All chemical reagents and solvents were purchased from commercial sources and used as received without further purification. Elemental analyses of C, H and N were recorded on a Carlo Erba EA 1108 analyzer. Infrared spectra were performed in the range 4000–500 cm⁻¹ on a Varian 670 FT/IR spectrophotometer in the ATR mode.

[Tb(NO₃)₃(Ph₃PO)₃] (1): To a solution of Tb(NO₃)₃·5H₂O (0.130 g, 0.30 mmol) in acetonitrile (10 mL), triphenylphosphine oxide, (0.167 g, 0.60 mmol) and acetonitrile (10 mL) were added. The mixture was stirred for 4 h at room temperature and the resultant colorless solution was left to slowly evaporate, until single crystals of 1·CH₃CN precipitated. The single crystals were filtered, and they lose the acetonitrile solvate on drying to yield 1. Yield (based on Ph₃PO): 0.22 g (93%). M.W.: 1179.80. Anal. calcd. for C₅₄H₄₅TbN₃O₁₂P₃: C 54.97, H 3.84, N 3.56%. Found: C 54.89, H 3.59, N 3.95%. IR (ATR, ν~/cm⁻¹): 1120, 1153 (P = O), 1305 (NO3-).

The same product is isolated when Tb(NO₃)₃·5H₂O and Ph₃PO are mixed in 1:3 molar ratio in acetonitrile.

[Dy(NO₃)₃(Ph₃PO)₃] (2) was obtained in a similar way to 1: amounts of Dy(NO₃)₃·6H₂O (0.23 g, 0.66 mmol) and Ph₃PO (0.360 g, 1.32 mmol). Single crystals of 2·CH₃CN were isolated in the same way as those of 1·CH₃CN, which lose the acetonitrile solvate on drying to yield 2. Yield (based on Ph₃PO): 0.403 g (77%). M.W.: 1183.37. Anal. calcd. for C₅₄H₄₅DyN₃O₁₂P₃: C 54.81, H 3.83, N 3.55%. Found: C 54.84, H 3.40, N 3.53%. IR (ATR, ν~/cm⁻¹): 1120, 1153 (P = O), 1305 (NO3-).

The same product is isolated when Dy(NO₃)₃·6H₂O and Ph₃PO are mixed in 1:3 molar ratio in acetonitrile.

[Dy(NO₃)₃(EtOH)(Ph₃PO)₂] (3): A hot solution of Dy(NO₃)₃·6H₂O (0.137 g, 0.30 mmol) in ethanol (5 mL) was added to a hot solution of triphenylphosphine oxide (0.170 g, 0.60 mmol) in ethanol (5 mL). The mixture was stirred at room temperature for 24 h, when a colorless powder precipitated. The solid was filtered and dried in air. Yield: 0.250 g (88%). M.W.: 951.15. Anal. calcd. for C₃₈H₃₆DyN₃O₁₂P₂: C 47.98, H 3.81, N 4.42%. Found: C 47.90, H 3.81, N 4.29%. IR (ATR, ν~/cm⁻¹): 1119, 1150 (P = O), 1308 (NO3-), 3346 (OH).

[Dy(NO₃)₃(EtOH)(Ph₃AsO)₂] (4) was obtained in the same way as 3: amounts of Dy(NO₃)₃·6H₂O (0.23 g, 0.66 mmol) and Ph₃AsO (0.425 g, 1.32 mmol). The mixture yields a solution that by slow evaporation gives rise to single crystals of 4. Yield: 0.313 g (46%). M.W.: 1039.05. Anal. calcd. for C₃₈H₃₆DyN₃O₁₂As₂: C 43.92, H 3.49, N 4.04%. Found: C 44.44, H 3.35, N 3.92%. IR (ATR, ν~/cm⁻¹): 902, 924 (As = O), 1313 (NO3-), 3334 (OH).

[DyCl₂(Ph₃AsO)₄]Cl·1.25H₂O (5·1.25H₂O): To an ethanol (10 mL) solution of DyCl₃·6H₂O (0.460 g, 1.22 mmol), Ph₃AsO (0.77 g, 2.395 mmol) was added. The mixture was stirred under reflux for 1 h and the colorless solution was concentrated in a rotaevaporator to ca. 2 mL, yielding an oil. After standing for 24 h, single crystals of 5·2.75EtOH·1.25H₂O, suitable for X-ray diffraction studies, were isolated. The crystals were filtered and dried in air, losing the ethanol solvate to give rise to 5·1.25H₂O. Yield (based on Ph₃AsO): 0.74 g (78%). M.W.: 1579.93. Anal. calcd. for C₇₂H_62.5AsCl₃DyO_5.25: C 54.68, H 3.95%. Found: C 54.27, H 3.33%. IR (ATR, ν~/cm⁻¹): 886 (As = O), 3264 (OH).

