how to calculate secondary valency and tell me while writing name of complex compounds for eg k4[Fe(CN)6] why we cant write tetrapotassium hexacyanoferrate(II) instead we write potassium hexacyanoferrate(II) please explain me

The secondary valencies are non-ionisable, and are satisfied by negative ions or neutral molecules. The secondary valency is equal to the coordination number of a metal, and remains fixed for a metal.

For example, in coordination compound [Pt (NH3)6]Cl4 , the secondary valencies are 6 for Pt4+; in Co[(NH3)6] (3+) the secondary valencies are 6.

The ligands satisfying secondary valencies are always directed towards fixed positions in space thereby giving a definite geometry to the complex but the primary valencies are non-directional.