The colour of $[Ni(NO_2)_6]^{4-}$ complex is ______.

Brown-yellow

The correct answer is Option (1) → Brown-yellow

The complex [Ni(NO₂)₆]⁴⁻ (hexanitronickelate(II)) is an octahedral Ni(II) (d⁸) species with nitrite ligands. NO₂⁻ is a moderate-field ligand in the spectrochemical series (positioned near NH₃), leading to d-d transitions that absorb light in the visible region, resulting in a brown-yellow colour.