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.

We present an overview of very recent advances in the engineering of magnetic networks based on octacyanometalates. The selected magnetic networks of Cu^IIW^V, Ni^IIW^V and Mn^IILNb^IV (L – organic bridging linker) illustrate the possible strategies for tuning of the magnetic characteristics. The combination of magnetic ordering for 2D (two-dimensional) and 3D (three-dimensional) networks together with the solvent sensitivity of a cyano-bridged framework resulted in the development of a novel 3D {[Mn^II(imH)]₂[Nb^IV(CN)₈]} assembly with magnetic sponge character, characterized by T_c of 62 K, the highest ever observed for octacyanometalate-based networks.

Unique two dimensional enantiopure cyanido-bridged {[Mn^II(R-mpm)₂]₂[Nb^IV(CN)₈]}·4H₂O and {[Mn^II(S-mpm)₂]₂[Nb^IV(CN)₈]}·4H₂O (1-S) (mpm = α-methyl-2-pyridine-methanol) ferrimagnets with T_C = 23.5 K were synthesized and characterized. They reveal natural optical activity (NOA) due to the chiral crystal structure, and magnetic optical activity (MOA) in the presence of an external magnetic field, with the strong enhancement in the magnetically ordered phase below T_C.