Structural determinations and magnetic properties of a "chiral at metal" complex and its resulting [Cu-Ln]2 compounds.

A chiral trianionic ligand possessing one amide, one imine, two phenol functions and one asymmetric carbon atom into its diamino chain reacts with CuII ions to yield anionic [LCu]- units that crystallize in a non-centrosymmetric space group as infinite 1D zig-zag chains in which a transmission of chirality to the CuII ion is effective. The distorted square planar environment of the CuII ion is large enough to induce the presence of a stereogenic CuII centre. Further reaction with LnIII ions in presence of ancillary ligands does not preserve such an arrangement but yields a tetranuclear complex made of two [LCu-Ln] units in a head-to-tail position. The tetranuclear [LCu-Ln]2 complexes made with the racemic and chiral LCu units crystallize in different space groups, so that racemization does not occur. The structural determinations confirm that a symmetry centre is present in the two structures, except for the methyl groups linked to the chiral carbon atoms, which appear as disordered in the (S-S) tetranuclear entity. Such an arrangement implies a conformation change of the diamino chain linked to the CuII ion in one [LCu-Ln] unit of the (S-S) entity, and cancels any chirality contribution of the CuII ions, as in the meso compound. Ferromagnetic Cu-Ln interactions, resulting from an alternate distribution of the CuII and LnIII ions, are the only ones to be active. Eventually the micro-Squid studies confirm that the hysteresis loops of the corresponding racemate and chiral tetranuclear [LCu-Dy]2 entities are slightly different.

Contribution of 155Gd Mössbauer data to the study of the magnetic interaction in heterodinuclear 3d-Gd (3d = Cu, Ni) coordination complexes.

The valence shell of the gadolinium element corresponds to 4f7 5d1 6s2, so that the trivalent GdIII ion possesses free 5d and 6s orbitals. It has been previously shown by CASSCF methods that the 5d orbitals, along with the 6s Gd orbitals, which are expected to be unoccupied, present a slight spin density and that the magnetic behaviour of CuII-GdIII complexes can only be reproduced if the 5d Gd orbitals are taken into account in the active space. 155Gd Mössbauer isomer shifts of 3d-Gd complexes, L1CuGd(NO3)3, L1NiGd(NO3)3·acetone, L2Cu(acetone)Gd(NO3)3, L2Ni(H2O)2Gd(NO3)3 where L1 and L2 are hexadentate Schiff base ligand, are almost unchanged (0.62-0.64 mm s-1 relative to 155Eu/SmPd3 source) though the values are slightly smaller than a typical ionic compound GdF3 (0.67 mm s-1). The very similar isomer shift values of the 3d-Gd complexes indicate that there is no change in the small electron density of the 6s orbital and that the spin delocalization or spin polarization concerns the 5d Gd orbitals, in agreement with the crystal structure and Mössbauer spectrum of the Gd complex of nitrogen-coordinating tridentate ligand PrnTBP, [Gd(PrnTBP)3](OTf)3. Thus the observed 155Gd Mössbauer isomer shifts of 3d-Gd complexes give an experimental proof for the participation of 5d Gd orbitals to the magnetic interaction in these 3d-Gd complexes.

Influence of ancillary ligands and solvents on the nuclearity of Ni-Ln complexes.

