Irving-Williams Order Research Articles

An infrared spectroscopic study of the metal binding in some valine chelates

The IR spectra of some metalvaline chelates have been investigated from 3600–250 cm −1 and empirical assignments for observed bands relevant to the chelation binding have been accomplished. The configurations of the chelates have been deduced. From the variations in the metalnitrogen stretching and NH 2 stretching frequencies, the periodic series trend in metalnitrogen bond strengths is shown to be Co(II)<Ni(II)<Cu(II)>Zn(II), in accord with the Irving-Williams order. The group variation, Zn(II)<Cd(II)<Hg(II), reflects an increase in metalnitrogen bond strengths with increasing softness of the metal ion. Using the frequency separation for the carboxylate stretching vibrations, the soft Hg(II) and Cd(II) are shown to bind relatively weakly with oxygen whereas the relatively hard Co(II) is more strongly bound.

Thermodynamic stabilities of cobalt(II), nickel(II), and copper(II) complexes of O2N2-donor macrocyclic ligands

Co-ordination compounds formed between two O2N2-donor macrocyclic ligands, one 14-membered (L1) and one 16-membered (L2), and H+, Co2+, Ni2+, and Cu2+ in 65% ethanol have been studied potentiometrically at 25.0 °C and I= 0.10 mol dm–3[N(CH3)4NO3]. For each of these ligands the formation constants of the bivalent metal ions follow the Irving–Williams order of stability. For each of the bivalent metal ions the stabilities follow the macrocyclic ring-size sequence 16 < 14. It can be deduced that an alternating sequence of five- and six-membered chelate rings increases complex stability.

The effect of metal ions on mitochondrial pyridine dinucleotide transhydrogenase

Bovine heart submitochondrial particle transhydrogenase is inhibited by cations in a concentration and pH-dependent manner, and non-energy-linked transhydrogenation is inhibited to a greater extent by metals than the energy-linked reaction. The inhibition of the enzyme by Mg2+ is competitive with the NADP substrate and non-competitive with the NAD substrate. Mg2+ stimulates inactivation of the enzyme by 5,5′-dithiobis(2-nitrobenzoic acid), and protects against thermal and proteolytic inactivation. This suggests that Mg2+ binding in the NADP site alters transhydrogenase to a more thermostable conformation, which is less susceptible to attack by trypsin and more reactive with 5,5′-dithiobis(2-nitrobenzoic acid). Other cation inhibitors mimic Mg2+ in these properties. The order of effectiveness of the inhibitors tested is La3+ > Mn2+ > Ca2+ ⋍ Mg2+ > Sr2+ > Na+ ⋍ K+. This order is described by the Irving-Williams order for the stability of metal-ligand complexes, suggesting that carboxylates or amines may comprise the inhibitory cation binding site.

Pyridine base adducts of manganese(II) and iron(II) complexes of TTA and STTA

Octahedral high-spin adducts of the type ML 2B 2, where M = Mn(II) and Fe(II), L = thenoyltrifluoroacetone and monothiothenoyltrifluoroacetone and B = pyridine and γ-picoline, were isolated in the atmosphere and the properties of these adducts were investigated to clarify the effect of the sulfur-coordination. The assignments of the metal-ligand stretching bands in IR spectra were performed. The frequencies of these bands followed the Irving-Williams order in most cases and suggested the soft character of Fe(II) to sulfur atom. The values of isomer shift and quadrupole splitting of Mössbauer spectra in Fe(II) complexes of STTA revealed the contribution of π back-donation to a great extent.

