d The cyano and hydroxo complexes are far more stable than those formed by halide ion. This is due to the fact that $C N^{-}$ and $O H^{-}$ are strong lewis bases (nucleophiles). Further $\left[F e(O H)_{5}\right]^{-3}$ is not formed. hence most stable ion is $\left[F e(C N)_{6}\right]^{-3}$
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