# Gadolinium(III) chloride

Gadolinium(III) chloride, also known as gadolinium trichloride, is GdCl$_3$. It is a colorless, hygroscopic, water-soluble solid. The hexahydrate GdCl$_3$∙6H$_2$O is commonly encountered and is sometimes also called gadolinium trichloride. Gd$^{3+}$ species are of special interest because the ion has the maximum number of unpaired spins possible, at least for known elements. With seven valence electrons and seven available f-orbitals, all seven electrons are unpaired and symmetrically arranged around the metal. The high magnetism and high symmetry combine to make Gd$^{3+}$ a useful component in NMR spectroscopy and MRI.

## Preparation

GdCl$_3$ is usually prepared by the "ammonium chloride" route, which involves the initial synthesis of (NH$_4$)$_2$[GdCl$_5$]. This material can be prepared from the common starting materials at reaction temperatures of 230 °C from gadolinium oxide:
       10 NH$_4$Cl + Gd$_2$O$_3$ → 2 (NH$_4$)$_2$[GdCl$_5$] + 6 NH$_3$ + 3 H$_2$O
from hydrated gadolinium chloride:
       4 NH$_4$Cl + 2 GdCl$_3$∙6H$_2$O → 2 (NH$_4$)$_2$[GdCl$_5$] + 12 H$_2$O
from gadolinium metal:
       10 NH$_4$Cl + 2 Gd → 2 (NH$_4$)$_2$[GdCl$_5$] + 6 NH$_3$ + 3 H$_2$
In the second step the pentachloride is decomposed at 300 °C:
       (NH$_4$)$_2$[GdCl$_5$] → GdCl$_3$ + 2 NH$_4$Cl
This pyrolysis reaction proceeds via the intermediacy of NH$_4$[Gd$_2$Cl$_7$].
The ammonium chloride route is more popular and less expensive than other methods. GdCl$_3$ can, however, also be synthesized by the reaction of solid Gd at 600 °C in a flowing stream of HCl.
       Gd + 3 HCl → GdCl$_3$ + 3/2 H$_2$
Gadolinium(III) chloride also forms a hexahydrate, GdCl$_3$∙6H$_2$O. The hexahydrate is prepared by gadolinium(III) oxide (or chloride) in concentrated HCl followed by evaporation.

## Structure

GdCl$_3$ crystallizes with a hexagonal UCl$_3$ structure, as seen for other 4f trichlorides including those of La, Ce, Pr, Nd, Pm, Sm, Eu. The following crystallize in theYCl$_3$ motif: DyCl$_3$, HoCl$_3$, ErCl$_3$, TmCl$_3$, YdCl$_3$, LuCl$_3$, YCl$_3$). The UCl$_3$ motif features 9-coordinate metal with a tricapped trigonal prismatic coordination sphere. In the hexahydrate of gadolinium(III) chloride and other smaller 4f trichlorides and tribromides, six H$_2$O molecules and 2 Cl$^−$ ions coordinate to the cations resulting in a coordination group of 8.

## Properties, with applications to MRI

Gadolinium salts are of primary interest for relaxation agents in magnetic resonance imaging (MRI). This technique exploits the fact that Gd$^{3+}$ has an electronic configuration of f$^7$. Seven is the largest number of unpaired electron spins possible for an atom, so Gd$^{3+}$ is a key component in the design of highly paramagnetic complexes. To generate the relaxation agents, Gd$^{3+}$ sources such as GdCl$_3$∙6H$_2$O are converted to coordination complexes. GdCl$_3$∙6H$_2$O can not be used as an MRI contrasting agent due to its low solubility in water at the body's near neutral pH. "Free" gadolinium(III), e.g. [GdCl$_2$(H$_2$O)$_6$]$^+$, is toxic, so chelating agents are essential for biomedical applications. Simple monodentate or even bidentate ligands will not suffice because they do not remain bound to Gd$^{3+}$ in solution. Ligands with higher coordination numbers therefore are required. The obvious candidate is EDTA$^{4−}$, ethylenediaminetetraacetate, which is a commonly employed hexadentate ligand used to complex to transition metals. In lanthanides, however, exhibit coordination numbers greater than six, so still larger aminocarboxylates are employed.
One representative chelating agent is H$_5$DTPA, diethylenetriaminepentaacetic acid. Chelation to the conjugate base of this ligand increases the solubility of the Gd$^{3+}$ at the body's neutral pH and still allows for the paramagnetic effect required for an MRI contrast agent. The DTPA$^{5−}$ ligand binds to Gd through five oxygen atoms of the carboxylates and three nitrogen atoms of the amines. A 9th binding site remains, which is occupied by a water molecule. The rapid exchange of this water ligand with bulk water is a major reason for the signal enhancing properties of the chelate. The structure of [Gd(DTPA)(H$_2$O)]$^{2−}$ is a distorted tricapped trigonal prism.
The following is the reaction for the formation of Gd-DTPA: