Search

The paramagnetic lanthanide complexes with polyaminopolycarboxylate (PAPC) ligands attract considerable attention from the standpoint of potential applications thereof as relaxation agents used in medical magnetic resonance imaging (MRI) and in luminescent materials, as well as owing to promising use thereof as paramagnetic labels for studying the properties of biopolymers since they exhibit thermodynamic stability, good solubility in aqueous media and moderate toxicity. For the last decades, the NMR methods have been used to determine the physical and chemical properties of paramagnetic Ln compounds. The studies concerning paramagnetic NMR lanthanide-induced shifts (LISs) in dissolved Ln complexes, as well as the analysis of band shape as a function of temperature make it possible to obtain valuable information on the structure, intra- and intermolecular dynamics and paramagnetic properties thereof. This review is devoted solely to the following features: firstly, the processes of intramolecular dynamics of lanthanide complexes with polyamino-polycarboxylate ligands such as DOTA, EDTA and DTPA and their derivatives studied by NMR; secondly, the LISs of lanthanide complexes with EDTA, DOTA, DTPA and some of their derivatives depending on temperature and pH. Moreover, in this review, for the first time, the dependence of the activation energy of molecular dynamics in complexes with polydentate ligands on the atomic number of the lanthanide cation is analyzed and a monotonic change in energy is detected, which is due to the effect of lanthanide contraction. It should be noted that this phenomenon is quite general and may also appear in the future in many other series of lanthanide complexes with both other multidentate ligands and with bidentate and monodentate ligands. In the future, it is possible to predict the dependence of the properties of certain lanthanide complexes on the ionic radius of the lanthanide cation based on the approaches presented in the review. In this review, we have also presented the dynamic NMR as the main research method widely used to analyze the processes of molecular dynamics, and the structural studies based on the NMR relaxation spectroscopy and LIS analysis.

Atomic hardness is an important periodic descriptor which can govern chemical reactivity and stability. A number of theoretical models are available to compute atomic hardness. In this report, we have suggested a new and simple approach to compute atomic hardness. Considering periodic relationship of atomic hardness with nucleophilicity index, effective nuclear charge and atomic radius, this model is derived to compute hardness of 103 elements of the periodic table. Our proposed scale satisfies all sine qua non of the periodic table. Characteristic periodic properties viz. lanthanide contraction, chemical inertness of noble gases, relativistic effect is quite distinct in our computed result. We have also calculated molecular hardness invoking Hardness Equalization Principle. A strong correlation between our computed data and their experimental counterparts justifies our study.

Reactions of lanthanide nitrate, nickel nitrate, and polyhydroxyl Schiff-base ligand 2-(((2-hydroxy-3-methoxyphenyl)methylene)amino)-2-(hydroxymethyl)-1,3-propanediol (H4L) gave rise to a family of butterfly heterotetranuclear 3d–4f clusters formulated as [Ln2Ni2(H2L)2(μ3-OCH3)2(CH3CN)2(NO3)4]·2CH3OH·2H2O [Ln = Pr (1), Sm (2), Eu (3), Tb (4), Er (5)]. Single-crystal X-ray diffraction (XRD) reveals these complexes crystallize in the monoclinic space group P21/c. Both Ln1 and its symmetric Ln1A are coordinated to nine oxygen atoms and consist of two ligands with the same coordination mode μ3.η1:η2:η1:η2:η1, two nitrate ions and one μ3-OCH3. The five heterotetranuclear $${\text{Ni}}^{\text{II}}_{2} {\text{Ln}}^{\text{III}}_{2}$$ complexes display a central planar butterfly topology. The vertices of the body positions of the butterfly are occupied by NiII ions in all five complexes, which have been characterized by single-crystal X-ray diffraction, infrared spectroscopy, element analyses, powder X-ray diffraction and thermal gravimetric analyses. The magnetic properties of the complexes have been studied. The results show that complexes 1-3, and 5 exhibit antiferromagnetic exchange between the paramagnetic species, and 4 shows strong ferromagnetic interaction. We have shown the successful design and assembly of a new family of tetranuclear, dicationic, heterometallic $$\left\{ {{\text{Ni}}^{\text{II}}_{2} {\text{Ln}}^{\text{III}}_{2} } \right\}$$ complexes characterized by a butterfly-type topology. The results might be promoting the rational design and synthesis of 3d–4f molecule-base magnets materials with fascinating topologies and magnetic behaviours.