The same product is isolated when DyCl₃·6H₂O and Ph₃AsO are mixed in 1:4 molar ratio in ethanol.

[Dy(OTf)₂(MePh₂PO)₄](OTf)·THF (6·THF): Dy(OTf)₃ (0.423 g, 0.694 mmol) was dissolved in THF (7 mL), and methyldiphenylphosphine oxide (0.600 g, 1.387 mmol) was added. The mixture was stirred at room temperature for 24 h, and the resultant colorless solution was concentrated in a rotaevaporator up to ca. 2 mL, yielding an oil. The oil was left to stand for 10 days at room temperature until single crystals of 6·THFwere obtained. Yield: 0.250 (47%). M.W.: 1546.63. Anal. calcd. for C₅₉H₆₀DyF₉O₁₄P₄S₃: C 45.78, H 3.88, S 6.21%. Found: C 45.79, H 3.65, S 5.91%. IR (ATR, ν~/cm⁻¹): 633 (δSO₃), 1020, 1262 (SO₃), 1156, 1220 (CF₃), 1132 (P = O).

The same product is isolated when Dy(OTf)₃ and MePh₂PO are mixed in 1:4 molar ratio in THF.

[Tb(hfa)₃(Ph₃PO)₂] (7): Terbium chloride hexahydrate (0.14 g, 0.36 mmol) was dissolved in distilled water (5 mL). An ethanol solution (20 mL) of hexafluoroacetylacetone (0.22 g, 1.08 mmol) was added to the aqueous solution. An ethanolic tetramethylammonium hydroxide solution (0.1 M) was added dropwise until pH 7 was reached. After stirring the mixture at room temperature for 6 h, the solvent was eliminated in a rotaevaporator. The white solid that precipitated was dissolved in methanol (30 mL), and triphenylphosphine oxide (0.10 g, 0.36 mmol) was added. The mixture was heated under reflux while stirring for 6 h. Slow evaporation of the resultant solution yields single crystals of 7, suitable for X-ray diffraction studies. The crystals were filtered and dried in air. Yield (based on Ph₃PO): 0.070 (29%). M.W.: 1336.65. Anal. calcd. for C₅₁H₃₃TbF₁₈O₈P₂: C 45.83, H 2.49%. Found: C 45.79, H 2.42%. IR (ATR, ν~/cm⁻¹): 1138 (P = O), 1650 (C = O), 1250, 1160 (CF₃).

The same product is isolated when TbCl₃·6H₂O, hfah and Ph₃PO are mixed in 1:3:2 molar ratio in water/ethanol.

[Dy(hfa)₃(Ph₃PO)₂] (8) was obtained in a similar way to 7: amounts of DyCl₃·6H₂O (0.400 g, 1.07 mmol), Ph₃PO (0.298, 1.07 mmol) and hfah (0.664 g, 3.21 mmol). Single crystals of 8 are isolated in the same way as those of 7. Yield 0.210 (29%). M.W.: 1340.22. Anal. calcd. for C₅₁H₃₃DyF₁₈O₈P₂: C 45.70, H 2.48%. Found: C 45.38, H 2.86%. IR (ATR, ν~/cm⁻¹): 1136 (P = O), 1652 (C = O), 1250, 1161(CF₃).

The same product is isolated when DyCl₃·6H₂O, hfah and Ph₃PO are mixed in 1:3:2 molar ratio in water/ethanol.

Magnetic susceptibility direct current (dc) and alternating current (ac) measurements for 1–8 were performed with a Quantum Design SQUID MPMS-XL-5 susceptometer. The magnetic susceptibility dc data were recorded at temperatures ranging from 2 to 300 K, under a magnetic field of 1000 Oe. Magnetization measurements under magnetic fields of 0–50000 Oe at 2.0 K were also recorded. Diamagnetic corrections were estimated from Pascal's Tables. ac susceptibility data were registered with an oscillating ac field of 3.5 Oe and ac frequency of 1400 Hz, under diverse applied static fields (H_dc = 0 or 1,000) for all the reported compounds. In the case of 2 and 8, ac susceptibility measurements were moreover recorded under a dc field of 1000 Oe at ac frequencies in the range 50–1400 Hz.

