1:1 Metal Complexes of Bivalent Cobalt,Nickel, Copper,Zink, and Cadmium with the Tripodal Ligand tris[2-(dimethylamino)ethyl]amine: Their stabilities and the X-ray crystal structure of its copper(II) complex sulfate

The complex formation by Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺ with tris[2-(dimethylamino)ethyl]amine (N(CH₂CH₂NMe₂)₃, Me₆tren) was investigated at 25° and at an ionic strength of 1, using VIS spectroscopy and potentiometric measurements. The stability constants of these complexes are compared with those of tris(2-aminoethyl)amine (N(CH₂CH₂NH₂)₃, tren), obtained under the same conditions. The values of the constants for Me₆tren are much lower than those for tren, due to the bulky Me substituents. The values of the constants can be correlated with the ability of the individual metal ions to adopt coordination number 5. This appears to be easier for Cu²⁺ and Co²⁺ than for Cd²⁺ and Zn²⁺ and is very difficult for Ni²⁺. The 1:1 complexes [ML(H₂O)]²⁺ are monoprotonic acids whose pK_s values are similar or lower than those of the corresponding aquametal ions. The X-ray crystal structure of the copper(II) complex [Cu(SO₄)(Me₆tren)] · 8H₂O reveals pentacoordination at the central ion. The UV/VIS spectra of the aqueous solutions of the Co²⁺, Ni²⁺, and Cu²⁺ 1:1 complexes confirm that the same coordination number is present also in these complexes.     

Dissociation and coordinating ability of 1,2,3,4-tetrahydro-8-hydroxyquinoline in aqueous solution

The state of 1,2,3,4-tetrahydro-8-hydroxyquinoline is studied spectrophotometrically in the near-UV and visible regions at different pH in an aqueous solution in the presence and absence of Cu²⁺, Zn²⁺, and Cd²⁺ cations. The dissociation constant (pK ₁ ≈ 5.6 (293 K)) is (293 K)) is estimated from the pH-metric data. The stability of the complexes formed decreases in the series Cu²⁺ > Zn²⁺ ≥ Cd²⁺. The influence of tetrahydro atoms on the dissociation, complexation, and change in the state of 1,2,3,4-tetrahydro-8-hydroxyquinoline in an aqueous solution is discussed.     

Copper(II) Complexes with Chiral Diaminodiamido Ligands: Solution and Structural Studies

Abstract

Three diaminodiamido ligands (S,S)-N,N′-bis(prolyl)ethanediamine (ProNN-2), (S,S)-N,N′-bis(N-methylvalyl)ethanediamine (Me₂ValNN-2), and (S,S)-N,N′-bis(N-methylphenylalanyl)-ethanediamine (Me₂PheNN-2) were synthesised and their complex formation equilibria with copper(II) investigated in aqueous solution by potentiometry and, for ProNN-2, by electronic spectrophotometry. ProNN-2 forms the species [CuLH]³⁺, [Cu₂L₂]⁴⁺, [Cu₂L₂H_?2]²⁺ and [CuLH_?2], Me₂PheNN-2 forms the complexes [CuLH]³⁺, [Cu₂L₂H_?2]²⁺ and [CuLH_?2], whereas Me₂ValNN-2 forms the monomer [CuLH_?1]⁺ but not the dimer. The dimeric cation [Cu₂L₂H_?2]²⁺, of Me₂PheNN-2 has severe steric requirements, as demonstrated by the X-ray crystal structure of the complex [Cu₂L₂H_?2]Cl₂· 12H₂O, of the corresponding non-methylated ligand. Since copper(II) complexes of the ligands examined are used as additives to the mobile phase to perform chiral resolution of D,L-amino acids in RP-HPLC, the present results provide valuable clues to an understanding of the mechanism of the enantiomeric separation.     

The effect of heavy metals on the anthracene–Me-β-cyclodextrin host–guest inclusion complexes

Anthracene was used to form an inclusion complex with methylated-β-cyclodextrin (Me-β-CD) in water. In aqueous Me-β-CD solution, typical fluorescence emission of anthracene was observed. Benesi–Hildebrand's method was used to obtain the stoichiometry of the anthracene–Me-β-CD complex. The Stern–Volmer quenching constants, K_sv, and fluorescence quantum yields were calculated according to changes in the fluorescence emission intensity of anthracene–Me-β-CD inclusion complexes by adding various amounts of Pb²⁺ and Cd²⁺ salts in water. The K_sv values and fluorescence quantum yields indicate that Pb²⁺ salts quench the anthracene–Me-β-CD inclusion complexes more efficiently than Cd²⁺ salts.     

Formation and Stability of Binary Complexes of Divalent Ecotoxic Ions (Ni, Cu, Zn, Cd, Pb) with Biodegradable Aminopolycarboxylate Chelants (dl-2-(2-Carboxymethyl)Nitrilotriacetic Acid, GLDA, and 3-Hydroxy-2,2′-Iminodisuccinic Acid, HIDS) in Aqueous Solutions

The protonation and complex formation equilibria of two biodegradable aminopolycarboxylate chelants {dl-2-(2-carboxymethyl)nitrilotriacetic acid (GLDA) and 3-hydroxy-2,2??-iminodisuccinic acid (HIDS)} with Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ ions were investigated using the potentiometric method at a constant ionic strength of I?=?0.10?mol·dm^?3 (KCl) in aqueous solutions at 25?±?0.1?°C. The stability constants of the proton?Cchelant and metal?Cchelant species for each metal ion were determined, and the concentration distributions of various complex species in solution were evaluated for each ion. The stability constants (log₁₀ K _ML) of the complexes containing Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ ions followed the identical order of log₁₀ K _CuL?>?log₁₀ K _NiL?>?log₁₀ K _PbL?>?log₁₀ K _ZnL?>?log₁₀ K _CdL for either GLDA (13.03?>?12.74?>?11.60?>?11.52?>?10.31) or HIDS (12.63?>?11.30?>?10.21?>?9.76?>?7.58). In each case, the constants obtained for metal?CGLDA complexes were larger than the corresponding constants for metal?CHIDS complexes. The conditional stability constants (log₁₀ $ K_{\text{ML}}^{'} $ ) of the metal?Cchelant complexes containing GLDA and HIDS were calculated in terms of pH, and compared with the stability constants for EDTA and other biodegradable chelants.     

Synthesis, characterization and liquid–liquid extraction properties of new methoxyaminobiphenylglyoxime derivatives and their complexes with some transition metals

In this study, three new aminobiphenylglyoximes, [L¹H₂] N-(2-methoxy)aminobiphenylglyoxime, [L²H₂] N-(3-methoxy)aminobiphenylglyoxime and L[³H₂] N-(4-methoxy)aminobiphenylglyoxime have been synthesized by the reaction of (E,E)-4′-biphenylchloroglyoxime with 2-Methoxyaniline, 3-Methoxyaniline and 4-Methoxyaniline in absolute ethanol. The preparation Ni^II, Co^II and Cu^II complexes of these ligands are described. The ligands and their complexes were characterized by elemental analyses, IR, mass, H¹ and ¹³C NMR spectra, thermogravimetric analyses (t.g.a) and magnetic susceptibility measurements. Ligands complexing properties were studied by the liquid–liquid extraction of selected alkali (Li⁺, Na⁺, K⁺, Cs⁺) and transition metals (Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺). It has been observed that all ligands show a high affinity to Cu²⁺ ions, whereas almost no affinity to alkali metals. The extraction equilibrium constants (K _ex) for complexes of ligands with Cu²⁺ metal picrates between dichloromethane and water have been determined at 25°C.     

Spectral, Magnetic, Electrical and Thermal Studies on Malonyl bis(thiosemicarbazide) Complexes

Mono- and binuclear complexes of malonyl bis(thiosemicarbazide), H₄ MBT, with VO²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺ and Pt⁴+ have been isolated. The elemental analyses, magnetic moments, spectra [u.v.–vis., i.r., e.s.r. (for Cu²⁺; VO²⁺) and mass], thermal and voltammetric measurements [for Co²⁺ and Ni²⁺] have been used to characterize the isolated complexes. The ligand behaves as binegative quadridentate with Cu²⁺, Co²⁺ and VO²⁺ ions, binegative pentadentate with Pt⁴+ and hexadentate, either as trinegative in [Cd₂(HMBT)(OC₂H₅)(C₂H₅OH)]H₂O or mononegative in [Ni₂(H₃MBT)(OAc)₃(C₂H₅OH)]. The lack of thiol and/or enol hydrogen during the complex formation was confirmed pH-metrically. The pK’s (10.70, 8.50 and 8.15) of H₄MBT reveal the removal of CSNH protons in one step and CONH in two steps. Also, the stability constants reveal a higher value for the Cu²⁺ complex and a lower one for VO²⁺. The TG analyses suggest high stability for most complexes followed by thermal decomposition in different steps. The kinetic and thermodynamic parameters for some decomposition steps in VO²⁺and Pt⁴⁺ thermograms have been calculated     

