*2.3.2. Tetracyano-metallates*

In recent years, great efforts have been devoted to research directed toward the synthesis and characterization of multidimensional metal complexes with cyanide bridge groups [25]. The most common and controlled strategy at the time of preparing this type of system is in autoassembly specifically designed predecessors. They are normally used as a complex cyanometallate that acts as a ligand and a complex of a metal transition coordination position for atoms of nitrogen from the groups free of cyanide (see Figure 3).

**Figure 3.** Assembly blocks of cyanometallate systems: (a) the anionic block [M (CN)6] 3-, (b) the cationic block [M´L2] n+.

Tetracyano-metallates and hexacyano-metallates (Prussian blue analogues) are part of the materials with cyanide bridge group. Anion cyano (CN-) is a functional group with the ability to be simultaneously coordinated by both atoms at different cations. This makes it an excellent molecular block to generate different lattices structural at 1, 2, and 3 dimensions. The chemical bond in the cyanometallate in networks is the linear bridge M-CN-M; M = metal, with a distance M–M approximately 5 to 6 Å.

When linear polyhedron [Ag (CN)] acts as a binding between two centers of greater coordi‐ nation, the others are connected to 2D or 3D networks [26]. In the union of blocks, the same metal center generates various structural patterns, showing the variability of crystal structures. There are other types of binders such as NH3 and H2O, which play an important role in the 3D structure stabilization and the volume of empty space that is occupied by species guests.

The initial structure of compounds including cyanometallate [Ni (CN) 4] 2 . The flat square is the clathrate of Hoffmann's formula [Ni (NH3) 2Ni (CN) 4], where a 2D network shares a corner in units of [Ni (CN) 4] 2 , separated by NH3 binders. The resulting structure is in 2-dimensional (2D) structure, with the distance between blades occupied by the guest of benzene molecules that are perpendicular to the blades.

Materials with transition metals have been developed, which provide specific features to the two-dimensional network for the sole purpose of including pillars to further the creation of the same form, providing structures with defined pore and known dimensions. Such is the case of the tetracyano-niquelates. Although they are based on the principle of a structure-type Hoffmann, including transition metals such as Co, Mn, and Ni, the result is to have water molecules in the central part or interlamellar, which serve as pillars of the main structure.

The formation of tetracyano-niquelates involves square plane anion [Ni (CN) 4]2 , with four sites attached to metal ions M = Ni, Co, Mn, via the nitrogen atoms of the cyano group. To have the octahedral geometry, the block has two further binders. For this particular case, those additional ligands bind to water molecules to complete the coordination sphere as shown in Figure 4. The consecutive joining molecular building blocks of this two-dimensional space can build a two-dimensional structure.

**Figure 4.** Schematic representation of the assembling process for obtaining K, L1, and L0 phases. The zeolite-like water molecules are highlighted with open circles in the interlayer region.
