Coordination Number of Gd in GdCl3.6H2O

The coordination number of a central metal atom or ion in a coordination compound is defined as the total number of sigma bonds between the ligands and the central atom/ion. It essentially tells us how many atoms or molecules are directly attached to the metal center.

The given compound is Gadolinium(III) chloride hexahydrate, represented by the formula GdCl$_{3}$.6H$_{2}$O. This formula indicates that for every Gadolinium (Gd) ion, there are three chloride ions (Cl$^{-}$) and six water molecules (H$_{2}$O).

Gadolinium (Gd) is a lanthanide element and typically exists as the Gd$^{3+}$ ion in its compounds. Both chloride ions (Cl$^{-}$) and water molecules (H$_{2}$O) can act as ligands, donating electron pairs to the central metal ion.

The structure of hydrated lanthanide chlorides can vary. For GdCl$_{3}$.6H$_{2}$O, crystallographic studies have shown that the structure typically involves the coordination of both water molecules and chloride ions to the Gadolinium ion. The common structure in the solid state is better represented as [Gd(H$_{2}$O)$_{6}$Cl$_{2}$]Cl$\cdot$H$_{2}$O.

In this structure, the species inside the square brackets, [Gd(H$_{2}$O)$_{6}$Cl$_{2}$]$^{+}$, represent the coordination complex ion. The Gadolinium ion (Gd$^{3+}$) is surrounded by:

• Six water molecules (H$_{2}$O) acting as neutral ligands.
• Two chloride ions (Cl$^{-}$) acting as anionic ligands.

These ligands are directly bonded to the central Gd$^{3+}$ ion. The remaining chloride ion (Cl$^{-}$) and one water molecule (H$_{2}$O) are outside the coordination sphere, balancing the charge and participating in the crystal lattice through ionic or hydrogen bonding, but they are not directly coordinated to the Gd$^{3+}$ ion.

To find the coordination number of Gd, we count the number of ligands directly bonded to it within the coordination sphere:

Number of coordinated water molecules = 6

Number of coordinated chloride ions = 2

Total coordination number = Number of coordinated water molecules + Number of coordinated chloride ions

Total coordination number = 6 + 2 = 8

Therefore, the coordination number of Gd in GdCl$_{3}$.6H$_{2}$O is 8.