Pyramidal Complexes Research Articles

15N NMR shielding tensors in bent nitrosyl complexes of cobalt

Ab initio calculations of nitrogen NMR chemical shielding tensors are presented for square–pyramidal cobalt model complexes with an apical nitrosyl ligand as a function of the geometry of the CoNO grouping. It is shown that the isotropic nitrogen shielding decreases with increasing N–O distance (in the range 101–116 pm) and with increasing CoN distance (in the range 173–193 pm). With increasing CoNO angle, in the models studied, the isotropic shielding initially decreases up to about 140° and then increases. For complexes with the same CoNO geometry and S-, N- or O-ligating coligands, the shielding increases in the order S<N,O. The variation of the shielding with CoNO geometry is mainly due to variations in the shielding tensor component parallel to the NO bond which mixes n(N) and π ∗(NO) orbitals. The calculations give no evidence for differing CoNO geometries producing similar isotropic shieldings but very different spans and skews of the shielding tensor, as given by the experiment in some series of compounds. It is likely that there is CoNO motional averaging (libration or spinning) in the solid state, as described for [Co(NO)(TPP)], in complexes for which anomalously small spans are observed.

The Abusir Drill Core Survey, Egypt

The Abusir Drill Core Survey was an investigation of the geoarchaeological deposits in the valley floor of the Abusir pyramid complex, at the apex of the Nile delta in Lower Egypt. The project, part of the author’s PhD research, was graciously funded by the London-based Egypt Exploration Society (E.E.S.). David Jeffreys of the Institute of Archaeology, UCL, supervised the survey, in conjunction with his on-going E.E.S. Survey of Memphis project.

Electron-transfer kinetics of tris(2-(methylthioethyl))aminecopper(II/I). A tripodal ligand complex exhibiting virtual C3v symmetry.

The tripodal ligand TMMEA (tris(2-methylthioethyl)amine) forms a trigonal bipyramidal complex with copper(II) in which the bridgehead nitrogen occupies one axial site, a solvent molecule (or anion) occupies the opposite axial site, and the three thioether sulfurs occupy the three planar sites. Upon reduction to copper(I), the axial solvent molecule (or anion) dissociates to leave a trigonal pyramidal complex with shortened Cu-S bonds and an elongated Cu-N bond. Therefore, both oxidation states maintain virtual C3v symmetry similar to that found in the type 1 blue copper protein sites. The electron-transfer cross-reaction rate constants have been determined for the Cu(II/I)(TMMEA) system reacting with three reductants and three oxidants. The Marcus cross relation was then utilized to generate apparent values for the Cu(II/I) electron self-exchange rate constant (k(11)) from the kinetic data for each of the six reactions. The median value obtained from the three reduction reactions is log k(11(Red)) = -1.5 while the median from the three oxidation reactions is log k(11(Ox)) = +0.9. This difference of 2.4 orders of magnitude is consistent with the dual-pathway square scheme mechanism which we have previously proposed for electron transfer in Cu(II/I) complexes. For this tripodal ligand system, however, the pathway involving a metastable Cu(II)L intermediate (pathway B) appears to be preferred over the pathway involving a metastable Cu(I)L intermediate (pathway A), which is opposite to the trend we have previously observed for a number of systems involving macrocyclic and acyclic tetrathiaethers. Both pathways exhibit relatively sluggish electron-transfer kinetics which is attributed to the rupture/formation of the strongly bound inner-sphere water molecule and the accompanying solvent reorganization.

Self-Assembly of Pyramidal Tetrapalladium Complexes with a Halide at the Apex.

Halide ions act as the template for the self-assembly of tetrapalladium macrocyclic pyramidal structures. These undergo easy inversion in which the halide ion apparently jumps across the macrocycle.

Self-Assembly of Pyramidal Tetrapalladium Complexes with a Halide at the Apex

Halogenidionen dienen als Templat für die Selbstanordnung von makrocyclischen pyramidalen Tetrapalladium-Systemen. Die Molekülmodelldarstellung der raschen Inversion dieser Verbindungen erweckt den Eindruck, das Halogenidion springe durch den Makrocyclus hindurch (siehe Schema).

