Recent progress in transition metal hexacyanometallates: From structure to properties and functionality

The coordination chemistry of transition metal hexacyanometallates is a field of continuous advances and emerging applications. In the last three decades, relevant progress has been achieved on the understanding of the crystal and electronic structures of these materials, including those compositions resulting from non-conventional preparative routes. In this review, we summarize such advances at the time that provide a comprehensive overview of the structure-properties relationship. Since an improvement of the functionality of a given material is facilitated when its crystal and electronic structures are known, we provide guidelines on how to understand and predict the properties of these coordination polymers. In this family of materials, the extended 3D framework is usually formed by interconnected ⋅⋅⋅-T-N≡C-M-C≡N-T-⋅⋅⋅ chains. When these chains have a linear geometry, the formed solids crystallize either with a cubic or pseudocubic unit cell, and these coordination polymers are known as Prussian blue analogs. In the last years, deviations from such regularity have been observed for an increasing number of T metals, where the N end of the CN– ligand forms a bifurcated coordination bonding with two T metal centers. Other less common coordination modes at the N end have been also documented and are herein considered. Since the first documented report on this family of coordination polymers (in 1704), a search of published documents, using Science Finder and Google Scholar under the keywords of Prussian blue and transition metal hexacyanometallates, produces above four thousand outputs, mainly scientific papers. But these papers mostly deal with potential applications, and only a small fraction of them includes structural studies in the context of coordination chemistry. This review is structured in three blocks of sections. Initially, the crystal and electronic structures are discussed, then, the related physical properties are analyzed, and finally, their implications on the applications, with the focus on the structure-properties relationship are discussed. The accompanying Supplementary Data file has been conceived as a source of useful structural information, which together with the main document will help the readers to understand better these compounds and devise new and promising applications.