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
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.
To determine the stability of cobalt complexes, we need to consider the nature and strength of the ligands attached to the central cobalt atom. In general, strong field ligands tend to form more stable complexes compared to weak field ligands.
Now let's analyze the ligands in each of the given complexes:
a. [Co(NH3)6]2+: In this complex, ammonia (NH3) is a weak field ligand. It donates electrons through its lone pair to the cobalt atom, but not very effectively. Therefore, [Co(NH3)6]2+ is relatively less stable.
b. [CO(NH3)6]3+: Here, the ligand is carbon monoxide (CO), which is a stronger field ligand compared to ammonia. Carbon monoxide has a larger π-acceptor ability, which allows it to form stronger bonds with the metal. So, [CO(NH3)6]3+ is expected to be more stable than [Co(NH3)6]2+.
c. [Co(en)3]3+: The ligand in this complex is ethylenediamine (en). Ethylenediamine is a bidentate ligand, which means it can donate two pairs of electrons to the central cobalt atom. Moreover, it has a stronger π-donor ability compared to ammonia. Therefore, [Co(en)3]3+ is expected to be even more stable than [CO(NH3)6]3+.
d. [CoF6]3–: In this complex, the ligand is fluoride (F-), which is a strong field ligand. Fluoride ions are small and highly electronegative, allowing for effective donation of electron pairs to the cobalt atom. Consequently, [CoF6]3– is likely to be the most stable among the given complexes.
In conclusion, the correct answer is d. [CoF6]3–. It is expected to be the most stable cobalt complex due to the presence of strong field fluoride ligands.