Chelate-stabilized zinc complexes of alcohols and carbonyl compounds: pyridylphenylketone and pyridylphenylmethanol

The reactions of various zinc salts with the chelating ketone pyridylphenylketone (PPK) and the chelating alcohol pyridylphenylmethanol (PPM) were studied. 2:1 reactions of PPK with ZnX 2 yielded octahedral complexes (PPK) 2ZnX 2 with X=OSO 2CF 3, Cl, Br, NCS. 1:1 reactions yielded the square pyramidal complexes [(PPK) 2ZnHal] 2ZnHal 4 with X=Br, I and the trigonal-bipyramidal complex [(PPK)Zn(NCS) 2] 2. With Zn(SC 6F 5) 2 the tetrahedral 1:1 complex (PPK)Zn(SC 6F 5) 2 resulted. Methanol as a coligand was incorporated in the octahedral complex [(PPK) 2Zn(CH 3OH) 2](BF 4) 2. Of the PPM complexes, only (PPM) 2Zn(OSO 2CF 3) 2 was analogous to the corresponding PPK complex. The zinc halides produced the octahedral complexes [(PPM) 3Zn]ZnHal 4 with X=Cl, Br. Zn[N(SiMe 3) 2] 2 and PPM yielded polymeric zinc pyridylphenylmethoxide.

Vibrational studies of [Ni(II)(DIARS) 2X]X, (DIARS o–C 6H 4(As(CH 3) 2) 2 and XCl, Br, I)

Infrared and laser Raman spectra of [Ni(II)(diars) 2X]X, (XCl, Br and I) have been used as probes to determine the structures of chelated diarsine molecules. It has been observed that the effects of metal chelation and coordination geometry give rise to frequency shifts in these complexes. The variation in vibrational spectroscopic features indicates reduction in the symmetry of the complexes in the crystalline environment. The effect of halogen on the Ni–halogen stretching frequency of these square pyramidal complexes is not as significant as observed in the case of octahedral complexes.

Intramolecular rearrangements in six-coordinate ruthenium and iron dihydrides.

Molecular orbital calculations at the ab initio level are used to study polytopal rearrangements in H2Ru(PH3)4 and H2Fe(CO)4 as models of 18-electron, octahedral metal dihydrides. It is found that, in both cases, the transition state for these rearrangements is a dihydrogen species. For H2Fe(CO)4, this is a square pyramidal complex where the H2 ligand occupies an apical position and is rotated by 45 degrees from its original orientation. This is precisely analogous to the transition state for Fe-olefin rotation in (olefin)Fe(CO)4 complexes and has a very similar electronic origin. Another transition state very close in energy is found wherein the basic coordination geometry is a trigonal bipyramid and the H2 ligand is coordinated in the axial position. For H2Ru(PH3)4, the former stationary point lies at a much higher energy and the latter clearly serves as the transition state for hydride exchange. The reason for this difference is discussed along with the roles of electron correlation in the two compounds.

Will an η3-Si3H3 Ligand Form Sandwich Compounds with Main Group Elements?

η3-Si3H3 sandwich compounds 5 and 6, with classical and H-bridged ligands, respectively, having the main group elements boron and carbon as central atoms are minima at B3LYP/6-311++G(2d,2p). The stability of these systems is assisted by transfer of charge from the ligands to the central atom and is reversed from that of cyclopentadienyl sandwiches. The C and B containing pyramidal complexes 7, containing both a η3-Si3H3 and a μ2-Si3H3 ligand, are more stable than 5 by 20.7 and 8.5 kcal/mol, respectively. The spiro compounds 8, in which the C and B atoms are sandwiched by two allylic μ2-Si3H3 ligands, are still more stable by 29.6 and 21.9 kcal/mol, respectively. All three types (face-face, face-side, side-side) of sandwich structures are considered viable targets for synthetic pursuit. The Be complexes deviate from the C and B analogues because Be is much more electropositive. In the preferred cluster structure 9 the Be atom sits in a Si6H6 basket.

