A series of mononuclear tetrahedral cobalt(II) complexes with the general molecular formula [Co(L)X] [where L = tetramethylthiourea ([(CH)N]C═S) and X = Cl (1), Br (2), and I (3)] were isolated, and their structures were characterized by single-crystal X-ray diffraction. The experimental direct-current magnetic data are excellently reproduced by fitting both χ T( T) and M( H) simultaneously using the spin Hamiltonian (SH) parameters D = -18.1 cm and g = 2.26, D = -16.4 cm and g = 2.33, and D = -22 cm and g = 2.4 for 1-3, respectively, and the sign of D was unambiguously confirmed from X-band electron paramagnetic resonance measurements. The effective energy barrier extracted for the magnetically diluted complexes 1-3 (10%) is larger than the barrier observed for the pure samples and implies a nonzero contribution of dipolar interaction to the magnetization relaxation dynamics. The SH parameters extracted for the three complexes drastically differ from their respective parent complexes that possess the general molecular formula [Co(L)X] [where L = thiourea [(NH)C═S] and X = Cl (1a), Br (2a), and I (3a)], which is rationalized by detailed ab initio calculations. An exhaustive theoretical study reveals that both the ground and excited states are not pure but rather multideterminental in nature (1-3). Noticeably, the substitution of L by L induces structural distortion in 1-3 on the level of the secondary coordination sphere compared to 1a-3a. This distortion leads to an overall reduction in | E/ D| of 1-3 compared to 1a-3a. This may be one of the reasons for the origin of the slower relaxation times of 1-3 compared to 1a-3a.

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Four mononuclear cobalt­(II) complexes with pseudo tetrahedral geometry were isolated with different counteranions; their structure solution reveals the molecular formula as [Co­(L1)4]­X2 [where L1 = thiourea (NH2CSNH2) and X = NO3 (1), Br (2), and I (3)] and [Co­(L1)4]­(SiF6) (4). The detailed analysis of direct-current (dc) magnetic data reveals a zero-field splitting (ZFS; D) with m S = ±3/2 as the ground levels (D < 0) for the four complexes. The magnitude of the ZFS parameter is larger, in absolute value, for 1 (D = −61.7 cm–1) than the other three complexes (−5.4, −5.1, and −12.2 cm–1 for 2–4, respectively). The sign of D for 1, 2, and 4 was unambiguously determined by X-band electron paramagnetic resonance (EPR) spectroscopy of the diluted samples (10%) at 5 K. For 3, the sign of D was naturally endorsed from the frequency-dependent out-of-phase signal (χM″) observed in the absence of an external dc magnetic field and confirmed by high-frequency EPR (70–600 GHz) experiments performed on a representative pure polycrystalline 3, which gave a quantitative D value of −5.10(7) cm–1. Further, the drastic changes in the spin Hamiltonian parameters and their related relaxation dynamics phenomena (of 2–4 compared to 1) were rationalized using ab initio complete-active-space self-consistent field/n-electron valence perturbation theory calculations. Calculations disclose that the anion-induced structural distortion observed in 2–4 leads to a nonfavorable overlap between the π orbital of cobalt­(II) and the π* orbital of the sulfur atom that reduces the overall |D| value in these complexes compared to 1. The present study demonstrates that not only the first but also the second coordination sphere significantly influences the magnitude of the ZFS parameters. Particularly, a reduction of D of up to ∼90% occurs (in 2–4 compared to 1) upon a simple variation of the counteranions and offers a viable approach to modulate ZFS in transition-metal-containing single-molecule magnets.

