Among [CoF6]^3-, [Co(NH3)6]^3+, [Co(en)3]^3+ and [Co(CN)6]^3-, the complex that absorbs light of the longest wavelength is

JEE Main levelSingle correct optionDifficulty 2 of 3cc027

Among $[CoF X_{6}] X^{3 -}$ , $[Co (NH X_{3}) X_{6}] X^{3 +}$ , $[Co (en) X_{3}] X^{3 +}$ and $[Co (CN) X_{6}] X^{3 -}$ , the complex that absorbs light of the longest wavelength is

All are Co(III)

d^{6}

, so the difference is the ligand field. Absorbed energy

\propto Δ_{o}

and

λ \propto 1/ Δ_{o}

. The spectrochemical order is

F X^{-} < NH X_{3} < en < CN X^{-}

, so

F X^{-}

has the smallest

Δ_{o}

and the longest absorbed wavelength.