The Structure of Prussian Blue Analogs

Prussian blue analogs are here defined as polynuclear transition metal cyanides of the composition M [M (CN)6]i xHzO a retallizing with a cubic unit cell. They are easily obtained as sparsely soluble precipitates by mixing solutions of a cyano complex M (CN)e with an appropriate salt of The compounds prepared by using the hexacyanometalate in the form of the most common potassium salt invariably contain different amounts of potassium, which in some cases can be exchanged by cesium 

The precipitates of Prussian blue analog cyanides always contain variable amounts of water, which can be removed without significant effects on the X-ray diffraction pattern. Some of these water molecules can be replaced by other molecules such as ammonia or alcohol 37). It was therefore assumed that the water is present partly as zeolitic, partly as surface water 3). Recent infrared spectroscopic studies, however, reveal that, in addition, coordinated water molecules are also present 33, 38). 

The first structural investigations of Prussian blue analogs date back to 1936 when Keggin and Miles (2) studied the X-ray powder patterns of iron cyanides. They found a luiit cell of the face-centered type and deduced a structural model from these geometrical data. This description, which will be briefly outlined below, has until recently been accepted by many authors (3, 39—44) for the discussion of the structural properties of cubic polynuclear cyanides Mfc[M (CN)e]i cHgO. In many cases the physically equivalent description has been used 3, 42, 

44) with the origin shifted by Y y) compared with the original paper (2). 

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In his compilation of structural data, Wyckoff 53) relates the structures of Prussian blue analogs to the K2PtCl6 type. Whereas this comparison is stoichiometrically obvious for the compounds M2M (CN)e. the ambident coordination behavior of the cyanide ligand is not considered as a structural element. The pol5mieric cyanide is here assumed... 

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