A Schiff base ligand resulting from the reaction of ovanillin and 2,2-dimethyl-1,3-diaminopropane allows the preparation of hetero-dinuclear [Ni-Ln]3+ or -trinuclear [Ni-Ln-Ni]3+ complexes. Although empirical parameters for rationalizing the strength of the ferromagnetic Ni-Gd interaction have already been discussed in several papers, no systematic study has been devoted to the control of the nuclearity of such complexes. With the help of structural determinations, we demonstrate the role of solvent and of the nature of ancillary ligands, linked to the Ln ions, in nuclearity. For instance, the presence of one chelating nitrato ligand is already sufficient to impede an increase in the nuclearity, while the replacement of nitrato ligands by chloride anions still yields dinuclear Ni-Ln complexes. This experimental result evidences the role of protic solvents. In contrast, the use of lanthanide salts, soluble in non-protic solvents, allows the isolation of dinuclear [Ni-Ln]3+ or trinuclear cationic [LNi-Ln-NiL]3+ complexes, depending on the Ni/Ln ratio. A further synthetic step can be overtaken by the reaction of a Ni-Ln complex, soluble in a non-protic solvent, with a LM complex (M = Cu, Zn). By doing so, a heterotrinuclear complex made of three different metal ions, two distinct 3d ions and a 4f one, has been isolated and structurally characterized. Note that the Ni coordination number decreases from 6 to 5 on going from the dinuclear complex to the trinuclear one. Also, the replacement of water molecules by chloride ligands in the hexacoordinate Ni complexes induces a net increase of the positive zero-field splitting parameter D to 20 cm-1, which is supported by ab initio calculations. Although the Ni-Ln (Ln = Gd, Tb, Dy) magnetic interactions are ferromagnetic, the corresponding trinuclear complexes are devoid of SMM properties in the absence of an applied magnetic field.

Effects of the Exchange Coupling on Dynamic Properties in a Series of CoGdCo Complexes.

Reaction of 2-hydroxy3-methoxybenzaldehyde ( o-vanillin) with 1,1,1-tris(aminomethyl)ethane, Me-C(CH2NH2)3, or with N, N', N''-trimethylphosphorothioic trihydrazide, P(S)[NMe-NH2]3, yields two tripodal LH3 and L1H3 ligands which are able to give cationic heterotrinuclear [LCoGdCoL]+ or [L1CoGdCoL1]+ complexes. The CoII ions are coordinated to these deprotonated ligands in the inner N3O3 site, while the GdIII ion is linked to three deprotonated phenoxo oxygen atoms of two anionic [LCo]- or [L1Co]- units. Air oxidation of these trinuclear complexes does not yield complexes associating CoIII and GdIII ions. With the first ligand, the structurally characterized resulting complex is the neutral mononuclear LCoIII compound, while in the second case, oxidation of the CoII ions turned out to be impossible. The [L1CoLnCoL1]+ complexes behave as single-molecule magnets with effective energy barriers for the reversal of magnetization varying from Ueff = 51.3 K, τo = 2 × 10-6 s for the yttrium complex to Ueff = 29.5, 29.4, 27.4 K and τo = 1.3 × 10-7, 1.47 × 10-7, 1.50 × 10-7 s for the gadolinium ones, depending on the used counteranions. The energy decrease is compensated by the suppression of quantum tunneling of magnetization in absence of applied field, thanks to the introduction of a ferromagnetic Co-Gd interaction. Current work also shows that uncritical use of conventional spin Hamiltonians, based on quenched orbital momenta, can be misleading and that ab initio calculations are indispensable for establishing the picture of real magnetic interaction. Ab initio calculations show that the CoII sites in the investigated compounds possess large unquenched orbital moments due to the first-order spin-orbit coupling resulting in strongly axial magnetic anisotropy. Although the CoII ions are not axial enough for showing slow relaxation of magnetization by themselves, blocking barriers of exchange type are obtained thanks to the exchange interaction with GdIII ions.

NiII-LnIII complexes with o-vanillin as the main ligand: syntheses, structures, magnetic and magnetocaloric properties.

ortho-Vanillin in the presence of nickel and lanthanide ions yields three types of heteronuclear NiII-GdIII complexes, ranging from dinuclear to tetranuclear defect dicubane complexes, as demonstrated by their structural determinations. These complexes are dependent on the solvents used during the reaction processes and on the retained lanthanide ions, as characterized by an increase in their Lewis acid character on moving from lighter to heavier lanthanide ions. Intramolecular ferromagnetic NiII-GdIII interactions are present in the heterodinuclear NiII-GdIII entities, whereas ferromagnetic NiII-NiII and NiII-GdIII interactions dominate above 2 K in the tetranuclear Ni-Gd compounds, devoid of any GdIII-GdIII interaction. The effect of a magnetic field on the magnetic entropy and adiabatic temperature changes is maximum near the liquid-helium boiling temperature, mainly determined by the relative weakness of the magnetic interactions.

