Question

In which of the following does the central atom exhibit an oxidation state of +2 ? 

• A. K₂ [Ni(CN)₄ ]
• B. K₃ [Fe(CN)₆]
• C. [Fe(C₂O₄)₃ ]^3-
• D. [Cu(NH₃)₄ ]²⁺

Answer 1

The Correct Option is A, D

K₂(Ni(CN)₄): 
In this the anion will have a charge of -2 as 2K+ is present. 

Then the charge on the central atom that is Ni can be calculated: 
CN has a charge of -1, therefore Nickel x
x+4(-1) = -2
x = +2 

[Cu(NH₃)₄ ]²⁺: 
In this NH3 is neutral thus has a charge of 0 and the compound has an overall charge of +2. 
Therefore the charge on the central atom that is Cu is +2.