Na+/K+-ATPase: Activity and inhibition

The aim of the study was to give an overview of the mechanism of inhibition of Na⁺/K⁺-ATPase activity induced by some specific and non specific inhibitors. For this purpose, the effects of some ouabain like compounds (digoxin, gitoxin), noble metals complexes ([PtCl₂DMSO₂], [AuCl₄]⁻, [PdCl₄]²⁻, [PdCl(dien)]⁺, [PdCl(Me₄dien)]⁺), transition metal ions (Cu²⁺, Zn²⁺, Fe²⁺, Co²⁺), and heavy metal ions (Hg²⁺, Pb²⁺, Cd²⁺) on the activity of Na⁺/K⁺-ATPase from rat synaptic plasma membranes (SPM), porcine cerebral cortex and human erythrocytes were discussed. The article is published in the original.     

过渡金属取代的钨铝杂多配合物的制备、表征和性质

用分步设计法合成了以铝为中心原子的过渡元素与钨的三元杂多配合物,经ICP、TG分析确定其化学式为K_4，6，7[AlM(OH₂)W₁₁O₃₉]·xH₂O(M=V⁵⁺、Cr³⁺、Mn²⁺、Fe³⁺、Co²⁺、Ni²⁺、Cu²⁺、Zn²⁺、Cd²⁺)。采用IR、UV、XRD、¹⁸³W和²⁷Al NMR等对配合物的结构进行了表征,表明配合物具有Keggin结构；循环伏安法对该系列配合物的氧化还原性质研究表明,其氧化还原过程为不可逆的两电子还原,配合物的磁化率测试均表现为顺磁性,还对其热稳定性进行了讨论。     

Synthesis of a tetrazine-based catecholamide derivative and its evaluation as a chelating agent for removal of Cd(II), Co(II), and Cu(II)

The synthesis and structural characterization of a tetrazine-based catecholamide (CAM) ligand, N,N′-bis(N″-(aminoethyl)-2,3-bis(hydroxy)benzamide)-1,2,4,5-tetrazine-3,6-diamine (5), were investigated. All compounds were characterized by ¹H NMR spectroscopy, ¹³C NMR spectroscopy, and FTIR spectroscopy. The protonation equilibria of 5 and complexation capacities (log β_pqr) of Cd²⁺, Co²⁺, and Cu²⁺ complexes of 5 were evaluated through potentiometric titration and spectrophotometric titration, respectively. Species independent pM value (=?log [M]_free) was used to compare metal affinities with the final sequence Cu²⁺ > Cd²⁺ > Co²⁺. Results show that 5 has potential for heavy metal removal.     

Azidinium-Salze. 25. Mitteilung. Symmetrische Azopyridinon-Farbsalze: Synthese,spektrophotometrische und acidobasische Eigenschaften,Metallkomplex-Bildung und kinetische Untersuchung der Azo-Kupplung

Symmetrical Azopyridinone Dyes: Synthesis, Spectrophotometrical and Acidobasic Properties, Metal-Complex Formation and Kinetical Investigation of the Azo-Dye Formation The 13 substituted symmetrical azopyridinone dyes 2a–n were synthesized and their VIS spectra measured. The pK*_a values of some dyes and of pyridinone coupling components were determined in MeOH/H₂O 64:36. The metal-complex formation of the dyes with Cd²⁺, Zn²⁺, Co²⁺, Ni²⁺, and Cu²⁺ was investigated, and complex-formation constants of the 1:1 complexes were determined in H₂O for 2k (Table 2) and in dimethylformamide/H₂O 1:1 for some other azopyridinone dyes (Table 3). The mechanism of the azo-dye formation was investigated and found to be much more complicated than expected. A mechanism of the coupling reaction was developed (see Scheme 4) and fitted by least-squares calculations.     