Electronic Structure and Magnetic Anisotropy in Lanthanoid Single-Ion Magnets with C3 Symmetry: The Ln(trenovan) Series.

We report the syntheses and the magnetic characterization of a new series of lanthanide complexes, in which the Ce, Nd, Gd, Dy, Er, and Yb derivatives show single-molecule magnet behavior. These complexes, named Ln(trenovan), where H3trenovan is tris(((3-methoxysalicylidene)amino)ethyl)amine, exhibit trigonal symmetry and the Ln(III) ion is heptacoordinated. Their molecular structure is then very similar to that of the previously reported Ln(trensal) series, where H3trensal is 2,2',2″-tris(salicylideneimino)triethylamine. This prompted us to use the spectroscopic and magnetic properties of the Ln(trensal) family (Ln = Nd, Tb, Dy, Ho, Er, and Tm) to obtain a set of crystal-field parameters to be used as starting point to determine the electronic structures and magnetic anisotropy of the analogous Ln(trenovan) complexes using the CONDON computational package. The obtained results were then used to discuss the electron paramagnetic resonance (EPR) and ac susceptibility results. As a whole, the obtained results indicate for this type of complexes single-molecule magnet behavior is not related to the presence of an anisotropy barrier, due to a charge distribution of the ligand around the lanthanoid, which results in highly mixed ground states in terms of MJ composition of the states. The crucial parameter in determining the slow relaxation of the magnetization is then rather the number of unpaired electrons (only Kramers ions showing in-field slow relaxation) than the shape of the charge distribution for different Ln(III).

Effect of Ligand Substitution around the Dy(III) on the SMM Properties of Dual-Luminescent Zn-Dy and Zn-Dy-Zn Complexes with Large Anisotropy Energy Barriers: A Combined Theoretical and Experimental Magnetostructural Study.

The new dinuclear Zn(II)-Dy(III) and trinuclear Zn(II)-Dy(III)-Zn(II) complexes of formula [(LZnBrDy(ovan) (NO3)(H2O)](H2O)·0.5(MeOH) (1) and [(L(1)ZnBr)2Dy(MeOH)2](ClO4) (3) (L and L(1) are the dideprotonated forms of the N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato and 2-{(E)-[(3-{[(2E,3E)-3-(hydroxyimino)butan-2-ylidene ]amino}-2,2-dimethylpropyl)imino]methyl}-6-methoxyphenol Schiff base compartmental ligands, respectively) have been prepared and magnetostructurally characterized. The X-ray structure of 1 indicates that the Dy(III) ion exhibits a DyO9 coordination sphere, which is made from four O atoms coming from the compartmental ligand (two methoxy terminal groups and two phenoxido bridging groups connecting Zn(II) and Dy(III) ions), other four atoms belonging to the chelating nitrato and ovanillin ligands, and the last one coming to the coordinated water molecule. The structure of 3 shows the central Dy(III) ion surrounded by two L(1)Zn units, so that the Dy(III) and Zn(II) ions are linked by phenoxido/oximato bridging groups. The Dy ion is eight-coordinated by the six O atoms afforded by two L(1) ligands and two O atoms coming from two methanol molecules. Alternating current (AC) dynamic magnetic measurements of 1, 3, and the previously reported dinuclear [LZnClDy(thd)2] (2) complex (where thd = 2,2,6,6-tetramethyl-3,5-heptanedionato ligand) indicate single molecule magnet (SMM) behavior for all these complexes with large thermal energy barriers for the reversal of the magnetization and butterfly-shaped hysteresis loops at 2 K. Ab initio calculations on 1-3 show a pure Ising ground state for all of them, which induces almost completely suppressed quantum tunnelling magnetization (QTM), and thermally assisted quantum tunnelling magnetization (TA-QTM) relaxations via the first excited Kramers doublet, leading to large energy barriers, thus supporting the observation of SMM behavior. The comparison between the experimental and theoretical magnetostructural data for 1-3 has allowed us to draw some conclusions about the influence of ligand substitution around the Dy(III) on the SMM properties. Finally, these SMMs exhibit metal- and ligand-centered dual emissions in the visible region, and, therefore, they can be considered as magnetoluminescent bifunctional molecular materials.

