in which of the following pair of complexes choose the one that absorbs light at a longer wavelength

a)[co(NH3)6]2+ ,[CO(H20)6]2+

Dear Aman,

  Energy of splitting (Δ) = hc/λ

      so if Δ↓ then λ↑

Δ has less value in case of weak lingand as compared to strong ligand.

here in compound (a) NH3 is a strong ligand so there will be higher value of Δ and thus lower value of λ.

therefore option (b) which contains weak  ligand H2O will absorb light at a longer wavelength

With Regards,

Abhishek Jain