Question

Which one of the following cobalt complexes should be the most stable?

a. [Co(NH3)6]2+







b. [CO(NH3)6]3+







c. [Co(en)3]3+







d. [CoF6]3–
I think the answer is c, but not really sure. Can Dr.Bob explain ? Thanks

Answer 1

I think you are correct; however, I couldn't confirm that CoF6^3- is weaker. I did find Kf for Co(NH3)6^3+ = 4.5E33 and Kf for Co(en)3^3+ = 4.8E48. Since the bidentate bonds are so much stronger than unidendate bonds (as shown by the two Kf above), then CoF6^3- surely must be weaker bonds than the en complexes.