The combination of 3,4-dimethylbenzoic acid (3,4-DMHBA) and 5,5′-dimethyl-2,2′-bipyridine (5,5′-DM-2,2′-bipy) with rare earth nitrate under the room-temperature conditions results in two novel complex formulated as [Ho(3,4-DMBA)3(5,5′-DM-2,2′-bipy)(H2O)] (1) and [La(3,4-DMBA)3(3,4-DMHBA)(5,5′-DM-2,2′-bipy)]2(2), respectively. The structures were confirmed by single-crystal X-ray diffraction. Two complexes are all characterized with elemental analysis and PXRD. Interestingly enough, because of the lanthanide contraction phenomena, the complex 1 is a mononuclear molecule with the coordination number of eight, but the complex 2 is a binuclear molecule with the coordination number of nine. The thermal decomposition mechanism for two complexes was discussed by TG-DTG technology. In addition, the three-dimensional infrared accumulation spectra of the evolved gas during the thermal decomposition for title complexes were investigated via the simultaneous TG/DSC-FTIR techniques. Beyond that, the heat capacities of complexes were measured by DSC. And the smoothed value of heat capacities and thermodynamic functions (HT-H298.15) and (ST-S298.15) were calculated according to the fitted polynomial and thermodynamic equations.

A series of new two-dimensional (2D) lanthanide(III) coordination polymers, namely {[Ln2(μ 2-HTFMIDC)3(DMA)4] · 2H2O} n [Ln = Pr (1); Nd (2); Sm (3); Eu (4); H3TFMIDC = 2-(trifluoromethyl)-1H-imidazole-4,5-dicarboxylic acid, DMA = N,N′-dimethylacetamide] for type I and {[Ln2(μ 2-HTFMIDC)3(DMA)2(H2O)2] · DMA} n [Ln = Eu (5); Gd (6)] for type II, have been successfully prepared under solvothermal conditions and structurally characterized for the first time. Both two types of structures exhibit similar 2D honeycomb-like networks, which are constructed by the linkages of μ 2-HTFMIDC2− bis-(bidentate) bridging ligands and Ln(III) metal centers. However, slightly different ABAB stacking fashions of the 2D layers and distinctly different hydrogen bonding interactions between the neighboring 2D layers are observed in crystal structures of type I and type II, which may be attributed to the lanthanide contraction effect. Meanwhile, the solid-state luminescent properties of 4 and 5 have been also investigated.

The ultrastable Y zeolite (USY) in fluid cracking catalyst is commonly stabilized by ion-exchange with rare earth (RE) cations. The RE-exchange provides hydrothermal stability to the zeolite by improving surface area retention, as well as inhibiting dealumination, resulting in greater preservation of acid sites. Though La and Ce are commonly used in fluid catalytic cracking (FCC) catalysts, we have observed that the stability of REUSY catalysts improves as the ionic radius of the RE cation decreases. In this paper, we compare the activity and selectivity of REUSY catalysts, stabilized with La and heavy (Ho, Er, and Yb) rare earth cations, the latter having a smaller ionic radius, due to the well-known phenomenon of lanthanide contraction. The experimental data show that a significant improvement in catalytic activity is achieved when RE elements having a smaller ionic radius are used to make the REUSY catalyst. Yttrium is even more effective than the heavier lanthanides in stabilizing Y-zeolite, leading to higher cracking activity and gasoline selectivity under a variety of deactivation conditions. These benefits of yttrium exchange does not only result from a larger resistance to dealumination, but also to an increase of the catalyst intrinsic cracking activity, which may be explained by changes in the adsorption of hydrocarbons at the active sites. Examples of commercial applications of yttrium-based FCC catalysts are given.

