Nature et stabilité des complexes métalliques de cryptands dinucléants en solution. I. Polyazapolyoxa macrotricycle cylindrique et monocycle constitutif

Nature and Stability of Some Metallic Complexes of Dinucleating Cryptands in Solution. I. A Polyazapolyoxa Cylindrical Macrotricycle and its Monocyclic Subunit pH-metry and UV spectrophotometry were used to study the complexing properties of the cylindrical macrotricycle, 1,7,13,19-tetraaza-4,16-dioxa-10,22,27,32-tetraoxatricyclo[17.5.5.5]tetratriacontane ( 1 ) and of its constitutive monocyclic subunit, 1,7-diaza-4,10-dioxacyclododecane ( 2 ) with some transition and heavy metal cations (Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Ag⁺), in aqueous medium 0.1M Et₄NClO₄, at 25°. The protonation constants of the ligands as well as the nature and the stabilities of the complexes formed in the pH-regions studied were determined. The tricycle 1 only formed dinuclear M₂L complexes with Cu²⁺, Zn²⁺, and Ag⁺, accompanied in the latter case by a protonated mononuclear MLH species, and with Cu²⁺ and Zn²⁺ at high pH-values by dinuclear hydroxo complexes. Only mononuclear complexes were evidenced with the other cations, ML being accompanied either by protonated or hydroxy mononuclear species. The mononuclear complexes of 1 , when they existed, were more stable than the corresponding complexes of 2 , except for cobalt which formed complexes of comparable stability with both ligands. In the other cases (Cd²⁺, Pb²⁺, Ag⁺), the stability differences between the complexes of 1 and 2 increased with the size of the cation.     

Complexation of iminodisuccinic acid (IDS) and 3-hydroxy-2,2′-iminodisuccinic acid (HIDS) with Cd2+, Hg2+, and Pb2+ in aqueous solution

In a search for environmentally friendly metal-chelating ligands for industrial applications, the protonation and complex formation equilibria of iminodisuccinic acid (IDS) and 3-hydroxy-2,2′-iminodisuccinic acid (HIDS) with Cd²⁺, Hg²⁺, and Pb²⁺ in aqueous 0.1?mol?L^?1 NaNO₃ solution were studied at 25°C by potentiometric titration. Models for complexation and stability constants of the different complexes were determined for each metal ion using the computer program SUPERQUAD. In all cases, complex formation was dominated by stable ML^2? complexes.     

Pyridinderivate als Komplexbildner. XI. Die Thermodynamik der Metallkomplexbildung mit Bis-, Tris- und Tetrakis[(2-pyridyl)methyl]-aminen

Pyridine Derivatives as Complexing Agents XI. Thermodynamics of Metal Complex Formation with Bis-, Tris- and Tetrakis[(2-pyridyl)methyl]-amines. The equilibria between H⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Pb²⁺, Hg²⁺ and Ag⁺, and the ligands bis(2-pyridylmethyl)-amine (=DPA), tris(2-pyridylmethyl)-amine (=TPA), tris(6-methyl-2-pyridylmethyl)-amine (=TLA) and N,N,N′,N′-tetrakis(2-pyridylmethyl)-ethylenediamine (=TPEN) have been studied. Only the stability constants of DPA and TLA with almost all these cations were obtained using the pH method. For the other ligands, the complexes are already formed in acid solutions and only the use of different ligand-ligand or metal-metal exchanges as well as of pM methods were successful. The protonation constants indicate that for DPA the protonation occurs firstly at the aliphatic nitrogen atom whereas in all other cases only the pyridine groups can be protonated. The thermodynamic functions of protonation are in agreement with this interpretation. The stability constants of the complexes are often similar in magnitude to those of the analogous aliphatic amines, in spite of the much lower basicities of the pyridine derivatives. The Fe(II)N₆ species of DPA and TPEN are appreciably more stable than those of the corresponding aliphatic ligands. This is due to the formation of low-spin complexes with an unexpected ΔH value. Comparison of the thermodynamic data of formation of the complexes with TPA and TLA shows the effect of the three bulky methyl groups of the second ligand. As a consequence of steric hindrance and of the major dehydration, ΔH and less ΔS are more positive for M(TLA)²⁺ than for M(TPA)²⁺. Therefore M(TLA)²⁺ is normally much less stable than M(TPA)²⁺. The data for MnTPA²⁺ and ZnTPA²⁺ appear to indicate that in these complexes the coordination number of the metal ion is seven and four respectively. In addition to the complexes ML²⁺, with these two ligands hydroxo complexes ML(OH)⁺ are formed at remarkably low pH. Further TPEN seems to be sexidentate in the 1:1 complexes with Mn²⁺, Co²⁺ and Ni²⁺ but quinquedentate in those with Cu²⁺ and Zn²⁺, also in agreement with the spectra in solution and of the solid complex salts. The reaction: M(DPA)₂²⁺ + TPEN → M(TPEN)²⁺ + 2DPA is for all metal ions favoured by ΔH and ΔS, whereas in the case of the corresponding aliphatic ligands only by the second term. This result is explained in terms of a different magnitude of hydration of the two sexidentate ligands as a consequence of the presence of the hydrophobic aromatic rings in TPEN.     

