While synthesizing a Prussian blue analogue using K3FeCN6 and nickel nitrate hexahydrate, I found 2 peaks of CN in the IR spectrum of the Prussian blue analogue. Some literature suggested it was an Fe3+ and Fe2+ bonded bridged CN group. But K3FeCN6 is a source of Fe³⁺ only, so how am I getting an Fe²⁺ peak in the IR spectra? And also, when I synthesize using K4FeCN6, I get only an Fe²⁺-bonded CN group peak.
It could be due to some of Fe ions in the K3FeCN6 sample is in reduced state (Fe2+) as a result of synthesis process used for the sample preparation. Thus, you have seen both Fe3+ and Fe2+ in the sample.
Can you clearly see the cyanide-bound Ni(2+) as a separate peak? If not, it may be that it substitutes for Fe(2+) in a kind of modified Prussian Blue. Ni(2+) generally can substitute for Fe(2+) in moderate amounts. Normally it prefers square planar coordination with cyanide, but when it's a minority component in an octahedral structure it may make an exception for you...
The appearance of two CN stretching peaks in the IR spectrum during the synthesis of a Prussian Blue analogue using K₃[Fe(CN)₆] and nickel nitrate hexahydrate likely indicates the presence of both terminal and bridging cyanide ligands. In Prussian Blue analogues, CN⁻ can coordinate in different modes—terminally to one metal center or bridging between two metal centers—which results in distinct vibrational environments. These different coordination modes lead to splitting of the CN stretching band, typically observed as two separate peaks in the 2000–2200 cm⁻¹ region of the IR spectrum.
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