Which of the following is more stable ?
(1) `K_(4)[Fe(CN)_(6)] or K_(3)[Fe(CN)_(6)]`
(2) `[Co(NH_(3))_(6)]^(3+) or [Co(en)_(2)(NH_(3))_(2)]^(3+)`
(3) `[NiCl_(4)]^(2-) or [Ni(CN)_(4)]^(2-)`

Explanation

(1) `K_(3)[Fe(CN)_(6)]` is more stable due to higher oxidation number of Fe(+3).
(2) `[Co(en)_(2)(NH_(3))_(2)]^(+3)` is more stable due to the formation of chelate:
(3). `[N (CN)_(4)]^(2-)` is more stable due to stronger ligand `CN^(-1)`