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.     

Studies on Complexation of Resorcinol with Some Divalent Transition Metal Ions and Aliphatic Dicarboxylic Acids in Aqueous Media

The binary and ternary complexes of Cu²⁺, Ni²⁺, Co²⁺ and Zn²⁺ metal ions with resorcinol (R) as primary ligand and some biologically important aliphatic dicarboxylic acids (adipic, succinic, malic, malonic, maleic, tartaric and oxalic acids) as secondary ligands were studied in aqueous solution at 25 °C and I=0.1 mol⋅dm⁻³ NaNO₃ using the potentiometric technique. The formation of different 1:1 and 1:2 binary complexes and 1:1:1 ternary complexes is inferred from the corresponding potentiometric pH-titration curves. The ternary complex formation was found to take place in a stepwise manner. The protonation constants of the ligands were determined and used for determining the stability constants of the different complexes formed in aqueous solutions. The lower stability of the 1:2 binary complexes compared to the corresponding 1:1 systems of all ligands studied were in accordance with statistical considerations. The order of stability of the complexes formed in solution was investigated in terms of the nature of the resorcinol, carboxylic acid, and metal ion used. The values of Δlog ₁₀ K, percentage of relative stabilization (% R.S.), and log ₁₀ X for mixed-ligand complexes studied have been evaluated and discussed. The concentration distribution of the various species formed in solution was evaluated. The mode of chelation of the ternary complexes was ascertained by conductivity measurements.     

溶液中N-乙酸基取代氮氧杂大环及其配合物稳定性研究

用pH电位滴定法在25℃,0.5mol·L^-1KNO₃水溶液中测定了三种大环化合物:H₂L¹(1,12-二氮杂-3,4:9,10-二苯并-5,8-二氧杂环十五烷-N,N'-二乙酸);H₃L²(1,12,15-三氮杂-3,4:9,10-二苯并-5,8-二氧杂环十七烷-N,N',N″-三乙酸)和H₂L³(1,15-二氮杂-3,4:12,13-二苯并-5,8,11-三氧杂环十八烷-N,N′-二乙酸)的逐级质子化常数.又测定了它们与Cu²⁺、Ni²⁺、Pb²⁺配合物的稳定常数,以及H₂L³与镧系金属La³⁺、Pr³⁺、Nd³⁺、Eu³⁺、Sm³⁺、Gd³⁺、Dy³⁺、Yb³⁺配合物的稳定常数.讨论了三种大环化合物质子化的一般顺序及其与各种离子配位时稳定性选择规律.说明了影响配位稳定性的有关因素.     

