Iron(III) spin-crossover (SCO) complexes [Fe(qsal)]BS·MeOH·HO (1), [Fe(qsal)](NS)·MeOH (2), [Fe(qnal)](NS) (3), and [Fe(qnal)]PS·MeOH·CHCl (4) (Hqsal, N-(8-quinolinyl)salicylaldimine; Hqnal, N-(8-quinolinyl)-2-hydroxy-1-naphthaldimine; BS, benzenesulfonate; NS, 1-naphthalenesulfonate; PS, 1-pyrenesulfonate) have been synthesized and characterized by X-ray structure determinations and temperature-dependent magnetic susceptibility measurements. The aromatic counteranions BS, NS, and PS can be used for the tuning of intermolecular coupling through a variety of weak interactions. All of the complexes show temperature-dependent SCO behavior. but the light-induced excited spin-state trapping (LIESST) effect was observed only for 1, 3, and 4 when the samples were illuminated (λ 808 nm) for 1 h at 5 K. In particular, 59% of the LS form of 1 was converted to the metastable HS state by illumination, equal to the highest degree of conversion yet known for LIESST in [Fe(qsal)] derivatives. The lack of a LIESST effect for 2 may be due to the relatively limited degree of interaction between the cations and anions in the lattice, reflected in a much longer minimum Fe···Fe separation in this complex in comparison to the others.

The cobalt(ii) complex incorporating π-conjugated substituent, [Co(Naph-C2-terpy)2](BF4)2 (1; Naph-C2-terpy = 4'-(2-naphthoxy(ethoxy))-2,2':6',2''-terpyridine), exhibits an abrupt spin transition (ST) behavior (cooperative factor C = 0.91) while its solvated product, 1·2MeOH, shows gradual spin crossover (SCO) behavior (C = 0.49). Single crystal X-ray structural analyses demonstrated that the octahedral coordination core [CoN6] in 1 shows larger distortion in both high-spin and low-spin states than solvated 1·2MeOH or another two derivatives, [Co(R-terpy)2](BF4)2 (R = 2-naphthyl (2), 9-anthracenyl (3)). The respective distortion parameters (Σ) are compared with those for previously reported SCO cobalt(ii) compounds. The highly-distorted [CoN6] core in 1 (Σ = 126 in the HS state and 101.6 in the LS state) was stabilized by strong intermolecular interactions and observed an abrupt ST behaviour.

CO2‐responsive spin‐state conversion between high‐spin (HS) and low‐spin (LS) states at room temperature was achieved in a monomeric cobalt(II) complex. A neutral cobalt(II) complex, [CoII(COO‐terpy)2]⋅4 H2O (1⋅4 H2O), stably formed cavities generated via π–π stacking motifs and hydrogen bond networks, resulting in the accommodation of four water molecules. Crystalline 1⋅4 H2O transformed to solvent‐free 1 without loss of porosity by heating to 420 K. Compound 1 exhibited a selective CO2 adsorption via a gate‐open type of the structural modification. Furthermore, the HS/LS transition temperature (T1/2) was able to be tuned by the CO2 pressure over a wide temperature range. Unlike 1 exhibits the HS state at 290 K, the CO2‐accomodated form 1⊃CO2 (PCO2 =110 kPa) was stabilized in the LS state at 290 K, probably caused by a chemical pressure effect by CO2 accommodation, which provides reversible spin‐state conversion by introducing/evacuating CO2 gas into/from 1.

This review discusses the correlation of the local and whole molecular structure of iron(iii) complexes with the magnetic properties including the light-induced excited spin-state trapping (LIESST) effect.