N-R¹-N′-R²-4,4′-Bipyridinium and N-R¹-4,4′-bipyridinium salts containing ferrocenyl substituents were prepared as donor–acceptor compounds (R¹ = ferrocenyl, ferrocenylphenyl; R² = ferrocenyl, ferrocenylphenyl, phenyl, methyl). Platinum(II)-bridged diferrocene compounds were prepared by using the N-R¹-4,4′-bipyridinium ligands. These compounds display multistage redox properties. The redox potentials of the ferrocenyl and bipyridinium moieties in these compounds are independently affected by molecular modifications, which indicates that conjugation between the donor and acceptor moieties is less effective. Single-crystal X-ray diffraction analyses revealed large twist angles between the two moieties in the solid state. The compounds exhibit broad charge-transfer absorption bands in the range 450–800 nm. No electronic communication was observed between the terminal ferrocenes in the diferrocene complexes.

Charge-transfer salts composed of biferrocene derivatives and X_n–TCNQ (X = F, Cl; n = 1, 2; TCNQ = tetracyanoquinodimethane) were prepared and crystallographically characterized. All nine salts exhibited a 1:2 donor/acceptor (D/A) ratio consisting of mixed-valence biferrocenium cations and anions with acceptor dimers. The packing patterns were correlated to the donor substituents. On the basis of the π–π interactions of the cations, the observed structures were classified into three categories: segregated-stack structures for the biferrocenium and dibromo-biferrocenium salts, mixed-stack structures [···(DA₂)_n···] for the diiodo-biferrocenium salts, and intermediate structures [···(DDA₂A₂)_n···] for the bromo-biferrocenium salts.

To develop solvent-recognition films, Nafion membranes incorporating cationic nickel-chelate complexes, that is, [Ni(L¹)(L²)]⁺ (HL¹=acetylacetone, 2,2,6,6-tetramethyl-3,5-heptanedione; L²=N,N-diethylethylenediamine, N-butyl-N,N′,N′-trimethylethylenediamine), were prepared. Immersion of the films in various solvents effected the color changes varying from red to pale blue green depending on the donor number of the solvents. The color change is based on an equilibrium shift between square-planar and solvent-coordinated octahedral geometries of the cations. The degree of the color change depended on the affinity of the incorporated complex to the solvent molecules. The films were robust and exhibited a reversible solvent response. The films exhibited thermochromism when a small amount of appropriate solvents were incorporated and changed from pale blue green at low temperatures to red at high temperatures.

Crystal structures and thermal properties of cobaltocenium salts with bis(perfluoroalkylsulfonyl)amide (C_nF_2n+1SO₂)₂N anions [n=0 (1), 1 (1 a), 2 (1 b), 3 (1 c), and 4 (1 d)] and the 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonylamide anion (2) were investigated. In these solids, the cations are surrounded by four anions around their C₅ axis, and stacking of these local structures forms two kinds of assembled structures. In the salts with even n (1, 1 b, and 1 d), the cation and anion are arranged alternately to form mixed-stack columns in the crystal. In contrast, in the salts with odd n (1 a and 1 c), the cations and anions independently form segregated-stack columns. An odd–even effect was also observed in the sum of the phase-change entropies from crystal to melt. All of the salts exhibited phase transitions in the solid state. The phase transitions to the lowest-temperature phase in 1, 1 a, and 2 are accompanied by order–disorder of the anions and symmetry lowering of the space group, which results in the formation of an ion pair. Solid-state ¹³C NMR measurements on 1 a and 1 b revealed enhanced molecular motions of the cation in the higher-temperature phases.