A Polarographic Study of Tl+, Pb2+ and Cd2+ Complexes with Aza-18-crown-6 and Dibenzopyridino-18-crown-6 in some Binary Mixed Non-aqueous Solvents

The complexation of Tl⁺, Pb²⁺and Cd²⁺ cations by macrocyclic ligands, aza-18-crown-6 (L1) and dibenzopyridino-18-crown-6 (L2) was studied in some binary mixtures of methanol (MeOH), n-propanol (n-PrOH), nitromethane (NM) and acetonitrile (AN) with dimethylformamide (DMF) at 22 °C using DC (direct current) and differential pulse polarographic techniques (DPP). The stoichiometry and stability constants of the complexes were determined by monitoring the shifts in half-waves or peak potentials of the polarographic waves of metal ions against the ligand concentration. In all of the solvent systems, the stability of the resulting 1:1 complexes was found to be L1 > L2. The selectivity order of the L2 ligand for the cations was found to be Pb²⁺ > Tl⁺ > Cd²⁺ and the selectivity of the L1 ligand for Pb²⁺ ion was greater than that of Tl⁺ ion. The results show that the stability of the complexes depends on the nature and composition of the mixed solvents. There is an inverse relationship between the stability constants of the complexes and the amount of dimethylformamide in the mixed solvent systems.     

Complexation of 3-hydroxy-2,2′-iminodisuccinic acid (HIDS) with Mg2+, Ca2+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ ions in aqueous solution

In a search for environmental-friendly metal chelating ligands for industrial applications, the protonation and complex formation equilibria of 3-hydroxy-2,2′-iminodisuccinic acid with Mg²⁺, Ca²⁺, Mn²⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ ions in aqueous 0.1 mol L^?1 NaCl solution were studied at 25°C by potentiometric titration. The model for complexation and the stability constants of the different complexes were determined for each metal ion using the computer program SUPERQUAD. In all cases, complex formation was dominated by stable ML ^{n ?4} complexes.     

Complexation of some environmentally friendly carboxylic acid ligands with La3+ ion in aqueous solution

In search for environmentally friendly metal chelating ligands for industrial applications, protonation, and complex-formation equilibria of iminodisuccinic acid (IDS), 3-hydroxy-2,2′ -iminodisuccinic acid (HIDS), N,N′-ethylenediaminedisuccinic acid (EDDS), and oxydisuccinic acid (ODS) with La³⁺ in aqueous 0.1?mol?L^?1 NaCl solution were studied at 25°C by potentiometric titration. The model for complexation and the stability constants of the different complexes were determined using the computer program SUPERQUAD. Complex formation was dominated by stable LaL^? complexes for all tested ligands.     

Spectrophotometric Study of Complexation of Tri-Aza Dibenzosulfide and Dibenzosulfoxide Macrocyclic Compounds with Heavy Metal Ions

The complexation reactions between 7,10,13-triaza-1-thia-4,16-dioxa-20,24-dimethyl-2,3;17,18-dibenzo-cyclooctadecane-6,14-dione ( TTD ) and 7,10,13-triaza-1-sulfoxo-4,16-dioxa-20,24-dimethyl-2,3;17,18-dibenzo-cyclooctadecane-6,14-dione ( TSD ) macrocycles with Ag⁺, Cd²⁺, Cu²⁺, Pb²⁺, Sr²⁺, Tl⁺, and Zn²⁺ ions have been studied in ethanol and methanol solutions at 25°C. The complexes formed between macrocycles ( TTD ) and ( TSD ) with these metals cations had a stiochiometry of 1:1 and 1:2, respectively. The stability constants of the resulting complexes were determined and found to decrease in the order Cu²⁺ > Zn²⁺ > Ag⁺ > Tl⁺ > Cd²⁺ > Pb²⁺ > Sr²⁺ with macrocycle ( TTD ) and Tl⁺ > Zn²⁺ > Cd²⁺ > Pb²⁺ > Cu²⁺ > Ag⁺ > Sr²⁺ with macrocycle ( TSD ).     

Copper(II)–nickel(II) complexes of novel 1,9-dioxa-3,6-dithiacyclotridecane-10,12-dione and 1,4-dioxa-7,10-dithiacyclododecane-2,3-dione ligands

1,9-Dioxa-3,6-dithiacyclotridecane-10,12-dione (L¹), and 1,4-dioxa-7,10-dithiacyclododecane-2,3-dione, (L²), two novel ligands, and their mono- and di-nuclear copper(II) and/or nickel(II) complexes have been prepared and characterized by elemental analyses, ¹H- and ¹³C-n.m.r., i.r., magnetic moments and mass spectral studies. Elemental analyses, stochiometric and spectroscopic data on the complexes indicate that the metal ions are coordinated to the nitrogen, oxygen and sulphur atoms and the data support the proposed structure for the diones and their mono–dinuclear copper(II) complexes. Moreover, copper(II) complexes of the dione ligands have a 1:1 metal:ligand ratio.     