Three new salen-type 3d–4f heteronuclear metal complexes incorporating with the semi-rigid ligand N,N′-bis(2-hydroxy-3-methoxybenzylidene)-1,2-diaminocyclohexane (H2L), namely, [(CuClL)LaCl2(CH3OH)3]·H2O (1), [(CuClL)DyCl2(CH3OH)2] (2) and [(CuClL)2GdCl2(CH3OH)2] (3), have been isolated. X-ray crystallographic analysis indicates that 1 and 2 possess the typical heterodinuclear structures, while, complex 3 has a trinuclear structure. In 1–3, the Cu2+ ion is in the N2O2 cavity of the ligand and the lanthanide ions are to the O2O2 set of the ligand. The coordination numbers of the lanthanide ions are reduced from 9 to 8 along with lanthanide contraction. These results further enrich the structures of salen type 3d–4f heteronuclear complexes. Pictogram

Simple salts of the tetracyanoplatinate (TCP) anions with the general formula M[Pt(CN)4]x·H2O have been described for over 200 years. A major structural feature of these compounds are pseudo one dimensional Pt···Pt interactions between the square planar tetracyanoplatinate anions. These interactions form quasi one-dimensional chains in the solid state and are believed to be the source of their unique spectroscopic and emission properties. The lanthanides offer the ability to tune the Pt···Pt spacing of these pseudo one dimensional chains, predictably by taking advantage of the lanthanide contraction. Here, we describe the emission and structural characteristics of new actinide tetracyanometallates.

The ternary complexation of neodymium(III) and samarium(III) with triethylene glycol (EO3) and picrate anion (Pic) were characterized by elemental analyses, FTIR (Fourier-transform infrared) spectroscopy, single crystal X-ray diffraction, and photoluminescence (PL). Both the [Nd(Pic)(H2O)2(NO3)(EO3)](Pic) and [Sm(Pic)(H2O)2(NO3)(EO3)](Pic)·H2O complexes were isostructural with a ten-coordination number. In both complexes, the picrate and nitrate anions were coordinated to Ln(III) in a bidentate manner, and with the the EO3 ligand in a tetradentate manner, the addition of two water molecules maintained a ten-coordination number. The lighter lanthanide-picrate complexes formed a ten-coordination number due to the lanthanide contraction effect, acyclic polyether chain length, and number of donor oxygen atoms. The acyclic EO3 ligand affected photoluminescent intensity and its conformation on the structure of the [Ln(Pic)(NO3)(H2O)2(EO3)]+ moiety. Photoluminescent measurement showed complex Nd(III) emissions at 403, 486, and 682 nm, with the strongest emission peak at 403 nm. Formation of these peaks occurred due to the intraligand π–π transitions of the Pic anion. The Sm(III) complex exhibited the emission characteristic of the Sm(III) ion in the red spectral region at 616.7 nm (4G5/2 → 6H9/2 transition), even though the ligand emissions were also observed in the PL spectrum. The emission intensity of the 4f–4f transitions in the Sm complex was significantly higher than that found in its salt. We noted that the [Sm(Pic)(H2O)2(NO3)(EO3)](Pic)·H2O complex was an excellent red-light-emitter and would be considered as a candidate material for organic light emitting diodes.