Crystal data and details of refinement are given in Table S1. Single crystals of 1·CH₃CN, 2·CH₃CN, 4, 5·2.75EtOH·1.25H₂O, 6·THF, 7 and 8 were obtained as detailed above. Data were collected at 100 K on a Bruker Kappa APEXII CCD diffractometer, employing graphite monochromated Mo-kα (λ = 0.71073 Å) radiation. Multi-scan absorption corrections were applied using SADABS (Blessing, 1995; Krause et al., 2015). The structures were solved by standard direct methods, using SHELXT (Sheldrick, 2015a), and then refined by full-matrix least-squares techniques on F², using the program package SHELXL (Sheldrick, 2015b) from the program package SHELX (Sheldrick, 2008). All non-hydrogen atoms were refined anisotropically, with the exception of some atoms with low occupation sites. Most of the hydrogen atoms were included in the structure factor calculations in geometrically idealized positions. However, non-disordered hydrogen atoms, which could be potentially involved in H-bonding schemes, were mostly located in the corresponding Fourier maps. These H atoms were freely refined, or with thermal parameters derived from their parent atoms.

CCDC 1850933–1850938 and 1851725 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/structures.

As previously stated, the chemistry of lanthanoids with phosphine oxides is quite well studied and many complexes with different metal ion:phosphine oxide ratios were isolated (Platt, 2017). These previous studies show that the stoichiometry of the complexes seems to be dependent on the nature of the metal salt employed, and on the molar quantities of salt and phosphine oxide in the reaction mixture, but also on the cone angle of the phosphine (Tolman, 1977). Recently, it has been reported that complexes of formulas [Ln(Cy₃PO)₂(H₂O)₅]X₃ (Ln = La, Dy, Er, Yb, Lu; X = Cl or Br) (Lees and Platt, 2014, 2015) can be easily obtained. These complexes have pentagonal bipyramidal geometry, and some of them are air stable single ion magnets, with high blocking temperatures (Chen et al., 2016). Nevertheless, similar pentagonal bipyramidal complexes were not reported for Ph₃PO, what would lead to evaluate the influence of the aromatic vs. the aliphatic ring in the magnetic behavior of the complexes. With these considerations in mind, different dysprosium and/or terbium salts were initially mixed with Ph₃XO (X = P or As) or MePh₂PO in 1:2 molar ratio, with the intention of isolating complexes of stoichiometry Ln(R₃XO)₂Y₅ (R₃ = Ph₃ or MePh₂; X = P or As; Y = OH₂ and/or monodentate anion).

The obtained results indicate that the salt:R₃XO molar ratio plays a secondary role in the stoichiometry of the isolated complexes (Figure 1), and that in this case the obtaining of compounds with coordination number 7 and pentagonal bipyramidal geometry was not possible. Thus, mixing of Ln(NO₃)₂·6H₂O (Ln = Dy or Tb) with Ph₃XO (X = P or As) in 1:2 molar ratio leads to isolate [Ln(NO₃)₃(Ph₃PO)₃] or [Dy(NO₃)₃(EtOH)(Ph₃XO)₂] compounds, as a function of the solvent employed in the reaction, as previously described for Ph₃PO (Cousins and Hart, 1967). Accordingly, it also seems that the nature of the X atom (P or As) in the Ph₃XO donor does not play any role in the stoichiometry of the isolated complexes. Besides, it should be noted that the compounds [Ln(NO₃)₃(Ph₃PO)₃] 1 and 2 are also obtained when the salts and oxides are mixed in 1:3 molar ratio, and that these complexes present the same stoichiometry as those reported for Ln = La, Nd, Eu, Er, Tm, and Yb with Cy₃PO (Hunter et al., 2007).

When the nitrate salt was changed by dysprosium chloride, and Ph₃AsO was added in 1:2 molar ratio, the hydrated complex [DyCl₂(Ph₃AsO)₄]Cl·1.25H₂O was obtained (Figure 1). Its stoichiometry is the same described for [DyCl₂(Ph₃PO)₄]Cl·2H₂O (Glazier et al. 2004), obtained by mixing of DyCl₃ and Ph₃PO in 1:4 molar ratio, but clearly differs from that of [Dy(Cy₃PO)₂(H₂O)₅]Cl₃ (Lees and Platt, 2014), which also has been obtained by mixing of DyCl₃ and Cy₃PO in 1:4 molar ratio in ethanol. Accordingly, comparison of this experiment with the reactions with dysprosium nitrate and with previous results (Glazier et al., 2004; Hunter et al., 2007; Lees and Platt, 2014) clearly show that the cone angle of the phosphine plays a fundamental role in the stoichiometry of phosphine or arsine oxide complexes, but that the similarity between the chemistry of Cy₃PO and Ph₃XO also depends on the nature of the anion of the salt employed.

