Gadolinium(III) chloride
# Gadolinium(III) chloride

Gadolinium(III) chloride, also known as gadolinium trichloride, is $GdCl_3.$ It is a colorless, hygroscopic, water-soluble solid. The hexahydrate $GdCl_3∙6H_2O$ 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 $NH_4Cl$ + $Gd_2O_3$ → 2 $(NH_4)_2[GdCl_5]$ + 6 $NH_3$ + 3 $H_2O$

from hydrated gadolinium chloride:

       4 $NH_4Cl$ + 2 $GdCl_3∙6H_2O$ → 2 $(NH_4)_2[GdCl_5]$ + 12 $H_2O$

from gadolinium metal:

       10 $NH_4Cl$ + 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 $NH_4Cl$

This pyrolysis reaction proceeds via the intermediacy of $NH_4[Gd_2Cl_7].$

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, $GdCl_3∙6H_2O.$ 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 $H_2O$ 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 $GdCl_3∙6H_2O$ are converted to coordination complexes. $GdCl_3∙6H_2O$ 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. $[GdCl_2(H_2O)_6]^+,$ 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 $H_5DTPA,$ 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 $[Gd(DTPA)(H_2O)]^{2−}$ is a distorted tricapped trigonal prism.

The following is the reaction for the formation of Gd-DTPA:

