principles and approaches of mathematical processing are shown. Differences of calcium(II) and magnesium(II) ions as complexing amino acids as ligands are indicated, a regularity in the complex formation of amino acids of various structures is established.

Prediction of stability constants of coordination compounds from their molecular graphs

Every molecular structure can be regarded as a graph, and each graph can be transformed into characteristic scalar – topological index. There are many, over 1000 topological indices used by chemists, but they are not equally successful in solving specific problems. For the prediction of stability of coordination compounds valence molecular connectivity index of the 3rd order (3χv) proved best. Stability of these compounds was expressed as log K (K stays for stability constant), which is proportional to the Gibbs free energy of reaction between metal (or metal complex) and ligand. Various molecular graphs were checked, from a simple graph of ligand, to the graphs of various metal-ligand complexes (aqua complex, complex with additional bonds etc.) leading to conclusion that chemically sounder graphs generally yield better models. The best results were obtained for the systems of one metal, mesured at the same experimental conditions. However, our graph-theoretical approach enables development of models for the simulateneous prediction of stability constants of few metals.

Estimation of Stability Constants of Coordination Compounds using Models Based on Topological Indices

The principal aim of our research was to develop theoretical models for estimating stability constants of coordination compounds, focusing on chelates of heavy metals, Cu(II) and Ni(II), with bioligands (amines, amino acids and peptides). We tried to solve the problem using graph-theoretical models. We also encountered a specific problem for coordination compounds--the choice of the constitutional formula, that is graph, of the complex species.

Make sense of nanochemistry and nanotechnology

A class in a Scientific-Technological Lyceum (age 17) decided to produce a PowerPoint presentation to introduce nanochemistry and nanotechnology to the students in lower grades. Because the subject is very new, there was nothing in the School textbooks and, therefore, the students had to cooperate in order to find materials, to use ICT sources and to take decisions, such as selecting information and choosing slide layouts. Furthermore, the Cooperative Learning methodology was employed to solve the problem of setting up the presentation. To make nanochemistry and nanotechnology a reality for the students, they used a link between these new frontiers of Chemistry and subjects currently tackled at the Secondary School level. This was the quantitative determination of Ca2+ ions by using calcein, a luminescent chemosensor, in which well known concepts, such as chemical equilibrium and stability constants of coordination compounds, are involved. The educational aims of the project were to promote both content knowledge and social skills in Secondary School students. The activity created a good class atmosphere and also led to the retention of content knowledge.

Studies on stepwise stability constants of coordination compounds from sulfonated sheoak tannin with metal ions

The stepwise stability constants of coordination compounds from sulfonated sheoak tannin with metal ions have been determined by pH potential method with catechol as model compounds at 20°C and constant ionic strength.

The influence of medium on stability of coordination compounds: I. The system Cu(SCN)+-NaNO3-C2H5OH-H2O

The stability constants of Cu(SCN)+ in the titled system have been determined by spectro-photometric measurements at 25°C for 0, 5, 10, 15, 20 and 25 wt% of ethyl alcohol in mixed solvents. The total ionic strength of the solution are 0.2-2.0 mol·dm−3 and pH 1.5-1.6. On basis of Pitzer's theory, a method of determining thermodynamic stability constants of coordination compounds in mixed solvents by curve fitting technique have been proposed. The stability constants of Cu(SCN)+ in mixed solvents have been correlated with composition and dielectric constant of the solvent mixture.

The stability of coordination compounds of some crown ethers with lanthanide(III) ions

