Of the following complex ions: [CoCl(NH₃)₅]²⁺, [Co(NH₃)₆]³⁺, [Co(en)₃]³⁺, [Co(H₂O)₆]³⁺, one appears pink, two are yellow, and one blue. Which is blue? A) [Co(NH₃)₆]³⁺ B) [CoCl(NH₃)₅]²⁺ C) [Co(H₂O)₆]³⁺ D) [Co(en)₃]³⁺ E) both [Co(en)₃]³⁺ and [Co(H₂O)₆]³⁺

Question

Quizplus · Accepted Answer

The blue color in this complex can be attributed to the presence of the cobalt ion in a high spin state, which means the d-electrons are in the eg set, with a larger energy gap between eg and t2g, leading to the absorption of light in the higher energy region of the visible spectrum (blue). This high spin state is favored in octahedral complexes with weak field ligands, such as HS(O) and Cl-. Therefore, we can eliminate A and D from contention, as they have strong field ligands (en and NH₃ respectively). B is also eliminated because it has Cl- as a ligand, which is a moderately strong field ligand. Therefore, the only option left is [Co(H₂O)₆]³⁺ (C).

Of the Following Complex Ions: [CoCl(NH3)5]2+, [Co(NH3)6]3+, [Co(en)3]3+, [Co(H2O)6]3+, One

Of the Following Complex Ions: [CoCl(NH₃)₅]²⁺, [Co(NH₃)₆]³⁺, [Co(en)₃]³⁺, [Co(H₂O)₆]³⁺, One