The synthetic and structural chemistry as well as some physical and chemical properties of cyanide-bridged complexes containing two or more metal building blocks are discussed. Emphasis is laid on molecular shape and design and their consequences, including the effects of cyanide-isocyanide isomerism. Redox phenomena and molecular magnetism have been dealt with in relation to electronic communication across the cyanide bridges and metal-metal charge transfer. Long-range interactions between remote metal centers are discussed, and the few cases where they lead to remote chemical reactions are listed.Prussian Blue, [Fe 4 {Fe(CN) 6 } 3 ]ؒxH 2 O, which actually is the oldest known co-ordination compound, 1 has attracted the skills of many scientists seeking an understanding of its formation and composition, 2 structure, 3 colour 4 and physical properties like magnetism 5 and electrical conductivity. 6 Although it was soon obvious that the linking of two different metal ions by the cyanide ligand is the basis of all these phenomena, this did not initiate a variation of the theme. The systematic synthetic and physical exploration of Prussian Blue-like compounds had to await the popularity of the chemistry of materials. Only in recent years has a number of attractive two-and three-dimensional co-ordination polymers comprised of M᎐CN᎐MЈ units been prepared and subjected to the appropriate measurements. 7 While bulk magnetism and conductivity require extended interactions their basic fundamental property, the electronic communication between two metals across the cyanide bridge, is local and can be studied in simple dinuclear complexes. But classical co-ordination chemistry was not interested in this phenomenon, and hence the literature of more than 25 years ago contains only rare examples of cyanide-bridged systems 8 amongst the multitude of ligand-bridged dinuclear complexes. 9 It took the emerging discussion of mixed valence 10-12 and innersphere electron transfer 13,14 to move the M᎐CN᎐MЈ systems into the limelight. They were among the first to be studied in this respect, 15,16 and since then the electronic interactions between two metals across a cyanide bridge have proven to be a fertile area of research.In the last 15 years many di-and tri-nuclear complexes with bridging cyanide ligands have been investigated for the electronic communication and the electron transfer between their metal centers by means of electronic spectroscopy, electrochemistry, IR spectroscopy, fast kinetic methods and molecular orbital (MO) theory. Among others, the research groups of

Cyanide-bridged trinuclear complexes containing square-planar platinum at the center were synthesized and identified by structure determinations. Their chemical building blocks were cis- and trans-PtL2X2, Cp(dppe)FeX, Cp(PPh3)2RuX (with X = Hal or CN), Pt(CN)4 2-, and Fe(CN)6 3-. Several of the intermediate dinuclear complexes were isolated and used for reference purposes. The molecular structures, the ν(CN) IR data, the cyclic voltammograms, and the UV−vis−near-IR spectra were used to probe the effects of the geometry at platinum (cis vs trans), of cyanide−isocyanide isomerism (Pt(CN)2 vs Pt(NC)2), of the nature and ligation of the terminal metals (Fe vs Ru), and of one- or two-electron oxidation. The redox properties and the observed intervalence transfers indicate that there is electronic communication between the outer metal atoms along the linear M−CN chains containing trans-configured platinum, but not along the bent chains containing cis-configured platinum.

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