Scheme 1. Typical 2D (a) and 3D (b) Hofmann-Type Structures Based on Equatorial [μ₄-M^II(CN)₄]^2– Bridging Ligands. Reverse 3D Hofmann-Type Structure Based on the Axial [μ₂-M^II(CN)₄]^2– and μ₄-Tetradentate-Pyridine-Like Equatorial Bridging Ligands (c). Fragment of a Charged {Fe(μ₂-[M^II(CN)₄])₂}_n^2n– Layer Based on Equatorial [μ₂-M^II(CN)₄]^2– Ligands, and (d) (The Protonated Axial Ligand Is Partially Shown).