Stable complex based on EAN rule
i) K₄[Fe(CN)₆] ii) [Co(NH₃)₅Cl]Cl₂ iii) [Ni(CO)₄] iv) K₂[Ni(CN)₄]

EAN stands for Effective Atomic number.

It is given by formula:

EAN= Z-X+Y

where Z= Atomic number

            X=number of electrons loosed by central metal ion

            Y=Number of electrons shared by ligands

EAN value of complexes
atomic number of Fe , Co and  Ni are 26, 27 and 28 respectively 

each ligand contributes 2 electrons 

1. =26 −2+12 =36
2. =27−3+12 =36
3. =28 +8=36
4. =28 −2+8=34
5. so its not stable as its EAN is not equal to 36.

i, ii and iii obey  EAN rule