Question

What is the primary valence of iron in the coordination entity of compound Fe4[Fe(CN)6]3?

(a) 0

(b) +2

(c) +3

(d) +4

The question was asked during an interview.

My query is from Werner’s Theory of Coordination Compounds topic in section Coordination Compounds of Chemistry – Class 12

Answer 1

Correct answer is (b) +2

For explanation: This compound, also known as Prussian blue, dissociates into 4 Fe^3+ and 3[Fe(CN)6]^4- ions, of which the latter is the coordination entity having a charge of -4. The Fe atom in question is the one inside the bracket. Since CN has a charge of -1 and there are 6 of them, the primary valence of Fe will be 6 – 4 = +2.