Ni(NO₃)₂(aq) → Ni²⁺(aq) + NO₃^–(aq)

NaCN(aq) → Na⁺(aq) + CN^–(aq)

Ni²⁺(aq) + 4 CN^–(aq) → ← Ni(CN)₄^2–(aq)

complex ion formation