By using 1,4-benzene-dicarboxylic acid and 1,2,4,5-benzenetetracarboxylic acid as mixed-linkers, three novel 3D lanthanide coordination polymers, [Ln(BDC)0.5(BTEC)0.5(H2O)] (Ln = Eu (1) and Gd (2)) and [Tb(BDC)0.5(BTEC)0.5(H2O)] (3) (H2BDC = 1,4-benzene-dicarboxylic acid, H4BTEC = 1,2,4,5-benzenetetracarboxylic acid) were synthesized via hydrothermal reaction. The X-ray single crystal analyses reveal that the three coordination polymers present two types of different crystal cell parameters and hereby exhibited two kinds of different metal–organic frameworks, although they have the similar chemical formula [Ln(BDC)0.5(BTEC)0.5(H2O)]. Of these complexes, complexes 1 and 2 are isomorphous, crystallizing in monoclinic, space group P21/n. While 3 is a 3D eight-coordinated terbium complex with triclinic crystal system and P-1 space group. The occurrence of the different structures of these complexes under the similar preparation method may be related to the lanthanide contraction effect. Additional, the thermogravimetric analyses and photoluminescent properties for complexes 1 and 3 were discussed in detail. Three novel coordination polymers, [Ln(BDC)0.5(BTEC)0.5(H2O)] (Ln = Eu (1) and Gd (2)) and [Tb(BDC)0.5(BTEC)0.5(H2O)] (3) (H2BDC = 1,4-benzene-dicarboxylic acid, H4BTEC = 1,2,4,5-benzenetetracarboxylic acid) have been firstly synthesized hydrothermally. Complexes 1 and 2 are isomorphous, crystallizing in monoclinic, space group P21/n. While 3 is a novel eight-coordinated Tb(III) complex with triclinic crystal system and P-1 space group.

A gas electron diffraction study of PrI3 and GdI3 has been carried out in combination with mass spectrometric vapour monitoring at 1110(10) K and 1100(10) K, respectively. Up to 3 mol.% of dimeric species was observed in addition to the dominating monomeric molecules. The change of the thermal-averaged r g configuration parameters of the molecules in the series LaI3 → LuI3 reflects the lanthanide contraction. A low value of the shrinkage δ(I···I) even at such a high temperatures may be considered due to vibration effects in molecule whose equilibrium geometric nuclear structure is planar and which correspond to configurationally averaged 4f n electronic state. B3LYP calculations performed in this study with large core potential for lanthanide atoms also resulted in equilibrium geometry of D 3h symmetry. According to the quantum chemical calculations, the potential function the non-planar vibration is essentially anharmonic, which is therefore to be taken into account to correctly describe nuclear dynamics in molecules such as LnI3.

The heavy rare earth elements crystallize into hexagonally close packed (h.c.p.) structures and share a common outer electronic configuration, differing only in the number of 4f electrons they have. These chemically inert 4f electrons set up localized magnetic moments, which are coupled via an indirect exchange interaction involving the conduction electrons. This leads to the formation of a wide variety of magnetic structures, the periodicities of which are often incommensurate with the underlying crystal lattice. Such incommensurate ordering is associated with a 'webbed' topology of the momentum space surface separating the occupied and unoccupied electron states (the Fermi surface). The shape of this surface-and hence the magnetic structure-for the heavy rare earth elements is known to depend on the ratio of the interplanar spacing c and the interatomic, intraplanar spacing a of the h.c.p. lattice. A theoretical understanding of this problem is, however, far from complete. Here, using gadolinium as a prototype for all the heavy rare earth elements, we generate a unified magnetic phase diagram, which unequivocally links the magnetic structures of the heavy rare earths to their lattice parameters. In addition to verifying the importance of the c/a ratio, we find that the atomic unit cell volume plays a separate, distinct role in determining the magnetic properties: we show that the trend from ferromagnetism to incommensurate ordering as atomic number increases is connected to the concomitant decrease in unit cell volume. This volume decrease occurs because of the so-called lanthanide contraction, where the addition of electrons to the poorly shielding 4f orbitals leads to an increase in effective nuclear charge and, correspondingly, a decrease in ionic radii.