Steric control of the reactivity of moderately hindered tris(pyrazolyl)borates with copper(II) salts †

The co-ordination chemistry of the anions tris(3-phenylpyrazolyl)borate ([TpPh]−), tris(3-cyclohexylpyrazolyl)borate ([TpCy]−) and tris(3,5-diphenylpyrazolyl)borate ([TpPh2]−) with Cu(O2CMe)2·H2O, CuCl2 and Cu(BF4)2·6H2O has been investigated. The complex [Cu(O2CMe)(TpPh)] (1) transforms in solution to the B–N cleavage product [Cu(O2CMe)(HpzPh)(TpPh)] (2), whose crystal structure shows two square pyramidal Cu(II) centres in the asymmetric unit, each with a monodentate acetate ligand. The analogous complex [Cu(O2CMe)(TpCy)] (3) does not undergo this reaction. Reaction of K[TpCy] or K[TpPh2] with one equivalent of CuCl2 in CH2Cl2 yields mixtures of [CuCl(TpR)] (R = Cy, Ph2) and [CuCl(HpzR)(TpR)] (R = Cy, 5; R = Ph2, 6). Reaction of Cu(BF4)2·6H2O with one equivalent of K[TpPh] in CH2Cl2 gives [Cu(HpzPh)4](BF4)2 (7) in low yield as the only isolable product. An identical reaction with K[TpCy] affords [Cu(HpzCy)2(TpCy)]BF4 (8) in moderate yield. The single crystal structure of 8·CHCl3 contains a square pyramidal complex with two N–H⋯FBF3 and one Cl3C–H⋯FBF3 hydrogen bonds within each formula unit. Complexation of CuCl2 by two equivalents of K[TpPh] in MeOH affords [Cu(TpPh)2] (9) in high yield. In contrast, identical reactions employing K[TpCy] or K[TpPh2] yield [Cu(pzCy)(HpzCy)(TpCy)] (10) or 6 as the major products. The single crystal structure of 10 shows a square pyramidal Cu(II) centre with metric parameters very similar to 8; although not crystallographically located, the presence of a N–H⋯N hydrogen bond between the HpzCy and [pzCy]− ligands can be inferred from the close approach of the pzCy pyrrolic N atoms. All complexes were characterised by FAB mass spectrometry, microanalysis, IR, UV/vis and EPR spectroscopies.

THE REMAINS OF QUEEN WERET

In the summer of 1995 the Metropolitan Museum of Art's expedition to Dahshur discovered an undisturbed burial of the Middle Kingdom and one of the authors was asked to assist with the moving of the mummy that was still in its sealed sarcophagus. When the lid was removed, the mummy and sarcophagus contents were revealed to be extremely fragile, mostly because of the high humidity (95%) in the tomb. It was clear that a lengthy and careful excavation and recording of the sarcophagus contents would be needed before the mummy could be removed. With a day free, the author was asked to examine the human remains found during excavations of several previous seasons. This paper presents the examination of Queen Weret of Dynasty XII. It should be noted that there were two Queen Werets of Dynasty XII. The body we report here is the wife of Sesostris III. Because of extensive tomb robbing during the Second Intermediate Period and subsequent periods of lawlessness, there are no complete mummies of the kings and queens of the Middle Kingdom; thus any physical remains of these rulers are extremely important. Some of the kings and queens of the Twelfth Dynasty were buried at Dahshur, about fifteen miles southwest of Cairo. The mudbrick pyramids of Amenenhat II, Sesostris III, and Amenenhat III were first excavated by Jacques De Morgan (1894-5) and in addition to these monuments, De Morgan also unearthed royal burials of Dynasty XIII. In 1990 the Metropolitan Museum of Art's team, under the direction of Dieter Arnold, began reexamining the pyramid complex of Sesostris III. Around this 60 meter high pyramid runs a brick enclosure wall. In the area between the wall and the pyramid several royal women, including Queen Weret, were buried far beneath ground level.