Moncyclic polyethers, the ‘crown ethers’, are noted for their affinity and high selectivity towards alkali and alkaline earth elements, and a large amount of work has been published concerning the coordination chemistry of crown ethers with these elements. However. information about the coordination behavior of crown ethers towards the lanthanides is relatively scarce and is badly needed because of its practical as well as theoretical significance. t001 Stability Constants, β, Coordination Compounds of Crown Ethers with Na + and Ln 3+ (Solvent: MeOH; Ionic Strength: 0.1 M Me 4NCl). Crown 18C6 DB18C6 26 ± 0.5 °C DCH18C6 26 ± 0.5 °C Logβ 15 ± 0.5 °C 26 ± 0.5 °C 35 ± 0.5 °C Izatt [2] Ion Na + 4.35 ± 0.05 4.17 ± 0.02 4.01 ± 0.05 4.33 4.18 ± 0.01 3.32 ± 0.01 La 3+ 3.21 +- 0.05 3.25 ± 0.01 3.31 ± 0.03 3.29 ± 0.03 no 2.49 ± 0.03 Ce 3+ 2.76 ± 0.02 2.81 ± 0.01 2.93 ± 0.04 3.57 ± 0.20 no 2.05 ± 0.05 Pr 3+ 2.36 ± 0.02 2.47 ± 0.01 2.57 ± 0.04 2.63 ± 0.28 no 1.62 +- 0.07 Nd 3+ 2.05 ± 0.11 2.16 ± 0.01 2.30 ± 0.09 2.44 ± 0.16 no <1.3 Sm 3 - <1.6 - 2.03 ± 0.07 no no Eu 3+ - <1.5 - 1.84 ± 0.14 no no Gd 3+ - <1.5 - 1.34 ± 0.12 no no Tb 3+ - <1.2 - no <1.5 no Dy 3+ - <1.2 - no <1.7 no Ho 3+ - <1.5 - no <1.7 no Er 3+ - <1.4 - no <1.5 no Tm 3+ - <1.3 - no <1.6 no Yb 3+ - no - no <1.4 no Lu 3+ - no - no <1.4 no t002 Thermodynamic Functions for the Coordination of 18-C-6 with Na + and Ln 3+ in Methanol at 25 °C Ion ΔG° (kcal/mol) ΔH° (kcal/mol) TΔS° (kcal/mol) Na + −5.72 −8.01 −2.3 La 3+ −4.47 2.30 6.7 Ce 3+ −3.89 3.84 7.7 Pr 3+ −3.38 4.45 7.8 Nd 3+ −2.96 6.14 9.1 A pNa method is developed for the determination of the stability constants of the coordination compounds of crown ethers and lanthanide ions, Ln 3+, based on the competition between Na + and Ln 3+ ions towards coordination with a given crown. It consists of setting up a cell using Na + as the competing ion, Na +-selective electrode as the indicating electrode and AgAgCl electrode as the reference electrode. The electromotive forces of such a cell (C = drown ether) AgAgCl¦0.1 M Me4NCl, x M Na +, y M C, z M Ln 3+¦ Na +-electrode are measured successively under those conditions: (1) only Na +, but neither crown nor Ln 3+ is present; (2) crown ether is present but not Ln 3+ ion is present; (3) both crown ether and Ln 3+ are present. From the electromotive forces measured, concentrations of free ions and then stability constants can be calculated. Details of this method were published in a short note [1]. With this method, stability constants of coordination compounds of 18-crown-6 (18-C-6), dibenzo 18-crown-6 (DB 18-C-6), dicyclohexyl 18-crown-6 (DCH 18-C-6), 15-crown-5 (15-C-5) and benzo 15-crown-5 (B 15-C-5) with all Ln 3+ ions except Pm 3+ in methanol are determined. The results are given in Table I and agree quite well with those reported by Izatt et al. [2] except for Ce 3+. It can be seen that for the lighter lanthanides, the stability constants of the coordination compounds with 18-C-6 or DCH 18-C-6 decrease with increasing atomic number of the lanthanides, a fact which can be explained by the better matching of the crown ether cavity size with cation diameter. The 18-C-6 also shows weak coordination with the heavier lanthanides whereas the DB 18-C-6 shows weak coordination only with the heavier lanthanides and no coordination is indicated for 15-C-5 or 15-C5 with any lanthanides. The stability constants of Na +, La 3+, Ce 3+ Pr 3+ and Nd 3+ are determined at three different temperatures for the determination of ΔH° calculated by plotting log β against 1/T, Δ° is calculated from the relation ΔG° = RTlnβ and then ΔS° calculated from the relation ΔG° = ΔH° - TΔS°. The results obtained are given in Table II. From the ΔH° and ΔS° values it can be seen that the observed stability is entropic in origin.

New method for the determination of the stability constants of coordination compounds

A method for the determination of the stability constants of very stable complexes of a metal with a ligand has been developed. In this method an antagonist ion is used to form complexes of known stability with the ligand. By measuring the e.m.f. of concentration cells the values of the stability constants for the silver-dipyridyl system are obtained. Using silver antagonist ions the stability constants for the cadmium-dipyridyl system are determined. The values obtained by this method agree with those obtained by different techniques and those reported by other authors. The values of the thermodynamics functions relative to various steps of formation of the silver-dipyridiyl and cadmium-dipyridyl complexes are reported.