Relaxation Dynamics and Magnetic Anisotropy in a Low-Symmetry Dy(III) Complex.

The magnetic behaviour of a Dy(LH)3 complex (LH(-) is the anion of 2-hydroxy-N'-[(E)-(2-hydroxy-3-methoxyphenyl)methylidene]benzhydrazide) was analysed in depth from both theoretical and experimental points of view. Cantilever torque magnetometry indicated that the complex has Ising-type anisotropy, and provided two possible directions for the easy axis of anisotropy due to the presence of two magnetically non-equivalent molecules in the crystal. Ab initio calculations confirmed the strong Ising-type anisotropy and disentangled the two possible orientations. The computed results obtained by using ab initio calculations were then used to rationalise the composite dynamic behaviour observed for both pure Dy(III) phase and Y(III) diluted phase, which showed two different relaxation channels in zero and non-zero static magnetic fields. In particular, we showed that the relaxation behaviour at the higher temperature range can be correctly reproduced by using a master matrix approach, which suggests that Orbach relaxation is occurring through a second excited doublet.

Does the Sign of the Cu-Gd Magnetic Interaction Depend on the Number of Atoms in the Bridge?

Several theoretical investigations with CASSCF methods confirm that the magnetic behavior of Cu-Gd complexes can only be reproduced if the 5d Gd orbitals are included in the active space. These orbitals, expected to be unoccupied, do present a low spin density, which is mainly due to a spin polarization effect. This theory is strengthened by the experimental results reported herein. We demonstrate that Cu-Gd complexes characterized by Cu-Gd interactions through single-oxygen and three-atom bridges consisting of oxygen, carbon, and nitrogen atoms, present weak ferromagnetic exchange interactions, whereas complexes with bridges made of two atoms, such as the nitrogen-oxygen oximato bridge, are subject to weak antiferromagnetic exchange interactions. Therefore, a bridge with an odd number of atoms induces a weak ferromagnetic exchange interaction, whereas a bridge with an even number of atoms supports a weak antiferromagnetic exchange interaction, as observed in pure organic compounds and also, as in this case, in metal-organic compounds with an active spin polarization effect.

Analysis of the Role of Peripheral Ligands Coordinated to Zn(II) in Enhancing the Energy Barrier in Luminescent Linear Trinuclear Zn-Dy-Zn Single-Molecule Magnets.

Three new Dy complexes have been prepared according to a complex-as-ligand strategy. Structural determinations indicate that the central Dy ion is surrounded by two LZn units (L(2-) is the di-deprotonated form of the N2 O2 compartmental N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato) Schiff base. The Dy ions are nonacoordinate to eight oxygen atoms from the two L ligands and to a water molecule. The Zn ions are pentacoordinate in all cases, linked to the N2 O2 atoms from L, and the apical position of the Zn coordination sphere is occupied by a water molecule or bromide or chloride ions. These resulting complexes, formulated (LZnX)-Dy-(LZnX), are tricationic with X=H2 O and monocationic with X=Br or Cl. They behave as field-free single-molecule magnets (SMMs) with effective energy barriers (Ueff ) for the reversal of the magnetization of 96.9(6) K with τ0 =2.4×10(-7)  s, 146.8(5) K with τ0 =9.2×10(-8)  s, and 146.1(10) K with τ0 =9.9×10(-8)  s for compounds with ZnOH2 , ZnBr, and ZnCl motifs, respectively. The Cole-Cole plots exhibit semicircular shapes with α parameters in the range of 0.19 to 0.29, which suggests multiple relaxation processes. Under a dc applied magnetic field of 1000 Oe, the quantum tunneling of magnetization (QTM) is partly or fully suppressed and the energy barriers increase to Ueff =128.6(5) K and τ0 =1.8×10(-8)  s for 1, Ueff =214.7 K and τ0 =9.8×10(-9)  s for 2, and Ueff =202.4 K and τ0 =1.5×10(-8)  s for 3. The two pairs of largely negatively charged phenoxido oxygen atoms with short DyO bonds are positioned at opposite sides of the Dy(3+) ion, which thus creates a strong crystal field that stabilizes the axial MJ =±15/2 doublet as the ground Kramers doublet. Although the compound with the ZnOH2 motifs possesses the larger negative charges on the phenolate oxygen atoms, as confirmed by using DFT calculations, it exhibits the larger distortions of the DyO9 coordination polyhedron from ideal geometries and a smaller Ueff value. Ab initio calculations support the easy-axis anisotropy of the ground Kramers doublet and predict zero-field SMM behavior through Orbach and TA-QTM relaxations via the first excited Kramers doublet, which leads to large energy barriers. In accordance with the experimental results, ab initio calculations have also shown that, compared with water, the peripheral halide ligands coordinated to the Zn(2+) ions increase the barrier height when the distortions of the DyO9 have a negative effect. All the complexes exhibit metal-centered luminescence after excitation into the UV π-π* absorption band of ligand L(2-) at λ=335 nm, which results in the appearance of the characteristic Dy(III) ((4) F9/2 →(6) HJ/2 ; J=15/2, 13/2) emission bands in the visible region.