Complexing Properties of Phenolic Diazacrown Ethers with Transition and Heavy Metal Ions

The complexation equilibria of the phenolic diazacrown ether derivatives L1–L11 with transition and heavy metal ions (Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺) have been studied in methanol using UV absorption spectrophotometry. A majority of the systems studied formed only ML complexes. Using a ligand with a different position of the substituents on the phenolic side arms (denoted L7) leads to ML₂ formation with most of the metal ions. Every ligand forms very strong ML and ML₂ complexes with Pb²⁺, and, in nearly all cases, only a lower limit could be derived for the stability constant. The stability of the complexes generally increases as the length of the para-substituents on the phenol groups increases. Among the metal ions tested, Zn²⁺ and Hg²⁺ are the least preferred by alkyl and alkoxy derivatives, respectively.     

Competitive heavy metal removal by poly(2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid‐co‐itaconic acid)

The competitive removal of Pb²⁺, Cu²⁺, and Cd²⁺ ions from aqueous solutions by the copolymer of 2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid (AMPS) and itaconic acid (IA), P(AMPS‐co‐IA), was investigated. Homopolymer of AMPS (PAMPS) was also used to remove these ions from their aqueous solution. In the preparation of AMPS–IA copolymer, the molar percentages of AMPS and IA were 80 and 20, respectively. In order to observe the changes in the structures of polymers due to metal adsorption, FTIR spectra by attenuated total reflectancetechnique and scanning electron microscopy (SEM) pictures of the polymers were taken both before and after adsorption experiments. Total metal ion removal capacities of PAMPS and P(AMPS‐co‐IA) were 1.685 and 1.722 mmol Me²⁺/g_polymer, respectively. Experimental data were evaluated to determine the kinetic characteristics of the adsorption process. Competitive adsorption of Pb²⁺, Cu²⁺, and Cd²⁺ ions onto both PAMPS and P(AMPS‐co‐IA) was found to fit pseudo‐second‐order type kinetics. In addition, the removal orders in the competitive adsorption of these metal ions onto PAMPS and P(AMPS‐co‐IA) were found to be Cd²⁺ > Pb²⁺ > Cu²⁺ and Pb²⁺ > Cd²⁺ > Cu²⁺, respectively. Copyright © 2008 John Wiley & Sons, Ltd.     

Spectroscopic Characterization of <Emphasis Type="Italic">N,N-bis</Emphasis>(2-{[(2,2-Dimethyl-1,3-Dioxolan-4-yl)Methyl]Amino}Ethyl) N′,N′-Dihydroxyethanediimidamide and Its Complexes

A new dioxime ligand, N,N-bis(2-{[(2,2-dimethyl-1,3-dioxolan-4-yl)methyl]amino} ethyl)N′,N′-dihydroxyethanediimidamide (H₂L), and its mononuclear complexes with Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ are synthesized. H₂L forms transition metal complexes [Co(LH)₂(H₂O)₂] and [M(LH)₂] (M = Ni²⁺, Cu²⁺) with a metal : ligand ratio of 1 : 2. Complexes [M(H₂L)(Cl)₂] (Zn²⁺, Cd²⁺) have a metal : ligand ratio of 1 : 1. The mononuclear Co²⁺, Ni²⁺, and Cu²⁺ complexes indicate that the metal ions coordinate ligand through its two N atoms, as the most of dioximes. In the Co²⁺ complex, two water molecules and in the Zn²⁺ and Cd²⁺ complexes two chloride ions are also coordinated to the metal ion. The structures of these compounds are identified by elemental analyses, IR, ¹H and ¹³C NMR, electronic spectra, magnetic susceptibility measurements, conductivity, and thermogravimetric analysis.__________From Koordinatsionnaya Khimiya, Vol. 31, No. 7, 2005, pp. 540–544.Original English Text Copyright © 2005 by Canpolat, Kaya.The text was submitted by the authors in English.     