Metal Complexes with Macrocyclic Ligands. Part XXXI. Protonation studies and complexation properties of tetraazamacrocyclic methylenephosphonates with earth-alkali ions

The three ligands 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylenephosphonic acid) ( 1 ), 1,4,7,11-tetraazacyclotridecane-1,4,7,11-tetrakis(methylenephosphonic acid) ( 2 ), and 1,4,8,11-tetraazacyclotetradecane-1,4,8,1 1-tetrakis(methylenephosphonic acid) ( 3 ) have been synthesized by condensation of the corresponding macrocycles with formaldehyde and phosphorous acid. The protonation and stability constants with the earth-alkali ions have been determined at 25° and I = 0.1 M (Me₄)N(NO₃) by potentiometric titrations. Because of the high values of the first two protonation constants, ¹H-NMR measurements were necessary to determine them. Titrations in different supporting electrolytes (NaNO₃, KNO₃, RbNO₃, CsNO₃, and Me₄N(NO₃)) show that their choice is of paramount importance, as the above ligands can form complexes with alkali-metal ions. The potentiometric results for the earth-alkali ions show that beside mononuclear complexes of different degrees of protonation ([MLH_n], n = 0–4), also binuclear species are formed ([M₂LH_m], m = 0–2). It is interesting that 1 with the smallest macrocyclic ring has the greatest tendency to form binuclear complexes, which are so stable that they partially prevent the formation of the corresponding mononuclear species. For [ML], [MLH], [M₂L], and [M₂LH], the stability sequence is Mg²⁺ < Ca²⁺ > Sr²⁺ > Ba²⁺, whereas for [MLH₂], [MLH₃], and [MLH₄], the stability steadily decreases from Mg²⁺ to Ba²⁺.     

Synthesis,structure, and solvent-extraction properties of tridentate oxime ligands and their cobalt(II), nickel(II), copper(II), zinc(II) complexes

Synthesis of four different types of ligands Ar[COC(NOH)R] _n (Ar = biphenyl, n = 1, HL¹; Ar = biphenyl, n = 2, H₂L²; Ar = diphenylmethane, n = 1, HL³; Ar = diphenylmethane, n = 2, H₂L⁴; R = furfurylamine in all ligands) and their dinuclear Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ complexes is reported herein. These compounds were characterized by elemental analysis, ICP-OES, FT-IR spectra, and magnetic susceptibility measurements. The ligands were further characterized by ¹H NMR. The results suggest that dinuclear complexes of HL¹ and HL³ have a metal to ligand mole ratio of 2: 2 and dinuclear complexes H₂L² and H₂L⁴ have a metal to ligand mole ratio of 2: 1. Square pyramidal or octahedral structures are proposed for complexes of oxime ligands. Furthermore, extraction abilities of the four ligands were also evaluated in chloroform using selected transition metal picrates such as Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺. The ligands show strong binding ability towards Hg²⁺ and Cu²⁺ ions.     

Cryptates XXIV: Structure and stability of mononuclear and binuclear cation inclusion complexes of cylindrical macrotricyclic ligands

The cylindrical macrotricyclic ligands 1–3 yield inclusion complexes, [3]-cryptates, with various metal cations. NMR. studies indicate the successive formation of a mononuclear and a binuclear complex. The former is probably unsymmetrical undergoing fast intramolecular cation exchange; the latter is symmetrical, with a cation located on each macrocyclic subunit of the macrotricyclic system. A heteronuclear (Ag²⁺, Pb²⁺) complex has been observed. The stability constants of the mononuclear and binuclear alkali and alkaline-earth cation complexes of ligands 1–3 have been determined by potentiometric methods. The stabilities are comparable to those of the complexes of the isolated macrocyclic subunit 5b . The binuclear complexes are almost as stable as the mononuclear one even in highly charged species like for instance the complex of ligand 2 with two barium cations. Cylindrical macrotricyclic ligands are topologically well suited for the designed positioning of two metal cations in a binuclear inclusion complex.     

