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The novel mononuclear Co(II) complex cis-[Co(II)(dmphen)(2)(NCS)(2)]·0.25EtOH (1) (dmphen = 2,9-dimethyl-1,10-phenanthroline) features a highly rhombically distorted octahedral environment that is responsible for the strong positive axial and rhombic magnetic anisotropy of the high-spin Co(II) ion (D = +98 cm(-1) and E = +8.4 cm(-1)). Slow magnetic relaxation effects were observed for 1 in the presence of a dc magnetic field, constituting the first example of field-induced single-molecule magnet behavior in a mononuclear six-coordinate Co(II) complex with a transverse anisotropy energy barrier.

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This contribution is concerned with the synthesis and characterization of two new iron(II) spin-crossover dinuclear compounds of the formula [Fe(bpym)(NCSe) 2 ] 2 (bpym) (2) and [Fe(bt)(NCSe) 2 ] 2 (bpym) (4), where bpym ) 2,2′bipyrimidine and bt ) 2,2′-bithiazoline. Evidence for a S ) 2 (high spin, HS) T S ) 0 (low spin, LS) spincrossover behavior is provided by variable-temperature magnetic susceptibility measurements and 57 Fe Mo ¨ssbauer spectrometric data. Compound 2 shows only one step which involves 50% of the iron(II) ions. The transition is very abrupt and takes place at T c ≈ 120 K when cooling and shows a hysteresis loop of 2.5 K. A two-step spin transition takes place for 4. The first one occurs at 223 K and the second one at 265 K. Differential scanning calorimetry exhibits two peaks in the case of 4, a sharp one at 225 K and a broad one with a maximum at 262 K. ∆H and ∆S variations, per mol of dinuclear units, have been evaluated accurately for the first peak only, 5.7 ( 0.5 kJ mol -1 and 25.5 ( 1 J mol -1 K -1 , respectively. A rough estimate of the global variations gave 16 kJ mol -1 and 65 J mol -1 K -1 , respectively. Thermodynamic parameters, per mol of dinuclear units, for 2 are ∆H ) 3.0 ( 0.1 kJ mol -1 and ∆S ) 25 ( 1 J mol -1 K -1 . X-ray absorption spectroscopy has been carried out for 2 and 4 as well as for the parent compounds [Fe(bpym)(NCS) 2 ] 2 (bpym) (1) and [Fe(bt)(NCS) 2 ] 2 (bpym) (3). The X-ray absorption near edge structures spectra of the two spin isomers are interpreted and the extended X-ray absorption fine structures spectra are analyzed quantitatively at the first coordination shell. The average Fe-N bond distance is close to 0.2 Å larger for the HS state than that of the LS state. The spin transitions in these dimeric compounds are interpreted in terms of LS-LS T LS-HS and LS-HS T HS-HS intramolecular processes.