Since three nitrate donors or two of the chloride anions are coordinated to the lanthanoid metal center in 1-4 and related complexes (Glazier et al., 2004; Hunter et al., 2007; Bowden et al., 2014), a salt of a non-coordinating anion, such as triflate, was chosen, in order to see the influence of the coordinating ability of the anion of the salt in the final stoichiometry, and in the hope of isolating complexes of stoichiometry [Dy(R₃XO)₂(H₂O)₅]Y₃. In this way, mixing of dysprosium triflate with MePh₂PO in 1:2 molar ratio, and in a non-dried non-coordinating solvent such as THF, yields [Dy(OTf)₂(MePh₂PO)₄]OTf, which also has a molar ratio metal:oxide of 1:4. Thus, this experiment shows that, in spite of the quantity of water present in the non-dried THF solvent, the dysprosium atom links to two poorly coordinating triflate ions, preventing the coordination of water to the metal center. Accordingly, this result resembles in part those obtained for Ce, Nd and Lu triflate complexes of Ph₃PO (Fawcett et al., 2002; Berthet et al., 2003;), where the stoichiometry is the same, but for the Ce and Nd derivatives the coordination number is 7, with one triflate acting as a bidentate chelate ligand and the other one as a monodentate donor.

Finally, the reactivity of Dy and Tb toward Ph₃PO, in 1:1 molar ratio and in the presence of hexafluoroacetylacetone, was studied, given that a [Eu(tmh)₃(RPh₃PO)] (tmh = 2,2,6,6-tetramethylheptane-3,5-dione, R = H, m-Me or p-Me) complex, with coordination number seven, was previously reported (Yanagisawa et al., 2017). As Eu³⁺ is significantly bigger than Tb³⁺ and Dy³⁺, the isolation of Tb and Dy complexes with coordination number 7 was expected even in the presence of the less sterically hindered hexafluoroacetylacetone donor. Nevertheless, this synthesis leads to complexes [M(hfa)₃(Ph₃PO)₂] (M = Tb, Dy) where the metal:oxide ratio is 1:2, as it occurs in the previously described [Eu(hfa)₃(Ph₃PO)₂] and related compounds (Hasegawa et al., 2013), suggesting that for this kind of complex the size of the lanthanoid ion has a poor influence in the stoichiometry of the isolated compound.

As a summary, comparison of the results described herein with those previously described for Cy₃PO (Chen et al., 2016, 2017) seems to indicate that the lower cone angle for triphenylphosphine compared with tricyclohexylphosphine prevents the isolation of pentagonal bipyramidal complexes of triphenylphosphine oxide with dysprosium or terbium. In the same way, hexafluoroacetylacetone allows coordination of two phosphine oxide ligands, leading to coordination number of 8 for the lanthanoid atom while the more sterically hindered 2,2,6,6-tetramethylheptane-3,5-dione only allows linking one phosphine ligand, leading to coordination number 7 (Yanagisawa et al., 2017). Therefore, this study clearly shows that not only the cone angle of the phosphine oxides but also the volume of the auxiliary ligands are fundamental factors in controlling the coordination number in this type of complex, the salt:oxide metal ratio, and even the size of the lanthanoid ion, playing secondary roles.

All the compounds described herein were fully characterized by analytical techniques, IR spectroscopy and by single crystal X-ray diffraction studies, except 3. In addition, direct and alternating current magnetic measurements were recorded for all the compounds.

The IR spectra of all the phosphine oxide complexes show strong bands in the range 1119–1153 cm⁻¹, which can be assigned to the P = O stretching vibration (Bowden et al., 2012). For complexes 1, 2, and 3 there are two bands in this range, indicating the existence of two types of non-equivalent phosphine oxide. Nevertheless, complexes 6-8 present just one band at ca. 1135 cm⁻¹, which suggest that all the phosphine oxide ligands present in the compounds are equivalent.

Complexes 4 and 5·1.25H₂O show strong bands about 900 cm⁻¹, which can be assigned to As = O vibrations (Levason et al., 2001). In 4 two bands are observable, also suggesting two non-equivalent arsine oxide ligands, while 5·1.25H₂O only shows an As = O band, in agreement with the equivalence of the four arsine oxide donors present in the complex.