Bond distances, vibrational frequencies, electron affinities, ionization potentials, dissociation energies and dipole moments of the title molecules in neutral, positively and negatively charged ions were studied by use of density functional method. Ground electronic state was assigned for each molecule. The bonding patterns were analyzed and compared with both the available data and across the series. It was found that besides ionic component, covalent bonds are formed between the metal s, d and f orbitals and oxygen p orbitals. Contrary to the well known lanthanide contraction, the bond distance is not regular from LaO to LuO for both neutral and charged molecules. An obvious population at 5d orbital was observed through the lanthanide series. 4f electrons also participate the chemical bonding for CeO to NdO and TbO to TmO. For EuO, GdO, YbO and LuO, 4f electrons tend to be localized. The spin multiplicity is regular for neutral and charged molecules. The spin multiplicity of the charged molecules can be obtained by −1 (or +1 for TbO+, DyO+, YbO− and YbO+) compared with the corresponding neutral molecules.

Measurements of the heat capacity by quasi-adiabatic, intermittent energy increments from 5 to 350 K show small high heat-capacity anomalies near 7 and 10 K which are attributed to superconducting transitions seen by magnetic measurements on the same carefully synthesized and well-characterized sample of (Hf0.934Zr0.057)(N0.97). Although no previous heat capacity measurements over the cryogenic region are known, the estimated 298.15 K standard entropy values (S/R) vary in the literature from about 200 per cent higher to 5 per cent lower than our measured value of (5.28±0.01)R−1 when the formula is represented as above. A simple scheme to represent and predict values based on both molar volumes and atomic masses for related materials is presented which seems more reliable on a limited sample than do others despite the intrusion of lanthanide contraction.

We present a systematic study of the structural and bonding properties of selected lanthanide trihalide molecules, LnX3 (Ln = La, Gd or Lu, X = F or Cl). A topological analysis of the electron localization function has been carried out, revealing typical ionic bonding properties. The increasing ionic character of the Ln–X bonds through the rare-earth series has been clearly emphasized. Moreover, we have pointed out a strong distortion of the outer core shell of the metal. This singularity induces a typical tetrahedral arrangement of the lanthanide outer core basins, thus favoring a pyramidal equilibrium geometry. On the other hand, this structural effect is counterbalanced by increasing ligand repulsions through the series due to the well-known lanthanide contraction; therefore, these repulsions favor a planar arrangement of the molecular system with the appearance of a new core basin in the outer core shell of the metal.

A variable-temperature (79, 145, and 293 K) extended x-ray absorption fine structure study, using rare-earth LIII absorption edges, is reported for phosphate glasses doped with rare-earth elements (R, where R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) with compositions close to metaphosphate, R(PO3)3. The results yield nearest-neighbor R–O distances that demonstrate the lanthanide contraction in a glassy matrix and an R–O coordination intermediate between 6 and 7 for rare-earth ions with smaller atomic number (Z) and 6 for rare-earth ions with largerZ. Thermal parameters show no significant changes in R–O distances or coordination numbers between 293 and 79 K. There is evidence of an R–P correlation between 3.3 and 3.6 Å and the beginning of a second R–O correlation at approximately 4 Å. No R–R correlations up to a distance of approximately 4 Å were observed.

This paper is concerned with the estimation of heat capacities in the IVA 3d-transition element compounds using especially Zr and Hf compounds as examples. Most prediction schemes routinely tacitly assume that volumes and masses trend ‘in parallel’. However, the lanthanide contraction here ensures for ZrX/HfX systems — and generally elsewhere — that this is not so in this portion of the periodic table. Available methods such as Latimer's, Volumetric Priority, Komada-Westrum, Grimvall's, and Sommers' are compared on IVA elements and compounds. Only the Sommers approach has volumetric input. It provides the best prediction.