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The mechanistic investigations between Cu(II) and the anisotropic lanthanides (Ln(III)) are not much explored to date. This is due to the complicated energy spectrum which arises due to the orbital angular momentum of anisotropic lanthanides. Interestingly, the exchange coupling J in Ln(III)−Cu(II) systems was found to be antiferromagnetic for <4f 7 metal ions and ferromagnetic for ≥4f 7 metal ions, while the net magnitude of J Total strength gradually decreases moving from f 1 to f 13 . While this is established in several examples, the reason for this intriguing trend is not rationalized. In this article, we have taken up these challenging tasks by synthesizing a family of complexes with the general molecular formula [Cu 2 Ln(HL) 4 (NO 3 )](NO 3 ) 2 , where Ln = La (1 -La ), Ce (2 -Ce ), Pr (3 -Pr ), Gd (4 -Gd ), Tb (5 -Tb ), Dy (6 -Dy ), and Ho (7 -Ho ) and HL = C 15 H 15 N 1 O 3 ; (2methoxy-6-[(E)-2′-hydroxymethyl-phenyliminomethyl]-phenolate) is a monodeprotonated tridentate Schiff base ligand. Detailed dc magnetic susceptibility measurements performed for all the complexes reveal that the Cu(II) ion is coupled ferromagnetically to the respective Ln(III) ion, which has more than seven electrons in the 4f shell, while an antiferromagnetic coupling is witnessed if Ln(III) has less than seven electrons. The strength of the exchange coupling constant was quantitatively determined for representative complexes from the high-field/high-frequency electron paramagnetic resonance spectroscopy which follows the order of 4 -Gd (1.50(10) cm −1 ) > 5 -Tb (1.18(10) cm −1 ) > 6 -Dy (0.56(10) cm −1 based on the −

To investigate the influence of the coordination geometry on the magnetization relaxation dynamics, two geometric isomers of a fivecoordinate low-spin Co(II) complex with the general molecular formula [Co(DPPE) 2 Cl]SnCl 3 (DPPE = diphenylphosphinoethane) were synthesized and structurally characterized. While one isomer has a square pyramidal geometry (Co-SP (1)), the other isomer figures a trigonal bipyramidal geometry (Co-TBP (2)). Both complexes were already reported elsewhere. The spin state of these complexes is unambiguously determined by detailed direct current (dc) magnetic data, X-band, and high-frequency EPR measurements. Slow relaxation of magnetization is commonly observed for systems with S > 1/2. However, both 1 and 2 show field-induced slow relaxation of magnetization. Especially 1 shows relaxation times up to τ = 35 ms at T = 1.8 K, which is much longer than the reported values for undiluted Co(II) low-spin monomers. In 2, the maximal field-induced relaxation time is suppressed to τ = 5 ms. We attribute this to the change in g-anisotropy, which is, in turn, correlated to the spatial arrangement of ligands (i.e., coordination geometry) around the Co(II) ions. Besides the detailed electronic structure of these complexes, the experimental observations are further corroborated by theoretical calculations.

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A phenomenological lattice theory of elastic dielectric based on the Lundqvist potential of ionic cohesion is developed. In this theory the lattice is considered to be initially in the deformed and polarised state. The theory yields the expressions for the second order elastic constant, third order elastic constants, dielectric constants, and its strain derivatives. The many body lattice theory (MLT) of elastic dielectric so developed is applied to calculate the strain derivatives of the high and low frequency dielectric constants of the alkali halide solids. The calculated values of these constants are better than those obtained in all earlier models and are in excellent agreement with the experimental values.Es wird eine phanomenologische Gittertheorie elastischer Dielektrika entwickelt, die auf dem Lundqvistpotential fur die Ionenkohasion beruht. In dieser Theorie wird angenommen, daI3 das Gitter anfanglich im deformierten und polarisierten Zustand ist. Die Theorie liefert die Ausdrucke fur die elastischen Konstanten zweiter Ordnung, elastischen Konstanten dritter Ordnung, dielektrischen Konstanten und ihre Spannungsableitungen. Die so entwickelte Vielkorper-Gitter-Theorie (MCT) elastischer Dielektrika wird benutzt, um die Spannungsableitungen der Hoch-und Niederfrequenz-Dielektrizitatskonstanten der Alkalihalogenide zu berechnen. Die berechneten Werte dieser Konstanten sind besser als die, die mit allen fruherenModellen erhalten wurden, und stimmen ausgezeichnet mit den experimentellen Werten uberein.