Synthesis and structures of bis(dithiolene)molybdenum complexes related to the active sites of the DMSO reductase enzyme family.

Structural analogues of the reduced (Mo(IV)) sites of members of the DMSO reductase family of molybdoenzymes are sought. These sites usually contain two pterin-dithiolene cofactor ligands and one protein-based ligand. Reaction of [Mo(MeCN)3(CO)3] and [Ni(S2C2R2)2] affords the trigonal prismatic complexes [Mo(CO)2(S2C2R2)2] (R = Me (1), Ph (2)), which by carbonyl substitution serve as useful precursors to a variety of bis(dithiolene)molybdenum-(IV,V) complexes. Reaction of 1 with Et4NOH yields [MoO(S2C2Me2)2]2- (3), which is readily oxidized to [MoO(S2C2Me2)2]1- (4). The hindered arene oxide ligands ArO- afford the square pyramidal complexes [Mo(OAr)(S2C2R2)2]1- (5, 6). The ligands PhQ- affordthe trigonal prismatic monocarbonyls [Mo(CO)(QPh)(S2C2Me2)2]1- (Q = S (8), Se (12)) while the bulky ligand ArS- forms square pyramidal [Mo(SAr)(S2C2R2)2]- (9, 10). In contrast, reactions with ArSe- result in [Mo(CO)(SeAr)(S2C2R2)2]1-(14, 15), which have not been successfully decarbonylated. Other compounds prepared by substitution reactions of 1 and 2 include the bridged dimers [Mo2(mu-Q)2(S2C2Me2)4]2- (Q = S (7), Se (11)) and [Mo2(mu-SePh)2(S2C2Ph2)4]2- (13). The complexes 1, 3-5, 7-10, 12-14, [Mo(S2C2Me2)3] (16), and [Mo(S2C2Me2)3]1- (17) were characterized by X-ray structure determinations. Certain complexes approach the binding arrangements in at least one DMSO reductase (5/6) and its Ser/Cys mutant, and in dissimilatory nitrate reductases (9/10). This investigation provides the initial demonstration of the new types of bis(dithiolene)molybdenum(IV) complexes available through [Mo(CO)2(S2C2R2)2] precursors, some of which will be utilized in reactivity studies. (Ar = 2,6-diisopropylphenyl or 2,4,6-triisopropylphenyl.)

A Cu(II)-mediated C-H oxygenation of sterically hindered tripyridine ligands to form triangular Cu(II)3 complexes.

Two sterically hindered tris-pyridyl methane ligands, tris(6-methyl-2-pyridyl)methane (L1) and bis(6-methyl-2-pyridyl)pyridylmethane (L2), are newly synthesized. Under aerobic conditions, Ln (n = 1 or 2) reacts with CuX2 (X = Cl or Br), oxygenated at the methine position to LnOH or LnOMe. The former alcoholate ligand creates trinuclear Cu(II) complexes [Cu3(X)(LnO)3](PF6)2 [(X, n) = (Br, 1) 1, (C1, 1) 2, (Br, 2) 3, or (C1, 2) 4] in which the alkoxide oxygen atoms bridge copper centers. The crystal structures of 1-4 are presented along with their magnetic susceptibility data. The weak antiferromagnetic coupling between the Cu(II) centers in this trinuclear arrangement is due to weak interaction of the magnetic orbitals (dz2) which are oriented along three alternate sides in a hexagon of the Cu3O3 core in 1-4. Under anaerobic conditions, L1 reacts with CuBr2 to form a square pyramidal complex [CuL1Br2] (9) with the ligand facially capping. [Cu(Br)2(L1OMe)] (10) was obtained after the suspension of 9 in MeOH was stirred under air for 48 h. In the presence of cyclohexene, 9 is converted to [Cu(Br)(L1)]m (m = 1 or 2) 5 quantitatively to give trans- 1,2-dibromocyclohexane, indicating that Br2 is generated during the reaction. The FAB MS spectrum of [18O]-1 prepared by the reaction of L1 with CuBr2 under 18O2 shows that the ligand of [18O]-1 is L1(18O-.) L1(18OH), L1OCD3, and bis(6-methyl-2-pyridyl) ketone were obtained from reaction of L1 with CuBr2 in CD3OD under 18O2. These results indicate that the origins of the O atom in L1OH and L1OMe are O2 and MeOH, respectively. On the basis of these results, a mechanism of the oxygenation of L1 in the present system will be proposed.