Goldschmidt combined a number of widely separated sciences to synthesize a new structural crystal chemistry. Although his work on the relative abundances of the elements, atomic and ionic radii, interionic distances, the effect of radius ratio on coordination number in crystals, replacement of ions in minerals, and the lanthanide contraction is found in almost every textbook of general and inorganic chemistry and has provided the basis for modern crystal chemistry and the use of size relationships for interpreting properties of inorganic substances, Goldschmidt’s name, life, and career remain relatively unknown to most chemical educators and practicing chemists. Goldschmidt used the basic properties of matter to provide simple and elegant explanations for the composition of our environment. Throughout his relatively brief career, filled with sorrow and tragedy, he continued to maintain his intense interest in the elements and their genesis, affinities, and associations despite his changes from one method to another in his attempts to obtain new and more complete data. Thus, although he used petrology, crystallography, and chemistry and enriched all these fields greatly, to him they were only tools for exploring the earth and its history.

Experimental information and theoretical estimates for the enthalpy of formation at 298 K for the RM2 (whereM = Fe, Co,orNi andR = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) lanthanide Laves phases with the MgCu2 (C15) structure are reviewed and discussed in the light of information on lattice parameters and atomic volume. The purpose of the present study is to analyze all the available experimental information on enthalpies of formation for transition-metal lanthanide Laves phases and then plot the data in order to reveal tendencies and allow reliable estimates of data that have not yet been measured. It is concluded that as the lanthanide atomic number increases the enthalpy of formation is almost constant or becomes slightly more negative. The accompanying volume contraction is slightly larger than expected from the lanthanide contraction, 7.6 %. From the so-called Gschneidner correlation one would have expected the contrary behavior, that is, that the bonding strength would become slightly lower when going from LaM2 to LuM2. An increase in the transition-metal atomic number has a stronger effect on the enthalpy of formation and atomic volume than an increase in the lanthanide atomic number.

The bonded radii of anions obtained in topological analyses of theoretical and experimental electron density distributions differ from atomic, ionic and crystal radii in that oxide-, fluoride-, nitride- and sulfide-anion radii are not constant for a given coordination number. They vary in a regular way with bond length and the electronegativity of the cation to which they are bonded, exhibiting radii close to atomic radii when bonded to a highly electronegative cation and radii close to ionic radii when bonded to a highly electropositive cation. The electron density distributions show that anions are not spherical but exhibit several different radii in different bonded directions. The bonded radii of cations correlate with ionic and atomic radii. But unlike ionic radii, the bonded radius of a cation shows a relatively small increase in value with an increase in coordination number. In contrast to atomic and ionic radii, the bonded radius of an ion in a crystal or molecule can be used as a reliable and well-defined estimate of its radius in the direction of its bonds.

Following the discovery of Nd2Fe14B compound [1] which has been treated as the third generation of the rare earth permanent magnets, the Sm2Fe17 carbides and nitrides [2] with the rhombohedral Th2Zn17-type structure, and Nd(Fe,Ti)12 nitrides [3] with the ThMn12type structure have been found possessing excellent intrinsic permanent magnetic properties. So the iron rich rare earth-transition metal compounds (RnTm) become new potential candidates for permanent magnet applications, and the study of these compounds is attracted intensively world wide. To understand the influence of these interstitial atoms it is necessary to first understand better the interactions that are present in the parent compounds. In this letter, the magnetic properties like the Curie temperature and the saturation magnetization in RFe9Mn3 (R = Y, Tb, Dy, Ho and Er) compounds have been investigated. The mean-field constants of DyFe9Mn3 and HoFe9Mn3 compounds have also been calculated. The Mn element has been chosen because it can show some very interesting magnetic behaviors such as large magnetic moment in some compounds. All the RFe9Mn3 samples with R = Y, Tb, Dy, Ho and Er were prepared by argon arc melting from starting materials of at least 99.5% purity. The alloys were melted five times to ensure homogeneity. After arc melting, the polycrystalline specimens were sealed into evacuated quartz tubes and annealed at 1073 K for 50 h and then quenched into water. The phase composition, the structure and the lattice parameters for the RFe9Mn3 compounds with R = Y, Tb, Dy, Ho and Er were determined by means of the X-ray diffraction (XRD) patterns of free powder samples. Fig. 1 shows the X-ray diffraction patterns of RFe9Mn3compounds with R = Y, Tb, Dy, Ho and Er. The lattice constants a, c and the unit-cell volume V are shown in Table I. It can be seen that all of the investigated compounds are single phase and crystallized in ThMn12-type structure. The lattice constants a, c and the unit cell volume V decrease with increasing atomic number from Tb to Er, reflecting the lanthanide contraction. The specific magnetization was measured on free powder with an extracting-sample magnetometer at 5 K

