I recently came across this question while studying Solid State.
The coordination no. of [Ni(CN)4]2- is 4 as Ni is bonded to 4 CNs and is planar in structure.Hence its Limiting Radius Ratio must be between 0.225 and 0.414 . But the answer given in the book is 0.414 to 0.732 (which is the limiting radius ratio for coordination no. 6) .
How is this true?Some sites agree with my answer .
Is the answer given in the book wrong or have I made any mistake in the reasoning?

• I've always calculated limiting radius ratio for ionic crystals. I'm not sure if such a thing exists for coordination compounds. May 10, 2017 at 8:51
• I have a hunch that the charge 2- on the ion has some role to play. I am not sure. And yeah you are right . Even I calculated only for ionic crystals not for coordination compounds May 10, 2017 at 8:52
• If you insist, a cursory search tells me that the crystal radius of $\ce{CN-}$ is $\pu{1.77 Å}$, and the ionic radius for $\ce{Ni^2+}$ ion is $\pu{0.69 Å}$. May 10, 2017 at 9:23
• Then the radius ratio will be 0.3898 which agrees with my answer. May 10, 2017 at 9:28

Here, limiting radius ratio is for geometry of this complex is square planar because $\ce{CN-}$ is a strong field ligand and causes pairing of unpaired electrons. So we can relate that $\ce{Ni^2+}$ occupies a square planar void of the $\ce{CN-}$ lattice. The square planar void is a cross section of the octahedral void.