**2.4. Prussian blue analogues**

cyanometallate that acts as a ligand and a complex of a metal transition coordination position

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

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 clathrate of Hoffmann's formula [Ni (NH3) 2Ni (CN) 4], where a 2D network shares a corner

(2D) structure, with the distance between blades occupied by the guest of benzene molecules

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.

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

The formation of tetracyano-niquelates involves square plane anion [Ni (CN) 4]2

, separated by NH3 binders. The resulting structure is in 2-dimensional

3-, (b) the cationic block [M´L2]

2

. The flat square is

, with four

n+.

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]

The initial structure of compounds including cyanometallate [Ni (CN) 4]

M–M approximately 5 to 6 Å.

50 Advances in Petrochemicals

in units of [Ni (CN) 4]

2

that are perpendicular to the blades.

Historically, the "iron blue" is known as (ferric ferrocyanide) "soluble" Prussian blue and the blue is known as "insoluble" Turnbull (ferrous ferrocyanide); however, all recent studies conclude that both are identical. The formation of this pigment can be written as

$$4\left[\text{Fe}\right]\_{\text{(ac)}}^{3+} + 3\left[\text{Fe}\left(\text{CN}\right)\_{\text{(ac)}}\right]\_{\text{(ac)}}^{4-} \rightarrow \text{Azul de Prusia},\\\text{Fe}\_4\left[\text{Fe}\left(\text{CN}\right)\_6\right]\_3 \text{xH}\_2\text{O}\left(\text{x} = 14-16\right).$$

Its low solubility evidence is the polymeric nature of the solid product. Due to the 4:3 ratio of Fe (III) and Fe (II), 25% of the sites of Fe (II) are vacant. The occurrence of more than one oxidation state of the same element in a particular compound is defined as a mixture of valence. Due to the partial relocation of electrons from valence, the Prussian blue is a semiconductor. Depending on the properties desired in the product, many routes of preparation are known. Given its porous nature, it has a certain zeolitic character, which allows you to store small molecules in their cavities [27].

They have now developed materials under the same principle of formation of Prussian blue by changing the formation of anionic block metals. The hexacyano-cobaltates (of the family of the hexacyano-metallates) are an example of these materials considered as Prussian blue analogues.