In addition, the synthetized complexes also present bands related to the auxiliary nitrate, triflate or hexafluoroacetylacetonate ligands. Thus, complexes 1-4 show a strong band at ca. 1305 cm⁻¹, assigned to the ν₁(N-O) vibration of the nitrate ligand, acting as bidentate (Bowden et al., 2012). Complex 6 show many bands (633, 1020, 1262, 1156, and 1220 cm⁻¹) related to S-O or C-F vibrations, which suggest the presence of triflate (Johnston and Shiver, 1993). Finally, the IR spectra of 7 and 8 present strong bands at 1652 cm⁻¹, assigned to the stretching frequency of the C = O group, and at 1250 and 1161, in agreement with vibrations of the CF₃ and C-H moieties, all of which clearly suggest the presence of the hexafluoroacetylacetonate ligand (Richardson et al., 1968).

The direct current (dc) magnetic susceptibility studies of all the complexes (1-8) were recorded under a magnetic field of 1000 Oe in the temperature range 2–300 K. The plots of χ_MT vs. T for the complexes are shown in Figure S3. At room temperature (300 K), the χ_MT products for the terbium complexes 1 and 7 are in the range 11.81–12.37 cm³kmol⁻¹ while for the dysprosium complexes 2-6 and 8 vary from 14.05 to 14.93 cm³kmol⁻¹. All these values are similar to the expected ones for one isolated Tb^III (4f⁸, ⁷F₆) or Dy^III (4f⁹, ⁶H_15/2) ion of 11.82 and 14.17 cm³kmol⁻¹, respectively.

The experimental χ_MT values remain basically constant (1, 2), decreases very slowly (6-8) or decrease a bit quickly (3-5·1.25H₂O) upon cooling up to 50 K, and more rapidly below this temperature in all cases. This behavior appears to be mostly owing to the depopulation of the M_J sublevels of the lanthanoid ion, and possibly to very weak intermolecular interactions, which could be the cause of the more marked decrease in χ_MT at low temperature.

The variation of the magnetization with the field at 2 K for 1-8 (Figure S3) are akin in all cases, and shows a quite fast increase of the M/Nμ_B value at low field, without attaining saturation at 5 T. The M/Nμ_B value at the largest applied field of 5 T (in the 5.07–5.77 Ms/Nμ_B range) is remarkably lower than the predictable one for isolated Tb^III or Dy^III ions (Ms/Nμ_B = g_jJ), which can be ascribed to the crystal-field effects that promote a significant magnetic anisotropy.

Alternating current (ac) magnetic susceptibility measurements were also done for microcrystalline samples of compounds 1-8, with the aim of knowing their low-temperature magnetic relaxation behavior. No one of the compounds show out-of-phase ac susceptibilities (χ″) peaks at a frequency of 1400 Hz at zero field, indicating the absence of SMM behavior. Nevertheless, it should be noted that, sometimes, application of an external static field (Ruiz et al., 2012), in order to eliminate the mixing of the ground ±Ms levels, can partially or completely suppress the quantum tunneling of the magnetization (QTM), allowing to observe the slow relaxation process through the real thermally activated energy barrier (U). Therefore, this approximation was tested with the aim of checking the quantum tunneling effects.

Accordingly, variable temperature alternating current (ac) susceptibilities were measured for all the compounds at 1400 Hz and under a small dc field of 1000 Oe. In these conditions, the χ” vs. T curve only shows a clear net peak for 2, 4, and 8 (Figure S4), but in the case of 4 the peak appears at 2.5 K at a frequency of 1400 Hz. Thus, in view of these results, given that the magnetic response of 4 seems too weak, variable frequency and temperature ac susceptibilities were measured only for 2 and 8 with the application of an experimental optimal dc field of 1000 Oe. As shown in Figure 9, in these conditions 2 and 8 show frequency and temperature-dependence of the out-of-phase susceptibility, with maxima for χ” below 5 and 5.8 K for 2 and 8, respectively.

This indicates that both complexes (2 and 8) are field-induced single ion magnets. In addition, it should be noticed that Figure 9 reveals that χ” goes to zero below the maxima at low temperature for 2, which suggest that the fast relaxation of the magnetization by a quantum tunneling mechanism has been completely suppressed by the application of the external magnetic field of 1000 Oe. Nevertheless, this does not occur for 8, where the QTM does not seem fully suppressed at low temperatures.