On the importance of ferromagnetic exchange between transition metals in field-free SMMs: examples of ring-shaped hetero-trimetallic [(LnNi2){W(CN)8}]2 compounds.

Rare cases of genuine (i.e. field-free) SMM have been found for mixed 3d-4f-5d ring-shaped compounds with Tb(III) (Ueff/kB = 23.0 K) or Dy(III) (Ueff/kB = 26.4 K). The ferromagnetic interactions between the transition metals are shown to play an essential role in this feature.

Determination of magnetic anisotropy in the LnTRENSAL complexes (Ln = Tb, Dy, Er) by torque magnetometry.

We report here a study about the magnetic anisotropy of the LnTRENSAL complexes (Ln = Tb, Dy, Er) performed by using cantilever torque magnetometry and electron paramagnetic resonance. For all of the compounds, we extracted a set of crystal-field parameters to obtain the energy-level splitting of the ground-state multiplet.

Antiferromagnetic Cu-Gd interactions through an oxime bridge.

The copper complex of a polydentate non-symmetrical Schiff base ligand [LCu]2, prepared by template synthesis, has been reacted with the series of lanthanide ions. This complex used as a ligand possesses two functions (phenol and oxime) able to coordinate the Ln ions and, according to the Ln ion, three types of complexes are obtained. From La to Eu, trinuclear [(LCu)2Ln(NO3)3] complexes with a double phenoxo-oximato bridge were isolated. From Gd to Ho, the complexes [(LCu)2Ln(NO3)3(H2O)] are still trinuclear, with a supplementary water molecule linked to the Ln ion but the Cu(II) and Ln(III) ions are only bridged by the oximato (N-O) pair, the phenoxo oxygen atom being hydrogen-bridged to the Ln-coordinated water molecule. Then, with heavier Ln ions, dinuclear [(LCu)Ln(NO3)3(H2O)2] complexes are characterized. The magnetic study demonstrates that the oximato bridge is responsible for the antiferromagnetic character of the Cu-Gd interaction, with JCuGd = -0.63 cm(-1) in [(LCu)2Gd(NO3)3(H2O)], in contrast to the ferromagnetic Cu-Gd interaction induced by the single oxygen atom phenoxo bridge.

Synthesis, structural characterization, and magnetic properties of a copper-gadolinium complex derived from a hydroxybenzohydrazide ligand.