The authors have shown in recent years that interatomic and interionic distances are sums of the radii of the adjacent atoms and/or ions. Many examples will be provided and it will be shown how the experimental bond lengths agree with the radii sums. The examples include inorganic compounds like alkali halides, metal hydrides, graphene, etc., organic like aliphatic and aromatic compounds and biochemical like nucleic acids, amino acids, caffeine-related compounds and vitamins.

The initial retentions (at ∼20°C) of (n, γ) activated rare earth bromates were studied using a252Cf fission neutron source with respect to80Br,80mBr and82Br. They lay over the ranges 19–23, 21–23 and 28–32%, respectively. On heating, retention progressively increases and closes on ∼100% for Sm-bromate while for other systems the optimum values reach

Compounds of the type CsLn(SO4)2, (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were prepared at 300°C. IR spectra were recorded in the range 4,000-250cm−1 and tentatively assigned. The number of observed bands exceeds the predicted number by site symmetry selection rules. Streching bands show bathochromic shifts whereas the deformations exibit hypsochromic shifts on going to heavier lanthanides. At the same time there is an increased splitting of these bands in the same direction. All these facts indicate an increased coupling in the layer structure which is in accordance with the lanthanide contraction.

The exchange constants observed in the rare earth iron garnets, obtained from fitting magnetization and paramagnetic susceptibility as a function of temperature have been analysed in terms of Anderson’s theory of superexchange. The transfer integrals and covalency parameters have been obtained for various rare earth ions overlapping with oxygen.

VALUES are now available for ΔH1, the heat content change in the reaction where Ln represents a lanthanide element and Y a complexing agent such as the ethylenediaminetetraacetate (EDTA) or nitrilo-triacetate (NTA) ion. The measurements usually refer to dilute solutions of molar concentrations of ∼ 0.01 or less. The dependence of ΔH1 on the atomic number of the lanthanide is often rather complicated, a plot of one quantity against the other showing both a maximum and a minimum. This is true, for example, of the EDTA1 and 1:1 NTA complexes2,3 and the 1:1 and 1:2 diglycollate and dipicolinate complexes4. There are, however, grounds for believing that a complicating factor may be a change somewhere in the middle of the series in the effective hydration number of the uncomplexed lanthanide ion, consequent on the lanthanide contraction. This factor can be eliminated by measuring the integral heats of solution of a series of isomorphous lanthanide salts to give suitably dilute solutions. Such series of salts covering all the lanthanides are not numerous, but two well known ones are the bromates and the ethylsulphates, both of which crystallize as enneahydrates. If, for example, ΔH2 is measured for the process then ΔH1 + ΔH2 = ΔH3, where ΔH3, is the heat content increase for the process from which the simple hydrated lanthanide ion has been eliminated.

FOR a given oxidation level the oxy-acids in a given sub-group of the periodic table are normally found to exhibit a regular variation in acid strengths. In group VII, for example, Cobble1 lists the logical pK values as −2.25, (−1.5) and −1.25 for HMnO4, HTcO4 (estimated, with due regard for the lanthanide contraction) and HReO4. The general behaviour of pertechnic acid in aqueous neutralimetry certainly is consistent with that of a monoprotic and ‘relatively’ strong acid.