Potentiometric studies in transition metal ion ( d5− d10) chelates of substituted salicylhydroxamic acids

The interactions of transition metal ions (Mn 2+Zn 2+) with salicylhydroxamic acid (SHA) and 5-methyl, 5-chloro, 5-bromo, 5-nitro, 4-chloro, 4-bromo and 3-chloro salicylhydroxamic acids were investigated potentiometrically by the Calvin-Bjerrum titration technique at 30±0.1°C and ionic strength μ = 0.1 M(NaClO 4) in 50% v v dioxane-water mixtures. It was observed that all the metal ions form only 1:1 chelates with these ligands. The dissociations of the -NH and the -OH groups are observed potentiometrically. The NOH group does not dissociate even at very low concentrations of hydrogen ions. In SHA and its derivatives, p K 1 is ascribed to the dissociation of the -NH group and p K 2 to the dissociation of the phenolic -OH group. Arguments are put forward for these assignments. The effects of various substituting groups on the p K 1 and p K 2 values are discussed. Irving-Williams order is not completely obeyed, though the latter part of the order, i.e. Ni 2+ < Cu 2+ > Zn 2+ is followed by the chelates. Explanations are suggested. The validity of the log K = a p K + b relation is examined for these chelates and the significance of dπ − pπ interaction in the case of Cu 2+-5-nitro-SHA chelate is discussed.

Stability constants of some bivalent first row transition metal chelates of dicarboxylic acids containing ether linkages

The stability constants of the Mn 2+, Fe 2+, Co 2+, Ni 2+, Cu 2+ and Zn 2+ complexes with four acids of the type HOOCCH 2O(CH 2CH 2O) n CH 2COOH ( n = 0–3) are reported. For n = 0 the constants are in the Irving-Williams order, but not for n = 1–3.

Thiamine pyrophosphate-metal ion complexation: equilibrium and kinetics

Abstract 1. 1. Acid dissociation constants of thiamine pyrophosphate have been determined from pH titration data at 25°C in 0.1 M KNO3. 2. 2. Stability constants of the metal ion-thiamine pyrophosphate complexes MTPP and MHTPP+, where M is Mg2+, Mn2+, Cu2+, Ni2+, Cu2+, or Zn2+, have been determined from pH titration data at 25°C in 0.1 M tetramethylammonium bromide utilizing a Newton-Raphson algorithm to solve simultaneous mass balance equations and a reiterative nonlinear least-squares approach to compute the constants. The stabilities of these complexes do not follow the Irving-Williams order, but instead are typical of the order generally found with phosphate ligands. 3. 3. The kinetics of complexation of Ni2+ and Mg2+ with thiamine pyrophosphate have been studied using the temperature-jump method. The concentration dependence of the relaxation times observed is not consistent with the common assumption that the concentration of the intermediate ion-pair fulfills the steady-state conditions, but is consistent with the more general expression of the two-step mechanism of complex formation. For the reaction, M(H 2 O ) 2+ n + L 2− ( (aq.) ) ⇌ K 12 M(H 2 O) n , L ⇌ K 32 K 23 M(H 2 O) n−x L +x H 2 O , at 25°C and an ionic strength of 0.1 M (tetramethylammonium bromide), the results are: M = Ni, k23 = 4.2·103 (± 6%) s−1, k32 = 250 s−1, K12 = 240 M−1; M = Mg, k23 = 7.5·104 (± 12%)s−1, k32 = 2400 s−1, K12 = 87 M−1; with the probable experimental errors being three times the reported standard deviations. 4. 4. These results are viewed as being in agreement with rate-determining loss of a water molecule from the inner coordination sphere of the metal ion. Nevertheless, for those cases in which the rate of water dissociation from the intermediate ion-pair is known experimentally, this rate invariably is slightly less than the rate of solvent exchange. This effect is examined and its implications are discussed. 5. 5. For systems where calculation of the fraction of ion-pair using experimentally determined values is possible, it can be concluded that the greater the charge at the binding site, the more predominant are the inner sphere complexes.

The Effect of Aromatic Amines on the Catalyzed Decarboxylation of 3-Oxo-glutaric Acid

Equilibrium constants have been measured for the formation of MA species, where M is divalent manganese, cobalt, nickel, or zinc, and A2− is the 3-oxo-glutarate ion. Equilibrium constants have also been measured for the reactions [Formula: see text], where B is 2,2′-bipyridyl or 1,10-phenanthroline. These constants were obtained by pH measurements at 25 °C and an ionic strength of 0.60. The results are compared with those for similar systems, especially as regards the tendency to form ternary mixed complexes of the type MAB.The rates of decarboxylation of the various MA and MAB species have been measured. The resulting rate constants follow the Irving–Williams order for stability constants, except that in both MAB species the cobalt compound decomposed slightly faster than the nickel compound. The aromatic base, which had been found earlier to have very little effect in copper complexes, increased the rate constants appreciably with other metals, especially with manganese and zinc. Usually 1,10-phenanthroline has a larger effect than 2,2′-bipyridyl. Some possible explanations of the results are considered.

