Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)₈]ⁿ⁻ ions comprise a series of complexes of heavy transition metals in high oxidation states, including Nb^IV, Mo^IV/V, W^IV/V, and Re^V. Since the discovery of the pioneering bimetallic {Mn^II ₄[M^IV(CN)₈]₂} and {Mn^II ₉[M^V(CN)₈]₆} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d–d or d–f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)₈]ⁿ⁻-based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)₈]ⁿ⁻ ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

An alternating bimetallic {(H₃O)[Ni^III(cyclam)][Fe^II(CN)₆]·5H₂O}_n chain undergoes reversible dehydration at 40 °C accompanied by electron transfer which leads to Ni^II–Fe^III in about 50% of metal centres. The hydrated dark blue form is a paramagnet while the dehydrated yellowish-green form shows ferromagnetic coupling between neighbouring Ni^II and Fe^III.

A new crystalline pseudo-polymorphic form of the 2D microporous honeycomb-like magnetic network {[Ni(cyclam)]₃[W(CN)₈]₂·solv}_n was obtained from the reaction between methanol-soluble {Ni[Ni(MeOH)₃]₈[W(CN)₈]₆} clusters and the [Ni(cyclam)]²⁺ complex in an acetonitrile-rich solvent mixture. In comparison to the three earlier characterised pseudo-polymorphs, which all crystallised in the triclinic space group P, the new form shows higher symmetry of C2/m, greater porosity of ca. 40% solvent accessible volume, and the highest magnetic ordering temperature of 12 K. It undergoes an irreversible single-crystal-to-single-crystal transformation to the hexadecahydrate form that is stable in air at ambient temperature. A new synthetic pathway leading directly to methanol solvate of the same network, which was previously obtained by sorption of MeOH into the anhydrous form, is also reported.

Reaction between [Ni(cyclam)]²⁺ and [W(CN)₈]⁴⁻ leads to the formation of a 3D diamond-like network [Ni(cyclam)]₂[W(CN)₈]·3.5H₂O (1). The structure is characterised by a network of intersecting channels of 3–4 Å in diameter, filled with crystallisation water, most of which is released upon drying in air, causing the crystals to collapse. Analogous compound [Ni(cyclam)]₂[Nb(CN)₈]·3.5H₂O (2), based on paramagnetic Nb(IV), could only be obtained as a powder, due to the decomposition of the [Nb(CN)₈]⁴⁻ complex under slow diffusion conditions. It shows long-range magnetic ordering with T_C = 11.8 K and magnetic hysteresis at 2 K. These properties are lost upon drying in air. After rehydration differently shaped hysteresis appears, which together with AC susceptibility measurements suggests the formation of a multiphase system. Subsequent dehydration–rehydration experiments show partial reversibility.