Statistical evaluations of single-ion Gibbs energies of transfer

A statistical approach for the evaluation of single-ion Gibbs energies of transfer of the cations Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Ba²⁺, Ag⁺, Tl⁺, Cu⁺, Zn²⁺, Cu²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ into 40 solvents based on the principal component analysis is presented. It is shown that the Gibbs energies of transfer depend both on the nature of the cation and on the donor site of the respective solvent molecule. Correlation of the data for the investigated cations required separating the solvents into subgroups according to their donor atoms in the solvent molecule. Gibbs energies of transfer into oxygen donor solvents could be correlated with the Born term [N _L(z _i e ₀)²/(8πε₀ r _i )]. Several cation parameters were investigated with respect to the transfer data into nitrogen and sulfur donor solvents. No correlations were found. Thus the use of cation parameters derived from the statistical analysis are proposed to account for the Gibbs energies of transfer into nitrogen and sulfur donor solvents. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 1, pp. 9–17. The text was submitted by the authors in English.     

Synthesis and properties of potassium salts of per-<Emphasis Type="Italic">O</Emphasis>-carboxymethyl-calix[4]pyrogallols and their complexes with Cu<Superscript>2+</Superscript>, Fe<Superscript>3+</Superscript>, and La<Superscript>3+</Superscript>

Synthesis of water-soluble potassium salts of carboxymethyl derivatives of calix[4]pyrogallols and dodeca(carboxylatomethyl)tetramethylcalix[4]pyrogallol (L) complexes with transition metal ions (Cu²⁺, Fe³⁺, La³⁺) is described. Their structures in the solid state and in solution were characterized by NMR spectroscopy, ESR, and IR spectroscopy. Calix[4]pyrogallol dodecacarboxylates exist in the rccc-configuration. Calix[4]pyrogallol with methyl substituents at the lower rim in a wide range concentrations exists in water predominantly in the dimeric form. The obtained polynuclear transition metal complexes possess less symmetric structure than potassium salt of calix[4]pyrogallol (K₁₂L). All studied complexes contain water molecules bound by rather strong hydrogen bonds. At room temperature the Fe₄L complex is characterized by the environment of the Fe³⁺ ions close to octahedral. The absence of signals in the ESR spectrum of the Cu₆L complex indicates the strong antiferromagnetic interaction Cu²⁺-Cu²⁺.     

Metal complexes of macrocyclic ligands. Part XXXIII. Effect of the side-chain length on the on/off equilibrium in the Ni2+ and Cu2+ complexes of 11-(2-aminoethyl)- and 11-(3-aminopropyl)-1,4,7,11-tetraazacyclotetradecane

Alkylation of 1,4,7-tritosyl-1,4,7,11-tetraazacyclotetradecane with CH₂O/KCN or acrylonitrile gave the corresponding cyanomethyl or 2-cyanoethyl derivatives, which, by treatment with Na in BuOH, were detosylated and reduced to the macrocyclic pentaamines 1 and 2 . The Ni²⁺ and Cu²⁺ complexes with 1 and 2 show a reversible pH-dependent change in geometry, in which the side-chain amino group is either coordinated to the metal ion (basic form) or protonated and, thus, non-coordinated (acidic form). The length of the side chain determines the log K_a values of these protonation/deprotonation equilibria: 1.89 and ～ 2.0 for the Cu²⁺ and Ni²⁺ complexes of ligand 1 with the 2-aminoethyl side chain, and 6.17 and 7.43 for the Cu²⁺ and Ni²⁺ complexes of ligand 2 with the 3-aminopropyl side chain.     

The Synthesis of New Triazole Ligands and Determination of Complex Stability Constants with Transition Metal Cations in Aqueous Media

Six new triazole compounds were synthesized. These compounds containing the substituted benzylidenamino group were obtained by reaction of 3-(pyridine-4-yl)-5-p-tolyl-4-amino-4H-l,2,4-triazole 1 with the corresponding aldehyde. The reduced forms were prepared with NaBH₄ in methanol. The structures of the compounds were determined by IR, ¹H NMR, and ¹³C NMR spectral data, and their interaction with cations such as Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Ba²⁺, Sr²⁺, Ca²⁺, Cu²⁺, Cr³⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Ag⁺ were investigated by using UV-visible spectrophotometry. Of the tested metal cations, Cu²⁺, Cr³⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Ag⁺ complexed with the ligands. The complex stability constants (log ₁₀ K) were measured in slightly acidic aqueous media at 25.0±0.1 °C. These stability constants were determined by measuring the increase in solubility of the nearly insoluble ligand molecule due to complex formation with a soluble cation, and this method is discussed. It was found that the position of chlorine atoms on the benzene ring strongly affects the complexation of Cu²⁺ ion with these ligands.     

