Animated Solution for Chemistry - Coordination Compounds: The number of hydrogen bonded water molecule(s) associated with stoichiometry ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$ is/are.

When we look at the chemical formula for copper sulfate pentahydrate, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$, it is easy to assume that all five water molecules are identical in their behavior and placement within the crystal lattice. However, the reality of coordination chemistry paints a much more intricate and beautiful picture.

Let's dive into the molecular architecture of this vibrant blue crystal, commonly known as blue vitriol.

The central character in this structure is the copper ion, ${\text{Cu}}^{2+}$. Like many transition metals, it possesses empty d-orbitals that are hungry for electron pairs. Water molecules, with their lone pairs on the oxygen atom, act as excellent ligands.

However, due to spatial constraints and the preferred geometry of the ${\text{Cu}}^{2+}$ ion, not all five water molecules can fit around it. Exactly four water molecules form direct coordinate covalent bonds with the central copper ion. These four ligands arrange themselves in a square planar geometry, creating the complex cation $[\text{Cu}({\text{H}}_2\text{O}{)}_4{]}^{2+}$.

So, what happens to the fifth water molecule? It is left outside the primary coordination sphere. But it doesn't just float aimlessly; it serves a critical structural purpose.

This fifth water molecule acts as a molecular bridge. It sits between the complex cation $[\text{Cu}({\text{H}}_2\text{O}{)}_4{]}^{2+}$ and the sulfate anion ${\text{SO}}_4^{2-}$. It achieves this by engaging in extensive hydrogen bonding.

The oxygen atom of this fifth water molecule accepts hydrogen bonds from the hydrogen atoms of the coordinated water molecules. Simultaneously, its own hydrogen atoms donate hydrogen bonds to the highly electronegative oxygen atoms of the sulfate ion.

Because of this unique structural arrangement, the chemical formula is more accurately represented as $[\text{Cu}({\text{H}}_2\text{O}{)}_4]{\text{SO}}_4\cdot {\text{H}}_2\text{O}$.

When asked how many water molecules are exclusively hydrogen-bonded (and not directly coordinated to the metal), the answer is exactly $1$. This subtle distinction is not just a trivia fact; it explains why heating blue vitriol causes it to lose water in distinct stages, shedding the hydrogen-bonded water molecule before the coordinated ones, leading to a fascinating loss of its signature blue color!