Question

How can we decide which can valency is suitable
for gold and silver in complexes

Answer 1

most probably it depends on the question

or u can use any of the valency

Answer 2

Since oxidation state of central atom is +2. Its primary valence is 2 and since coordination number of Fe is 6. Its secondary valence should be 6. Primary valency is detected by ion outside the coordination sphere and secondary by anionic or neutral ligand inside the coordination sphere .