The type of bond between copper(II) tetraamine and chlorine in [Cu(NH$_3$)$_4$]Cl$_2$

Analysis of the Question

The question asks for the nature of the chemical bond existing between the complex cation, $[Cu(NH_3)_4]^{2+}$, and the chloride anions, $Cl^-$, in the compound $[Cu(NH_3)_4]Cl_2$.

Concept Explanation

To identify the bond, we must look at the structure of the compound:

1. The Complex Ion: Inside the square brackets $[Cu(NH_3)_4]^{2+}$, the ammonia molecules are bonded to the copper ion via dative (coordinate) covalent bonds.
2. The Compound as a Whole: The entire entity $[Cu(NH_3)_4]^{2+}$ acts as a positively charged complex ion (cation), while the two $Cl^-$ ions act as negatively charged counter-ions (anions).
3. The Bond Type: In chemistry, the electrostatic force of attraction between a positively charged ion and a negatively charged ion is defined as an ionic bond.

Even though the internal structure of the complex ion involves coordinate covalent bonds, the bond between the complex cation and the chloride anions is purely ionic.

Step-by-Step Solution

1. Identify the components: The formula $[Cu(NH_3)_4]Cl_2$ dissociates in water as: $[Cu(NH_3)_4]Cl_2 \rightarrow [Cu(NH_3)_4]^{2+} + 2Cl^-$
2. Analyze the interaction: We have a large, positively charged complex ion $[Cu(NH_3)_4]^{2+}$ and two negatively charged chloride ions $Cl^-$.
3. Determine the bond: The force holding these oppositely charged species together in the crystal lattice is an electrostatic attraction. Electrostatic attraction between ions is the definition of an ionic bond.

Conclusion

While the ammonia ligands are attached to the copper via dative bonds, the bond connecting the complex unit to the chlorine atoms is ionic.

Exam Tip: Whenever you see a complex salt (a compound with square brackets), remember:

• Bonds inside the brackets are usually dative/covalent.
• Bonds between the bracketed part and the outside atoms are ionic.