$\pi$-acid ligands are

Which of the following complexes is octahedral,diamagnetic,and the most stable?

The stabilization of coordination compounds due to chelation is called the chelate effect. Which of the following is the most stable complex species?

$FeSO_4$ solution mixed with $(NH_4)_2SO_4$ solution in $1:1$ molar ratio gives the test of $Fe^{2+}$ ion,but $CuSO_4$ solution mixed with aqueous ammonia in $1:4$ molar ratio does not give the test of $Cu^{2+}$ ion. Explain why?

Ammonia forms the complex $[Cu(NH_3)_4]^{2+}$ with copper ions in alkaline solution but not in acidic solution. The reason for this is

Which among the following cations forms the most stable complex with the same ligand?