During the stepwise formation of [Cu(NH3)4]2+ from [Cu(H2O)4]2+, what ion is expected after the second step?

• A. [Cu(H2O)2(NH3)3]2+
• B. [Cu(H2O)3(NH3)]2+
• C. [Cu(H2O)2(NH3)2]2+
• D. [Cu(H2O)3(NH3)2]2+

Answer: (C)

The formation of the complex ion [Cu(NH3)4]²⁺ from [Cu(H₂O)₄]²⁺ involves a series of ligand exchange steps where water molecules are replaced by ammonia (NH₃) molecules. In the initial state, the copper(II) ion is coordinated to four water molecules.

In the first step, one NH₃ molecule replaces one H₂O molecule, resulting in the complex [Cu(H₂O)₃(NH₃)]²⁺.

In the second step, another NH₃ molecule replaces a second H₂O molecule. Thus, from the complex [Cu(H₂O)₃(NH₃)]²⁺, we would see the formation of the complex where there are now two NH₃ ligands and two remaining H₂O ligands. The correct representation of this ion after the second substitution would therefore be [Cu(H₂O)₂(NH₃)₂]²⁺.

Thus, after considering the sequence of ligand substitution, the presence of both water and ammonia in the specified ratios directly leads to the conclusion that the expected ion after the second step would indeed be [Cu(H₂O)