The reaction of hydroxybenzohydrazide with o-vanillin yields 2-hydroxy-N'-[(2-hydroxy-3-methoxyphenyl)methylidene]benzohydrazide (LH3), a ligand that is able to give mononuclear and tetranuclear copper complexes but also to associate copper and gadolinium ions in a Cu2-Gd2 heterotetranuclear complex. This synthesis is successful if the Gd ions, which are acidic in protic solvents, are introduced in a basic methanol solution of the mononuclear copper complex. In the absence of piperidine, the addition of Gd ions to a methanol solution of the mononuclear copper complex only yields a tetranuclear cubane-type copper complex. This work reports on the first structural characterization of a copper-gadolinium complex involving a benzohydrazide ligand. The resulting complex consists of two Cu-Gd pairs linked by a dihydroxo Gd-Gd bridge, in which the Cu and Gd ions are bridged by a nonsymmetric phenoxo-hydroxo bridge. The magnetostructural correlation between the ferromagnetic coupling constant and the hinge angle observed in symmetrical double-phenoxo Cu-Gd bridges remains valid for dissymmetric Cu-Gd bridges and confirms the preponderance of the structural factor over the nature of the bridge. This tetranuclear complex corresponds to two S = 4 units linked through a dihydroxo bridge introducing a weak antiferromagnetic Gd-Gd interaction and impeding the existence of a S = 8 ground state.

Beyond the anisotropy barrier: slow relaxation of the magnetization in both easy-axis and easy-plane Ln(trensal) complexes.

We present a spectroscopic and magnetic (both static and dynamic) characterization of two isostructural Dy and Er complexes evidencing that, despite the different types of magnetic anisotropy, the two molecules show similar slow relaxation of the magnetization in a static magnetic field.

Interplay of strongly anisotropic metal ions in magnetic blocking of complexes.

The key characteristic of single-molecule magnets (SMMs) is the anisotropy-induced blocking barrier, which should be as efficient as possible, i.e., to be able to provide long magnetic relaxation times at elevated temperatures. The strategy for the design of efficient SMMs on the basis of transition-metal complexes such as Mn12Ac is well established, which is not the case of complexes involving strongly anisotropic metal ions such as cobalt(II) and lanthanides (Ln). While strong intraionic anisotropy in the latter allows them to block the magnetization already in mononuclear complexes, the presence of several such ions in a complex does not result automatically in more efficient SMMs. Here, the magnetic blocking in the series of isostructural 3d-4f complexes Co(II)-Ln(III)-Co(II), Ln = Gd, Tb, and Dy, is analyzed using an originally developed ab initio based approach for the investigation of blocking barriers. The theoretical analysis allows one to explain the counterintuitive result that the Co-Gd-Co complex is a better SMM than terbium and dysprosium analogues. It turns out that the highly efficient magnetic blockage in the Co-Gd-Co complex results from a concomitant effect of unexpectedly large unquenched orbital momentum on Co(II) ions (ca. 1.7 µB) and the large spin on the gadolinium (S = 7/2), which provides a multilevel blocking barrier, similar to the one of the classical Mn12Ac. We conclude that efficient SMMs could be obtained in complexes combining strongly anisotropic and isotropic metal ions with large angular momentum rather than in polynuclear compounds involving strongly anisotropic ions only.

Field and dilution effects on the slow relaxation of a luminescent DyO9 low-symmetry single-ion magnet.

A mononuclear Dy(III) complex with a non-Schiff base compartmental ligand has been prepared and characterised by X-ray crystallography and ac magnetic susceptibility measurements. The complex exhibits SIM behaviour induced by dilution and/or magnetic field with two thermally activated relaxation processes.

Tetranuclear [Co-Gd]2 complexes: aiming at a better understanding of the 3d-Gd magnetic interaction.

Tetranuclear [Co-Gd](2) complexes were prepared by using trianionic ligands possessing amide, imine, and phenol functions. The structural determinations show that the starting cobalt complexes present square planar or square pyramid environments that are preserved in the final tetranuclear [Co-Gd](2) complexes. These geometrical modifications of the cobalt coordination spheres induce changes in the cobalt spin ground states, going from S = 1/2 in the square planar to S = 3/2 for the square pyramid environments. Depending on the ligand, the complexes display antiferromagnetic or ferromagnetic Co(II)-Gd(III) interactions. The temperature dependence of the magnetic susceptibility-temperature products indicate that the Co-Gd interaction is ferromagnetic when high spin Co ions are concerned and antiferromagnetic in the case of low spin Co ions. This different magnetic behavior can be explained if we observe that the singly occupied σ d(x(2)-y(2)) orbital is populated (S = 3/2 Co ions) or unoccupied (S = 1/2 Co ions). Such an observation furnishes invaluable information for the understanding of the more general 3d-4f magnetic interactions.