Polarographic and spectrophotometric studies of the spiro[indolin-pyridobenzopyrane] complexes with the heavy metal ions

By means of the cathode differential pulse polarography on a mercury drop electrode the reactions of Zn²⁺, Cd²⁺, and Pb²+ ions with the spiro[pyridobenzopyranes] of the indoline series in DMSO were studied. Tetrabutylammonium perchlorate was used as the background electrolyte. It is found that well soluble heavy metal complexes of the spiro[indolin-pyridobenzopyranes] are formed in DMSO. The number of ligands, the step and the general stability constants of the obtained complex compounds are evaluated. The distribution curves of all the complexes under investigation are plotted. The spiropyrane complexes were also studied by means of the spectrophometric method. Good agreement of the results obtained by two different methods was established.     

Stability Constants for Dimercaptosuccinic Acid with Bismuth(III), Zinc(II), and Lead(II)

Abstract

Stability constants for the complexation of zinc(II), lead(II), and bismuth(III) by the vicinal dithiolate chelating agent meso-dimercaptosuccinic acid (DMSA) have been determined by a combination of potentiometric titration and spectrophotometric competition at 25°C and 0.1 M ionic strength. The spectrophotometric studies use the shifts in the ultraviolet bands of the thiol groups to quantitate metal binding to DMSA in the presence of competitive aminocarboxylic acids. Bismuth(III) forms a bis(DMSA) chelate with an exceptionally high stability constant of 10^43,87. This complex undergoes a series of protonations over the pH range 10 to 2, but there appears to be no measurable dissociation of ligand over this pH range. The zinc-DMSA system is dominated by a Zn₂(DMSA)₂ dimer, which has a protonation constant of 10⁶ and dissociates completely at lower pH. No more than 20% of total zinc exists as a monomeric complex at any pH. Lead forms a 1:1 complex with a stability constant of 10^17,4. Insoluble protonated lead complexes precipitate at pH < 6.5. Speciation calculations have been used to evaluate the potential competition from serum zinc to the binding of Pb²⁺ and Bi³⁺ by DMSA. The results indicate that DMSA should be relatively effective for the in vivo chelation of both Bi³⁺ and Pb^{2 +}.     

Toxic metal complexes of macrocyclic cyclen molecule – synthesis,structure and complexing properties

Toxic metal (Cd²⁺, Hg²⁺, Pb²⁺, and Ag⁺) complexes with the tetradentate macrocyclic ligand - cyclen (1,4,7,10-tetraazacyclododecane, [12]aneN₄, L) were prepared and studied in the solid state by IR, X-ray diffraction, elemental and thermal analysis. Diffraction results have yielded three molecular structures, [Cd([12]ane-κ⁴N^1,4,7,10)(NO₃)₂)] (1), [Hg([12]ane-κ⁴N^1,4,7,10)(NO₃-κ²O,O`)]NO₃ (2), [Pb₂([12]ane-κ⁴N^1,4,7,10)₂][Pb(NO₃)₆] (3) and one polymeric structure {[Ag₂([12]ane-κ³N^1,4,7)(μ₂-[12]aneN¹⁰)](NO₃)₂?2H₂O)}_n (4) featuring a unique coordination mode not observed before with cyclen as a ligand. The monodentate (1) and chelate (with small bite angle 50.3(3)°, (2) coordination modes of nitrate ligands were confirmed. Stereochemically active 6s² lone pair was suggested in 3 and DFT results confirmed no significant metal–metal covalent bond. The stability constants of the complexes with Cd²⁺ and Pb²⁺ ions were determined by potentiometric methods in aqueous solutions. Additionally, the structures of complexes in solution were observed by ¹H NMR. Both methods confirm similar cyclen complexing properties toward Zn²⁺ biometal and Cd²⁺, Pb²⁺ toxic metals.     