According to this, the experimental data for 2 (Figure 9B) were initially fitted to the Arrhenius equation (thermally activated Orbach relaxation process), and the best fit affords a small U_eff barrier of 29.11 K. But the experimental data deviate from the linear Arrhenius plot, which can be due to the simultaneous presence of different relaxation processes.

In keeping with this, the Cole-Cole plot for 2 (Figure 10) between 2.5 and 5 K gives parameters in the range 0.19–0.04, suggesting more than one relaxation process at the lowest temperature. Therefore, the experimental relaxation times were fitted to equation 1, which takes into account contributions from Orbach and Raman processes.

The best fit with this equation (Figure 9B), which reproduces all the curve, leads to an U_eff barrier of 32.17 K.

In the case of 8, the best fit of the experimental relaxation times to the Arrhenius equation leads to an U_eff of 28.17 K. However, as in the case of 2, the experimental relaxation times deviate from linearity (Figure 9), indicating the possible existence of several relaxation processes. The Cole-Cole plot for 8 between 2.6 and 6 K (Figure 10), which renders parameters in the range 0.16–0.04, corroborate the presence of more than one relaxation process, especially in the low-temperature area. In view of this, the experimental data were also fitted to the equation 1. The best fit led to the parameters indicated in Figure 9D. Although for Kramers ion like Dy^III an n value equal to 9 is expected (Abragam and Bleaney, 1970), values between 2 and 7 are also realistic when both acoustic and optical phonons are present (Singh and Shrivastava, 1979; Shrivastava, 1983).

At this point it is worth mentioning that the free ion electronic density of the Dy^III ion has an oblate shape, which can be stabilized by an axial crystal field leading to an axial anisotropy in the ground state (M_J = ± 15/2). This bistable ground state favors the SMM behavior. The axial crystal field is relatively easy to attain in low-symmetry Dy^III complexes, and this is the reason why this type of complexes usually shows slow relaxation of the magnetization and SMM behavior, either at zero applied dc field or in the presence of a magnetic field (when a significant QTM is operative).

The non-Kramers Tb^III ion is also an oblate ion and therefore also requires an axial crystal field to reach an axial bistable ground state, and then to exhibit SMM behavior. However, to this end a rigorous axial symmetry must be maintained, which is not an easy task. Moreover, non-Kramers ions possess an intrinsic tunneling gap that promotes QTM at zero field. Therefore, the lack of SMM at zero field, and in the presence of a small field for complexes 1 and 7 is not unexpected.

The fact that, among the Dy^III complexes, only compounds 2, 4, and 8 exhibit slow relaxation of the magnetization can be explained by analyzing the Dy-O distances and their distribution in the Dy^III coordination sphere, as these factors play an essential role in dictating the SMM behavior. The required axial crystal field can be reached when the donors atoms with the largest electron densities (generally those involved in the shortest Dy-donor distances) are located above and below the equatorial plane, thus diminishing the repulsive interactions between the donor atoms of the ligands and the oblate charge cloud of the Dy^III ion. Nevertheless, the crystal field can also be generated when a Dy-donor distance is significantly shorter that the other Dy-donor distances. Taking into account these considerations, complexes 5 and 6, which exhibit an elongated octahedral geometry, with the four shortest Dy-O distances in the equatorial plane, present an equatorial crystal field. Therefore, the fact that these complexes do not present SMM behavior is not surprising. In the case of 2, the three Dy-O_{phosphine oxide} distances, with a near T disposition, are significantly shorter than the other ones. Nevertheless, two of them (Dy-O1 and Dy-O2) are slightly but significantly shorter that the other one. This disposition leads to a feeble axial crystal field that favors slow relaxation of the magnetization. The fact that at zero-field this compound does not show any peak in the ac measurements can be due to the fast QTM promoted transverse anisotropy, as well as to intermolecular and hyperfine interactions. For complex 4, the shortest Dy-O distances are also those involving the oxygen atoms of the arsine-oxide ligands. These distances are approximately 0.2 and 0.1 Å shorter than those of the coordinated nitrate anions and ethanol molecule, respectively. This disposition of the shortest Dy-O distances is similar to that of 2 and therefore the appearance of field induced slow relaxation in 4 is not unexpected. Finally, for compound 8, the two shortest Dy-O bond distances are those involving the Ph₃PO ligands, which are at opposite sides of the Dy^III ions, with a O1-Dy-O2 angle of 143.63(4)°. This distribution of the phosphine ligands around the Dy^III ions generates a clear axial crystal field that is responsible of the SMM behavior observed for this compound.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.