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The properties, as well as the stabilization of the long-lived metastable state present in the long alkylated spin-crossover cobalt(II) compound are analyzed. The relaxation of these compounds is shown to be very slow because of the presence of long alkyl chains leading to large structural transition.

The Fe(III) ion in the title complex, [FeCl(2)(C(20)H(13)N(2)O)], has a distorted square-pyramidal coordination formed by one O atom and two N atoms from a tridentate 1-(8-quinolylimino-meth-yl)-2-naphtholate ligand and two Cl atoms. In the crystal structure, mol-ecules form a column structure along the a axis through π-π stacking inter-actions, with centroid-centroid distances of 3.657 (1) and 3.818 (2) Å. Weak C-H⋯Cl inter-actions are observed between the columns.

Iron(III) spin-crossover compounds [Fe(pap)(2)]ClO(4) (1), [Fe(pap)(2)]BF(4) (2), [Fe(pap)(2)]PF(6) (3), [Fe(qsal)(2)]NCS (4), and [Fe(qsal)(2)]NCSe (5) (Hpap=2-(2-pyridylmethyleneamino)phenol and Hqsal=2-[(8-quinolinylimino)methyl]phenol) were prepared and their spin-transition properties investigated by magnetic susceptibility and Mössbauer spectroscopy measurements. The iron(III) compounds exhibited spin transition with thermal hysteresis. Single crystals of the iron(III) compounds were obtained as suitable solvent adducts for X-ray analysis, and structures in high-spin (HS) and low-spin (LS) states were revealed. Light-induced excited-spin-state trapping (LIESST) effects of the iron(III) compounds were induced by light irradiation at 532 nm for 1-3 and at 800 nm for 4 and 5. The activation energy E(a) and the low-temperature tunneling rate k(HL)(T-->0) of iron(III) LIESST compound 1 were estimated to be 1079 cm(-1) and 2.4x10(-8) s(-1), respectively, by HS-->LS relaxation experiments. The Huang-Rhys factor S of 1 was also estimated to be 50, which was similar to that expected for iron(II) complexes. It is thought that the slow relaxation in iron(III) systems is achieved by the large structural distortion between HS and LS states. Introduction of strong intermolecular interactions, such as pi-pi stacking, can also play an important role in the relaxation behavior, because it can enhance the structural distortion of the LIESST complex.