Investigation of the Metal Cation Binding Properties of Macrocyclic EDTA–Bis(Amide) Complexes by Potentiometry and Stopped-Flow Spectrophotometry

The protonation constants of macrocyclic EDTA–bis(amide), EDTA–EAM (1), EDTA–PAM (2), EDTA–BAM (3), and EDTA–PenAM (4), and the stability constants of theirLn³⁺and Cu²⁺complexes have been determined by potentiometric titration at 25.0 ± 0.1°C andI= 0.10M(KCl). The dissociation rates of someLn³⁺complexes have been measured by stopped-flow and conventional spectrophotometry at 25.0 ± 0.1°C andI= 0.10M(NaClO₄). The first protonation constants of the ligands were lower than that of EDTA, reflecting the strong electron-withdrawing ability of the carbonyl group. The stability constants and dissociation rates ofLn³⁺complexes followed the orderLn(1)⁺〈Ln(2)⁺〉Ln(3)⁺〉Ln(4)⁺. An increase in ring size from 13 (2) to 15 (4) by addition of the carbon chain between two amide nitrogens of the ligands leads to an increase in the thermodynamic and kinetic stability of the complexes. However,1shows a different behavior in the complexes because of its greater rigidity and basicity.     

Two‐Dimensional Holo‐ and Hemidirected Lead(II) Coordination Polymers: Synthetic,Spectroscopic, Thermal,and Structural Studies of [Pb(μ‐SCN)2(μ‐ebp)1.5]n and {[Pb(μ‐OAc)(μ‐ebp)](ClO4)}n (ebp=4,4′‐[(1E)‐Ethane‐1,2‐diyl]bis[pyridine]; OAc=Acetato)

Two new coordination polymers of Pb^II complexes with bridging 4,4′‐[(1E)‐ethane‐1,2‐diyl]bis[pyridine] (ebp), thiocyanato, and acetato ligands, [Pb(μ‐SCN)₂(μ‐ebp)_1.5]_n ( 1 ) and {[Pb(μ‐OAc)(μ‐ebp)](ClO₄)}_n ( 2 ), were synthesized and characterized by elemental analysis, FT‐IR, ¹H‐ and ¹³C‐NMR, thermal analysis, and single‐crystal X‐ray diffraction. In 1 , the Pb²⁺ ions are doubly bridged by both the ebp and the SCN⁻ ligands into a two‐dimensional polymeric network. The seven‐coordinate geometry around the Pb²⁺ ion in 1 is a distorted monocapped trigonal prism, in which the Pb²⁺ ions have a less‐common holodirected geometry. In 2 , the Pb²⁺ ions are bridged by AcO⁻ ligands forming linear chains, which are also further bridged by the neutral ebp ligands into a two‐dimensional polymeric framework. The Pb²⁺ ions have a five‐coordinate geometry with two N‐atoms from two ebp ligands and three O‐atoms of AcO⁻. Although ClO acts as a counter‐ion, it also makes weak interactions with the Pb²⁺ center. The arrangement of the ligands in 2 exhibits hemidirected geometry, and the coordination gap around the Pb²⁺ ion is possibly occupied by a configurationally active lone pair of electrons.     

Stabilité en solution aqueuse de complexes de métaux lourds avec des ligands diaza-polyoxamacrocycliques

Stability in aqueous solution of some complexes of heavy metals with diaza-polyoxamacrocyclic ligands Stability of metal complexes (Mⁿ⁺ = Cu²⁺, Ni²⁺, Co²⁺, Zn²⁺, Pb²⁺, Ag⁺ and Cd²⁺) with five diaza-polyoxamacrocycles (L = [2.1.1], [2.2.1], [2.2.2], [2.1] and [2.2] ) have been determined at 25°, in 0.1 M Et₄N⁺ClO aqueous solutions, by means of potentiometric titrations. All cations form MLⁿ⁺ complexes; Cu²⁺ also forms the MHL⁽ⁿ⁺¹⁾⁺ protonated species with both [2.2.1] and [2.1.1] ligands. The stability of these complexes has been discussed in terms of structure and by considering the ionic radii of the cations together with the radii of the macrocyclic cavities. Different behaviour is observed between some of these complexes and the well known alkali and alkaline-earth cryptates, partly due to the more covalent nature of bonds formed by the investigated cations and the donor sites of the ligands. The effect of the substitution of two oxygen by two sulfur atoms in the pentadentate ligand [2.1] on the stability of the complexes is reported.