Molecular Recognition of Activated and Unactivated Phosphate Diesters

The molecular recognition of two simple phosphate diesters by a terpyridine-copper (TCu) complex was studied by X-ray crystallography and potentiometry. Molecular recognition of the substrate is the first step in a hydrolysis reaction. The two phosphate diesters bis(p-nitrophenyl) phosphate (BNP) and diphenyl phosphate (DPP) coordinate axially to an approximate square pyramidal copper complex where a chloride and three terpyridine nitrogens form the corners of the base. The coordination geometries are almost identical. The molecular recognition constants for the formation of TCu-BNP and TCu-DPP were measured potentiometrically and have log values of 1.2 and 2.5, respectively. By a significant margin, the TCu complex favors DPP over BNP in solution. A pH versus rate constant profile shows that TCu-OH can hydrolyze BNP but not DPP. Activated and unactivated phosphate diesters are typically hydrolyzed by similar mechanisms. In this study the difference in reactivity lies outside the process of molecular recognition. Activating (electron-withdrawing) effects of the p-nitro groups on the nature of the leaving groups must play an important role in the susceptibility of phosphate diesters to hydrolysis.

Structural trends in a series of divalent transition metal triazole complexes

A series of bis bidentate complexes of 4-amino-3-methyl-1,2,4-triazole-5-thione, SN 4C 3H 6 (amt), with the divalent ions Mn, Fe, Co, Cu, and Zn have been crystallized by direct combination of the ligand and metal nitrate or perchlorate hydrate salt in ethanol. The structures were determined by single crystal X-ray diffraction techniques. In all cases, the triazole coordinates through the amine and thione substituents on the five-membered ring. The isostructural compounds [Mn(amt) 2(H 2O) 2](ClO 4) 2 ( 1) and [Fe(amt) 2(H 2O) 2](ClO 4) 2 ( 2) have the triazole and water ligands in a trans arrangement. The octahedral coordination shows a tetragonal distortion as the MN bonds are 0.15 Å longer than the MO bonds. The nitrate salts [Mn(amt) 2(H 2O) 2](NO 3) 2 ( 3), [Co(amt) 2(H 2O) 2](NO 3) 2 ( 4), and [Zn(amt) 2(H 2O) 2](NO 3) 2 ( 5), are also isostructural, but have the water molecules cis to each other and the triazoles arranged so that the amine groups are cis while the thiones are trans. The nitrate salt of the copper(II) complex, [Cu(amt) 2(H 2O) 2](NO 3) 2 ( 6), is similar in structure to 1 and 2; however, it is the water molecules which are more distant in the Jahn–Teller distorted coordination sphere. When sulfate is used in place of nitrate, a square pyramidal complex [Cu(amt) 2(SO 4)]·2H 2O ( 7) is obtained in which two pseudo-inverted triazole ligands form the base while the apex is a monodentate sulfato ligand. Neither water molecule is coordinated to the metal in this compound. These results are discussed in terms of the coordination behavior of the triazole ligand in these and similar systems.

