*2.4.1. Hexacyano-metallates*

The hexacyano-metallates of the transition metals are microporous molecular family, whose structures are based on a three-dimensional arrangement of chains M-C≡N-M. Metal M (inner metal), linked to the carbon atom, is always forming octahedral molecular blocks [Mn + (CN) 6] 6-n as the metal M (external metal) acts as an assembler of the octahedral blocks and possibly octahedral or tetrahedral coordination [28, 29].

The first studies of H2 adsorption in this polymer-coordinated family were published in 2005, simultaneous with the reports of Long et al. [30] and Kepert et al. [31], who studied the formula of general Prussian blue analogues M3 [Co (CN)6]2, known as hexacyano-cobaltates as the internal metal cobalt and being able to change the external metal; M= Mn, Fe, Co, Ni, Cu, Zn, and Cd.

Figure 5 shows the structure of a Prussian blue analogue in the cubic phase due to the octahedral coordination adopted by the external metal.

**Figure 5.** Porous framework for the cubic form, by a hexacyano-cobaltates and a transition metal [30].
