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 $${\left[ {Fe{{\left( {OH} \right)}_5}} \right]^{3 - }}$$ is not formed. hence most stable ion is $${\left[ {Fe{{\left( {CN} \right)}_6}} \right]^{3 - }}$$