The type of bonding in [Cu(NH$_3$)$^4$]$^{2+}$ is

Analyze the question The question asks about the type of bonding in the complex ion [Cu(NH₃)₄]²⁺, the tetraamminecopper(II) ion. It is formed when excess ammonia is added to a solution containing Cu²⁺ ions, giving a deep blue solution.

Explain the concept

• A coordinate (dative covalent) bond is formed when one atom supplies both electrons of the shared pair.
• In complex ions, the ligands (here NH₃) donate lone pairs to the empty orbitals of the central metal ion (here Cu²⁺).

Solution

1. Each NH₃ molecule has a lone pair of electrons on the nitrogen atom.
2. Cu²⁺ has vacant orbitals that can accept electron pairs.
3. Each of the four NH₃ molecules donates its lone pair to Cu²⁺, forming four N→Cu bonds.
4. Both electrons in each of these bonds come from nitrogen alone, so the bonds between NH₃ and Cu²⁺ are coordinate bonds.

(Within each NH₃ molecule, the N-H bonds are ordinary covalent bonds. But the bonding that characterises this complex ion is the coordinate bonding between the ligands and the copper ion.)

Why the others are wrong

• Electrovalent: this involves complete transfer of electrons between a metal and a non-metal. That is not how the NH₃ ligands attach to Cu²⁺.
• Metallic: this occurs between metal atoms in a lattice with a "sea" of electrons. It is not relevant here.
• Covalent: the N-H bonds are covalent, but the question asks about the bonding that defines the complex, which is the dative (coordinate) type. Coordinate is the more specific and correct answer.

Answer: A. coordinate

Exam tip: If you see a square bracket with a metal ion and neutral molecules like NH₃ or H₂O around it (a complex ion), think coordinate (dative) bonding. The ligand always has a lone pair to donate.