Synthesis, spectroscopic studies and molecular structure of original halogeno-[ S-methyl 3-(2-hydroxyphenylethylidene)dithiocarbazato]oxo–rhenium(V) complexes

A new neutral rhenium(V)–oxo complex with tridentate Schiff base (SNO-CH 3), derived from the condensation of 2-hydroxyacetophenone with dithiocarbazic acid methyl ester (H 2N–NH–C(S)SCH 3) was prepared by a substitution reaction on the square pyramidal ionic complex [ReOCl 4][NBu 4]. Reacting the rhenium complex [Re VO(SNO-CH 3)Cl] 1 with lithium iodide (LiI) allows the formation of the iodo homologous rhenium(V)–oxo complex [Re VO(SNO-CH 3)I] 2. Compounds 1 and 2 were characterized by elemental analysis, IR, NMR spectra and mass spectrometry (FAB). The structure of [ReO(SNO-CH 3)I] 2 was determined by single X-ray diffraction methods. The coordination around rhenium is distorted square pyramidal with an apical double bonded oxo atom [ReO: 1.660 Å] and the basal ligands bend away from the oxo group.

Synthesis and Characterization of Mesoporous Chromium-Containing Silica Tube Molecular Sieves CrMCM-41

A series of mesoporous chromium-containing silica tube molecular sieves (CrMCM-41) with variable Si/Cr ratios have been synthesized and characterized by powder X-ray diffraction (XRD), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance UV−visible spectroscopy (UV−vis), electron spin resonance (ESR), electron spin-echo modulation (ESEM), Raman spectroscopy, 29Si magic-angle-spinning nuclear magnetic resonance (MAS NMR), and N2 adsorption measurements. XRD, EPMA, UV−vis, and ESR show that the as-synthesized CrMCM-41 materials have the MCM-41 structure and contain only atomically dispersed Cr(III). FTIR, UV−vis, and Raman reveal that Cr(VI) monochromate exists in calcined CrMCM-41. 29Si MAS NMR and N2 adsorption show that part of the chromium is incorporated into the MCM-41 structure. ESR shows that Cr(VI)−O2- exists in calcined CrMCM-41 and transforms to Cr(V) after evacuation above 150 °C. The interaction of Cr(V) with O2, CO, C2D4, ND3, CD3OH, and D2O was studied. Cr(V) occurs at two different sites. At one site, Cr(V) coordinates with one molecule of O2, CO, and D2O to form square pyramidal complexes and reacts with C2D4, CD3OH, and ND3 to be reduced to Cr(III). At the other site, Cr(V) is inert to O2, CO, and C2D4 while it coordinates with two molecules of ND3 to form six-coordinated complexes and coordinates with one molecule of CD3OH.

The Synthesis, NMR Spectroscopy, and X-ray Structure of a New Rhenium N2S2 Chelate Complex

A new chelate, mercaptoacetyl-L-histidinyl-S-benzyl-L-cysteine methyl ester was synthesized by standard peptide coupling techniques and reacted with ReOCl(3)(PPh(3))(2) to give two diastereomers, 7a and 7b. The two isomers were separated by reversed-phase HPLC and characterized by NMR spectroscopy and electrospray mass spectrometry. An X-ray structure of one isomer, 7a, confirmed that the chelated complex was analogous to other Re-N(2)S(2) compounds in that it formed a square pyramidal complex where the four donor atoms were the base of the pyramid and the oxygen attached to the rhenium was at the apex. The S-benzyl group, as expected, was cleaved during the formation of 7, and the resulting complex was a zwitterion where the rhenium was formally -1 and the counterion was the protonated imidazole ring.

