Question

Which from following compounds contains complex anions?

• A. Sodium hexanitrocobaltate (III)
• B. Triamminetrinitrocobalt (III)
• C. Pentaammineaquacobalt (II)iodide
• D. Hexaamminecobalt (III) chloride

Hint:

Nomenclature shortcut: If the name of the central metal ends in the suffix "-ate" (like zincate, ferrate, cobaltate, aluminate), the complex coordination sphere is guaranteed to be a negatively charged Anion!

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A coordination compound has a complex anion if the coordination sphere carries a negative charge and is paired with a simple cation (like Sodium or Potassium).

Step 2: Formula Application:
The suffix "-ate" in the metal name (e.g., Cobaltate) indicates that the metal is part of a complex anion.

Step 3: Explanation:
In "Sodium hexanitrocobaltate(III)", Sodium ($Na^+$) is the simple cation. The coordination sphere $[Co(NO_2)_6]^{3-}$ is the anion. In options (c) and (d), the coordination sphere is the cation because the metal name ends in "Cobalt". Option (b) is a neutral complex.

Step 4: Final Answer:
Sodium hexanitrocobaltate(III) contains a complex anion.