O-(2-Thiazolylazo)-4-Chlorophenol Complexes and Their Utility as Metallo-Chromic Indicators for Nickel(II) Ion

Abstract The stability constants of the complexes formed between o-(2-thiazoiylazo)-4-chlorophenol (TACL) and bivalent metal ions obtained spectrophotometrically were: logKMnR = 2.86, logKCoR = 5.37, logKNiR = 6.12, logKNiR 2 = 5.44, logKCuR = 8.39, logKZnR = 4.54, logKZnR 2 = 4.43 and logKCuR = 5.65, (20°c, μ = 0.1, 10% v/v aqueous dioxane). These values coincided well with the Irving-Williams order. The acid dissociation constants of analogous new dyestuffs synthesized in our Laboratory, o-(2-thiazolylazo)-phenol, o-(2-thiazolylazo)-4-bromophenol and o-(2-thiazoly-lazo)-4- iodophenol were 7.36, 7.35, and 7.25, respectively (20°C, μ = 0.1, 10% v/v aqueous dioxane). TACL was found to be applicable as a metallochromic indicator for the titration of nickel(II) in weakly acidic media using EDTA as a titrant. No heating was required.

Thermodynamic considerations in co-ordination. Part VII. Solubility of the histidine–H+system and stability constants, free energies, enthalpies, and entropies of protonation of histidine and tryptophan and of formation of their manganese(II), iron(II), cobalt(II), nickel(II), copper(II), and zinc(II) complexes

Acidic or basic perchlorate solutions of histidine precipitate histidine hydroperchlorate (histidine HCIO4,H2O), solubility product –log Ks= 17·65 in 3M-perchlorate at 25·0°. Tryptophan has higher pK values than histidine in this medium. Near pH 7·4 (the pH of blood) the formation constants indicate that the complexes of MnII, FeII, CoII, and NiII with histidine are tridentate and that those with tryptophan are bidentate. The zinc–histidine complex is less stable than that of cobalt, whereas for all other amino-acids the reverse is true. Calorimetry shows that the main driving force behind metal ion–amino-acid complex formation is an enthalpy effect, the bond strengths obeying the Irving–Williams order. It is suggested that the zinc ion is tetrahedral, the carboxy-group not being chelated.

Determination and comparison of the stability constants of some metal complexes of 8-hydroxyquinoline N-oxide and 8-hydroxyquinoline

The stability constants of 8-hydroxyquinoline N-oxide (or "oxine N-oxide") complexes with MnII, Fell, CoII, Kill, CuII, ZnII, CdII, UO211, AlIII, FeIII, YIII, LaIII, CeIII, PrIII, NdIII, SmIII, GdIII, DyIII, HoIII, ErIII, TmIII, YbIII, and LuIII, and oxine complexes with LnIII and YIII have been determined using pH-metric titrations in 50% v/v aqueous dioxan in the presence of 0.30M sodium perchlorate as the constant ionic medium. With both the ligands, the "gadolinium break" is observed in the case of the lanthanon complexes, whereas the bivalent ion complexes follow the Irving-Williams order of stability constants in the case of the oxine complexes. The stability constants of the oxine complexes have been found to be larger than those of the N-oxide complexes, though the oxine anion is only five times as basic as the N-oxide anion. A possible explanation for the increased stability of the oxine complexes in comparison with the corresponding N-oxide derivative is proposed.

The estimation of the stabilities of bivalent transition metal complexes and deviations from the Irving-Williams order

The free energies of formation of a range of ligands with Mn II Fe II, Co II and Zn II have been related to ligand field stabilisations, and it has been found possible to predict on theoretical grounds deviations from the Irving-Williams order.