Vapor- and gas-responsive ionic liquids (ILs) comprised of cationic metal-chelate complexes and bis(trifluoromethanesulfonyl)imide (Tf₂N) have been prepared, namely, [Cu(acac)(BuMe₃en)][Tf₂N] (1 a), [Cu(Bu-acac)(BuMe₃en)][Tf₂N] (1 b), [Cu(C₁₂-acac)(Me₄en)][Tf₂N] (1 c), [Cu(acac)(Me₄en)][Tf₂N] (1 d), and [Ni(acac)(BuMe₃en)][Tf₂N] (2 a) (acac=acetylacetonate, Bu-acac=3-butyl-2,4-pentanedionate, C₁₂-acac=3-dodecyl-2,4-pentanedionate, BuMe₃en=N-butyl-N,N′,N′-tetramethylethylenediamine, and Me₄en=N,N,N′,N′-trimethylethylenediamine). These ILs exhibited reversible changes in color, thermal properties, and magnetic properties in response to organic vapors and gases. The Cu^II-containing ILs are purple and turn blue-purple to green when exposed to organic vapors, such as acetonitrile, methanol, and DMSO, or ammonia gas. The color change is based on the coordination of the vapor molecules to the cation, and the resultant colors depend on the coordination strength (donor number, DN) of the vapor molecules. The vapor absorption caused changes in the melting points and viscosities, leading to alteration in the phase behaviors. The IL with a long alkyl chain (1 d) transitioned from a purple solid to a brown liquid at its melting point. The Ni^II-containing IL (2 a) is a dark red diamagnetic liquid, which turned into a green paramagnetic liquid by absorbing vapors with high DN. Based on the equilibrium shift from four- to six-coordinated species, the liquid exhibited thermochromism and temperature-dependent magnetic susceptibility after absorbing methanol.

The piano-stool half-metallocenium cations [Fe(C₅R₅)(CO)₂L]⁺ (C₅R₅=C₅H₅, C₅Me₅, C₅Me₄Et; L=1-pentene, nBuCN, MeCN, Me₂S, NH₃, NMe₃, pyridine) provide ionic liquids (ILs) with the bis(trifluoromethanesulfonyl)imide (Tf₂N) anion without introducing long alkyl chains. Their melting points are affected by molecular symmetry, and their thermal stabilities reflect the strength of the metal–ligand bonding. These are reactive liquids that show solventless ligand exchange reactions by gas absorption. The direction of the ligand-exchange reaction is correlated with the stabilities. Based on the variation of the melting points, these ILs undergo transformations between the liquid and solid phases associated with the reaction.

Simple cationic sandwich complexes that contained alkyl- or halogen substituents provided ionic liquids (ILs) with the bis(perfluoroalkanesulfonyl)imide anion. Ferrocenium- and cobaltocenium ILs [M(C₅H₄R¹)(C₅H₄R²)][Tf₂N] (M=Fe, Co) and arene–ferrocenium ILs [Fe(C₅H₄R¹)(C₆H₅R²)][Tf₂N] were prepared and their physical properties were investigated. A detailed comparison of their thermal properties revealed the effects of molecular symmetry and substituents on their melting points. Their viscosity increased on increasing the length of the substituent on the cation and the perfluoroalkyl chain length on the anion. Upon cooling, ILs with low viscosities exhibited crystallization, whereas those with higher viscosities tended to exhibit glass transitions. Most of these salts showed phase transitions in the solid state. A magnetic-switching phenomenon was observed for the paramagnetic ferrocenium IL, which was associated with a liquid/solid transformation, based on the magnetic anisotropy of the ferrocenium cation. ⁵⁷Fe Mössbauer spectroscopy was applied to [Fe(C₅H₄nBu)₂][Tf₂N] to investigate the vibrational behavior of the iron atom in the crystal and glassy states of the ferrocenium IL.

This review describes the preparation and structures of ferrocene-containing coordination polymers with a focus on ligand design. Neutral ligands such as heteroaryl ferrocenes and anionic ligands such as ferrocenyl carboxylates can be used to construct one-dimensional coordination polymers of various structural types. The main-chain and side-chain polymer structures are designed, and the conformational flexibility of ferrocenes leads to structural variety. The use of additional bridging ligands along with the anionic ligands leads to extended one- to three-dimensional structures. These complexes exhibit various properties, including characteristic redox activities.