Herein, we have synthesised and characterised a novel organic–inorganic hybrid crystal, [CH ₃ NH ₃ ] ₂ KCr(CN) ₆ ( MACr ).

To gain a better insight into the factors affecting the enhancement of the energy barrier in single molecule (single ion) magnets, three hexacoordinate cobalt(II) complexes based on the tridentate ligand 2,6-bis(pyrazol-1-yl)pyridine (pypz) and pseudohalide ions have been synthesized and investigated. It was found that dicyanoamido and azido ligands act as bridges to form a one dimensional network based on a single μ_1,5-dca bridge [Co(pypz)(dca)(H₂O]·dca (1) and dimer [Co₂(pypz)₂(μ_1,1-N₃)₂(N₃)₂]·2CH₃OH (2), while tcm⁻ counterbalanced the charge of [Co(pypz)₂]²⁺ in [Co(pypz)₂](tcm)₂ (3), where dca⁻ = dicyanamide ion; tcm⁻ = tricyanomethanide ion, respectively. The DC magnetic data show a sizable magnetic anisotropy, which was confirmed by high-field/high frequency EPR measurements. Two of them are SIMs (1 and 3) and the other one is a SMM (2). All complexes exhibit field induced slow magnetic relaxation with two (1 and 2) or three relaxation channels (3) and an exceptionally long relaxation time for the low-frequency channel upon application of an external field B_DC = 0.4, 0.3, and 0.4 T at T = 1.9 K; τ(LF) = 1.9, 2.1 and 0.9 s, respectively. Additionally, the high spin-reversal barriers, U = 103 K for 1 and 95 K for 2, are among the largest for field-induced SIMs for cobalt(II) reported in the literature.

A mononuclear pentacoordinate iron(III) complex shows slow magnetic relaxation with three relaxation channels. The high-frequency relaxation time of the order of microseconds is prolonged on cooling in accordance with the direct and Raman processes. The low frequency relaxation time is little dependent on temperature and varies in the range τ(LF) = 0.52–0.79 s.

A series of mononuclear Co(II) complexes showing slow magnetic relaxation is assessed from the point of view of relaxation mechanisms. In certain cases, the reciprocating thermal behavior is detected: On cooling, the slow relaxation time is prolonged until a certain limit and then, unexpectedly, is accelerated. The low-temperature magnetic data can be successfully fitted by assuming Raman and/or phonon bottleneck mechanisms of the slow magnetic relaxation for the high-frequency relaxation channel. An additional term with the negative temperature exponent is capable of reproducing the whole experimental dataset.

The novel organic-inorganic hybrid salt [L]₂[CoCl₄] (1) and molecular complex [CoLCl₃] (2), where L⁺ is 2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridinium cation, feature simple {CoCl₄} and {CoCl₃N} tetrahedral environments of negligible (1) and a slightly higher distortion (2) that are responsible for rather low positive magnetic anisotropy of Co^II ion with D/hc = 12.1(6) (1) and 19.4(15) cm⁻¹ (2). Both compounds exhibit field-induced slow magnetic relaxation with three relaxation channels [low- (LF), intermediate- and high-frequency (HF) modes] that is frequency and field dependent. With the increased DC field, the peaks referring to the LF relaxation path are moved to lower frequencies so that the applied DC field causes prolongation of the relaxation time. The opposite is true for the HF relaxation branch: the peak is moved to higher frequencies. Considering the simplicity of the coordination environment and moderate magnetic anisotropy of the metal ion in 1 and 2, the compounds are unique with respect to the remarkably long relaxation time for a given applied DC field and temperature: τ_LF = 0.54(4) s at B_DC = 1.0 T and T = 2.0 K for 1, and τ_LF = 1.8(2) s at B_DC = 1.2 T and T = 1.9 K for 2.

A dinuclear [(H₂O)Cu(HLN₂O₂)Eu(NO₃)₃] complex exhibits paramagnetic behaviour with S = 1/2 ground state. At low temperature it displays slow magnetic relaxation that is supported by a small external magnetic field.