Resonant Magnetization Tunneling in the Trigonal Pyramidal MnIVMnIII3Complex [Mn4O3Cl(O2CCH3)3(dbm)3

The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)3(dbm)3], where dbm- is the monoanion of dibenzoylmethane, functions as a single-molecule magnet. High-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the MnIVMnIII3 complex. These data show that the complex has a S = 9/2 ground state, experiencing axial zero-field splitting (DŜz2) with D = −0.53 cm-1 and a quartic zero-field splitting (B40Ô40)with B40 = −7.3 × 10-5 cm-1. Magnetization versus external magnetic field data were collected for an oriented single crystal in the 0.426−2.21 K range. At temperatures below 0.90 K hysteresis is seen. Steps are seen on each hysteresis loop. This is clear evidence that each MnIVMnIII3 complex functions as a single-molecule magnet that is magnetizable. Furthermore, the steps on the hysteresis loops are due to resonant magnetization quantum mechanical tunneling. In response to an external field each molecule reverses its direction of magnetization not only by being thermally activated over a potential-energy barrier, but by the magnetization tunneling through the barrier. Additional evidence for resonant magnetization tunneling was found in the change in the temperature at which the out-of-phase ac magnetic susceptibility is observed as a function of an external dc field. The results of magnetization relaxation experiments carried out in the 0.394−0.700 K range are presented. These data are combined with the ac susceptibility data taken at higher temperatures to give an Arrhenius plot of the logarithm of the magnetization relaxation rate versus inverse absolute temperature. The temperature-dependent part of this plot gives an activation barrier of 11.8 K. Below 0.6 K the relaxation rate is independent of temperature with a rate of 3.2 × 10-2 s-1. This S = 9/2 single-molecule magnet exhibits a tunneling of its direction of magnetization at a rate of 3.2 × 10-2 s-1 in the 0.394−0.600 K range. Thus, resonant magnetization tunneling is seen for a half-integer-spin (S = 9/2) ground-state magnet in the absence of an external magnetic field. The transverse component of the small magnetic field from the nuclear spins is probably the origin of this tunneling.

13-Membered macrocycles and their complexometric properties: study of 7,11-bis(carboxymethyl)-1,4-dioxa-7,11-diazacyclotridecane

The protonation constants of L1[7,11-bis(carboxymethyl)-1,4-dioxa-7,11-diazacyclotridecane] and the stability constants of complexes formed by this ligand with the alkaline-earth metal ions, the divalent first-row transition metal ions, Cd 2+, and Pb 2+ were determined by potentiometric methods, at 25°C and ionic strength 0.10 M in tetramethylammonium nitrate. In general, the stability constant values for the complexes studied are lower than expected for a potential six-co-ordinate ligand with an N 2O 4 co-ordination sphere. The complexes of L1 with metal ions, forming mainly electrostatic interactions (the alkaline-earth metal ions, Mn 2+ and Pb 2+), have a sharp decrease of stability when compared with the corresponding complexes of 12-membered ligands, L3 and L4 [7,10-bis(carboxymethyl)-1,4-dioxa-7,10-diazacyclodecane and 4,10-bis(carboxymethyl)-1,7-dioxa-4, 10-diazacyclododecane, respectively]. However, the Cu 2+ and Ni 2+ complexes of L1 and L4 have about the same values of stability constants. Hence, the 13-membered ligand is more selective for the divalent first-row transition metal ions than the 12-membered ones are. To explain these results we suggest that probably all the donor atoms of the ligands ( L1, L3 and L4) are involved on the co-ordination to the metal ions when mainly electrostatic interactions take place (also to the Co 2+, as spectroscopic measurements have revealed) but the distances and principally the orientation of the lone-pairs of electrons of L1 are not in the better position for the co-ordination. Based on electronic and EPR spectroscopic measurements in solution and in magnetic moments of the complexes some explanations about the peculiarities of the Ni 2+ and Cu 2+ complexes of L1 and L4 were proposed. The Ni 2+ complexes of both ligands exhibit tetragonally distorted octahedral geometry, but this metal ion seems to fit better in the cage formed by L1 than that of L4, as indicated by the values of stability constants. Both ligands should adopt different arrangements around the metal ion, the distortion from the octahedron being more important for [Ni L4] reflecting the loss in terms of crystal field stabilisation energy. However, the Cu 2+ forms wit L1 a square pyramidal pentaco-ordinate complex while L4 prefers a distorted octahedral arrangement.