Ion-pair extraction of transition metal cations from aqueous media using novel N2O2-macrocyclic crown ligands

Three new macrocyclic crown ether ligands containing nitrogen–oxygen donor atoms were designed and synthesized from 1,4-bis(2′-formylphenyl)-1,4-dioxabutane and 4-nitro-o-phenylenediamine. Ion-pair extraction of metal picrates such as Ag⁺, Hg²⁺, Cd²⁺, Zn²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Co²⁺, and Pb²⁺ from aqueous phase to the organic phase was carried out using the novel ligands. The solvent effect over the metal picrate extractions was investigated at 25 ± 0.1 °C by using UV–visible spectrometry. The extractability and the values of the extraction constants (log K_ex) were determined for the extracted complexes.     

Metal-ion-coordinating properties of various phosphonate derivatives,including 9−[2−(phosphonylmethoxy)ethyl]adenine (PMEA) – an adenosine monophosphate (AMP) analogue with antiviral properties

The stability constants of the 1:1 complexes formed between Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, (in part) Zn²⁺, or Cd²⁺ and (phosphonylmethoxy)ethane (PME^2?) or 9?[2?(phosphonylmethoxy)ethyl]adenine (PMEA^2?) were determined by potentiometric pH titration in aqueous solution (I = 0.1M , NaNO₃; 25°). The experimental conditions were carefully selected such that self-association of the adenine derivative PMEA and of its complexes was negligibly small; i.e., it was made certain that the properties of the monomeric [M(PMEA)] complexes were studied. Recent measurements with simple phosphate monoesters, R–MP^2– (where R is a non-coordinating residue; S. S. Massoud, H. Sigel, Inorg. Chem. 1988 , 27, 1447), were used to show that analogously simple phosphonates (R? PO) – we studied now the complexes of methyl phosphonate and ethyl phosphonate – fit on the same log K/logK vs. pK/ pK straight-line plots. With these reference lines, it could be demonstrated that for all the [M(PME)] complexes with the mentioned metal ions an increased complex stability is measured; i.e., a stability higher than that expected for a sole phosphonate coordination of the metal ion. This increased stability is attributed to the formation of five-membered chelates involving the ether oxygen present in the ? O? CH₂? PO residue of PME^2? (and PMEA^2?); the formation degree of the five-membered [M(PME)] chelates varies between ca. 15 and 40% for the alkaline earth ions and ca. 35 to 65% for 3d ions and Zn²⁺ or Cd²⁺. Interestingly, for the [M(PMEA)] complexes within the error limits exactly the same observations are made indicating that the same five-membered chelates are formed, and that the adenine residue has no influence on the stability of these complexes, with the exception of those with Ni²⁺ and Cu²⁺. For these two metal ions, an additional stability increase is observed which has to be attributed to a metal ion-adenine interaction giving thus rise to equilibria between three different [M(PMEA)] isomers. These equilibria are analyzed, and for [Cu(PMEA)] it is calculated that 17(±3)% exist as an isomer with a sole Cu²⁺-phosphonate coordination, 34(±10)% form the mentioned five-membered chelate involving the ether oxygen, and the remaining 49(±10)% are due to an isomer containing also a Cu²⁺-adenine interaction. Based on various arguments, it is suggested that this latter isomer contains two chelate rings which result from a metal-ion coordination to the phosphonate group, the ether oxygen, and to N(3) of the adenine residue. For [Ni(PMEA)], the isomer with a Ni²⁺-adenine interaction is formed to only 22(±13)%; for [Cd(PMEA)] and the other [M(PMEA)] complexes if at all, only traces of such an isomer are occurring. In addition, the [M(PMEA)] complexes may be protonated leading to [M(H·PMEA)]⁺ species in which the proton is mainly at the phosphonate group, while the metal ion is bound in an adenosine-type fashion to the nucleic base residue. Finally, the properties of [M(PMEA)] and [M(AMP)] complexes are compared, and in this connection it should be emphasized that the ether oxygen which influences so much the stability and structure of the [M(PMEA)] complexes in solution is also crucial for the antiviral properties of PMEA.