Chiral crystallization of a heterodinuclear Ni-Ln series: comprehensive analysis of the magnetic properties.

Four heterodinuclear (H(2)O)(2)NiL-Ln(NO(3))(3) complexes (Ln = Tb, Dy, Er, Yb) with a double phenoxo bridge coming from the dideprotonated Schiff-base ligand are synthesized and characterized by crystal and powder X-ray diffraction studies. This series of compounds devoid of any chiral center, crystallizes in a noncentrosymmetric space group P2(1), as the previously described (H(2)O)(2)NiL-Gd(NO(3))(3) equivalent. All four complexes are ferromagnetically coupled. If this behavior is clearly shown by the χ(M)T increase at low temperature in the case of the Ni-Tb and Ni-Dy complexes, it necessitates the preparation of the Zn-Er and Zn-Yb equivalent entities to be evidenced in the case of the Ni-Er and Ni-Yb complexes. Out-of-phase susceptibility signals are found in the four cases, but the SMM behavior is neither confirmed, nor completely studied because of the presence of fast quantum tunnelling at zero field. Thorough ab initio multiconfiguration calculations are carried out, achieving a realistic account of ligand field effects, exchange coupling and magnetic anisotropy in the discussed systems. The calculations reveal the ferromagnetic intercenter exchange coupling, the interplay with spin-orbit effects leading to a Ising-like scheme of the lowest levels. The ab initio simulation of the magnetic susceptibility is in semiquantitative agreement with experimental data, certifying the reasonableness of the theoretical treatments in obtaining valuable information for the interacting mechanisms. The anisotropy is accounted for by drawing polar diagrams of state-specific magnetization functions, obtained by handling of the data resulting from ab initio calculations including the spin-orbit effects. Supplementary, Density Functional Theory (DFT) calculations are carried out, presenting new methodological clues and assessments. The DFT is not perfectly adequate for lanthanide systems because of orbital pseudodegeneracy issues. However, we show that in particular circumstances the DFT can be partly used, succeeding here in mimicking different orbital configurations of the Ni-Tb system. The DFT seems to offer reasonable estimations of exchange coupling parameters, while it remains problematic in the complete account of Ligand Field splitting. The Paper presents unprecedented methodological advances and correlations with phenomenological and heuristic interpretation of experimental data, taking into focus relevant d-f systems constructed with a prototypical binucleating ligand.

Pentacoordinate Ni(II) complexes: preparation, magnetic measurements, and ab initio calculations of the magnetic anisotropy terms.

Two novel mononuclear five-coordinate nickel complexes with distorted square-pyramidal geometries are presented. They result from association of a tridentate "half-unit" ligand and 6,6'-dimethyl-2,2'-bipyridine according to a stepwise process that highlights the advantage of coordination chemistry in isolating an unstable tridentate ligand by nickel chelation. Their zero-field splittings (ZFS) were studied by means of magnetic data and state-of-the-art ab initio calculations. Good agreement between the experimental and theoretical axial D parameters confirms that large single-ion nickel anisotropies are accessible. The synthetic process can also yield dinuclear nickel complexes in which the nickel ions are hexacoordinate. This possibility is facilitated by the presence of phenoxo oxygen atoms in the tridentate ligand that can introduce a bridge between the two nickel ions. Two different double bridges are characterized, with the bridging oxygen atoms coming from each nickel ion or from the same nickel ion. This coordination change introduces a difference in the antiferromagnetic interaction parameter J. Although the magnetic data confirm the presence of single-ion anisotropies in these complexes, these terms cannot be determined in a straightforward way from experiment due to the mismatch between the principal axes of the local anisotropies and the presence of intersite anisotropies.