Why co ordination number is fixed for a metal? How can we say that primary valencies are ionizable if we take the eg. Of K4[Fe(CN)6]

Werner theory states that every metal cation has a fixed coordination no. But if we consider K4[Fe(CN)6] and [Fe(Cl)4]^-2 the coordination no. of iron in both cases is different?

According to Werner`s Theory on coordination compounds, transition metals have two types of valency – 1. primary valency which is ionizable and 2.Secondary valency which is the C.N. of the transition metal / metal ion. In case of a particular​compound, C.N. of the transition metal is always fixed, bcz it states, the highest extent of coordination of that particular transition metal. But, you can`t say, the C.N.of a metal is always fixed as in CoCl4(–2), C.N. of Co(II) is 4 while in case of [Co(H2O)6]+3, C.N. of Co(III) is 6 ; again, in case of [Co(CN)5]–2, C.N. of Co(III) is 5... Again, you notice, the primary valency of Co I guess.In case of K4[Fe(CN)6], primary valency of iron is 3, which is its oxidation state (+3) too and this is ionizable valency and C.N. if Fe(III) is 6. Hope it helps...