Gadolinium(III) Chloride

Gadolinium(III) chloride, also known as gadolinium trichloride, is GdCl₃. It is a colorless, hygroscopic, water-soluble salt. The hexahydrate GdCl₃∙6H₂O is commonly encountered and is sometimes also called gadolinium trichloride. Gd³⁺ 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 Gd³⁺ a useful component in NMR spectroscopy and MRI.

Preparation

GdCl₃ is usually prepared by the "ammonium chloride" route, which involves the initial synthesis of (NH₄)₂ dCl₅ This material can be prepared from the common starting materials at reaction temperatures of 230 °C from gadolinium oxide:
::10 NH₄Cl + Gd₂O₃ → 2 (NH₄)₂ dCl₅ + 6 NH₃ + 3 H₂O

from hydrated gadolinium chloride:
::4 NH₄Cl + 2 GdCl₃∙6H₂O → 2 (NH₄)₂ dCl₅ + 12 H₂O

from gadolinium metal:
::10 NH₄Cl + 2 Gd → 2 (NH₄)₂ dCl₅ + 6 NH₃ + 3 H₂

In the second step the pentachloride is decomposed at 300 °C:
:: (NH₄)₂ dCl₅ → GdCl₃ + 2 NH₄Cl
This pyrolysis reaction proceeds via the intermediacy of NH₄ d₂Cl₇

The ammonium chloride route is more popular and less expensive than other methods. GdCl₃ can, however, also be synthesized by the reaction of solid Gd at 600 °C in a flowing stream of HCl.
::Gd + 3 HCl → GdCl₃ + 3/2 H₂

Gadolinium(III) chloride also forms a hexahydrate, GdCl₃∙6H₂O. The hexahydrate is prepared by gadolinium(III) oxide (or chloride) in concentrated HCl followed by evaporation.

Structure

GdCl₃ crystallizes with a hexagonal UCl₃ structure, as seen for other 4f trichlorides including those of La, Ce, Pr, Nd, Pm, Sm, Eu. The following crystallize in theYCl₃ motif: DyCl₃, HoCl₃, ErCl₃, TmCl₃, YdCl₃, LuCl₃, YCl₃). The UCl₃ 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 H₂O 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 Gd³⁺ has an electronic configuration of f⁷. Seven is the largest number of unpaired electron spins possible for an atom, so Gd³⁺ is a key component in the design of highly paramagnetic complexes.
To generate the relaxation agents, Gd³⁺ sources such as GdCl₃∙6H₂O are converted to coordination complexes. GdCl₃∙6H₂O 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. dCl₂(H₂O)₆sup>+, 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 Gd³⁺ in solution. Ligands with higher coordination numbers therefore are required. The obvious candidate is EDTA⁴⁻, 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 H₅DTPA, diethylenetriaminepentaacetic acid.
Chelation to the conjugate base of this ligand increases the solubility of the Gd³⁺ at the body's neutral pH ''and'' still allows for the paramagnetic effect required for an MRI contrast agent. The DTPA⁵⁻ 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 d(DTPA)(H₂O)sup>2− is a distorted tricapped trigonal prism.

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

References

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{{DEFAULTSORT:Gadolinium(Iii) Chloride
Gadolinium compounds
Chlorides
Lanthanide halides
Coordination complexes