An aqueous solution of NiCl₂ was heated with excess sodium cyanide in presence of strong oxidizing agent to form . The total change in number of unpaired electrons on metal centre is

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Strategy:
 Determine the unpaired electrons for the initial hydrated nickel species and the final complex formed under strongly oxidizing conditions.

Step 1: Initial state (aqueous )
 in water forms the hexaaquanickel(II) complex, .
 () with weak field  is a high-spin complex ().
Unpaired electrons = 2.

Step 2: Final state
The reaction creates , which requires a strongly oxidizing environment because standard  with  forms square planar .
The charge implies Nickel is in the +4 oxidation state:

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