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Gadolinium(III) chloride, also known as gadolinium trichloride, is GdCl3. It is a colorless, hygroscopic, water-soluble solid. 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.
Additional recommended knowledge
GdCl3 is usually prepared by the "ammonium chloride" route, which involves the initial synthesis of (NH4)2[GdCl5]. This material can be prepared from the common starting materials at reaction temperatures of 230 °C from gadolinium oxide:
from hydrated gadolinium chloride:
from gadolinium metal:
In the second step the pentachloride is decomposed at 300 °C:
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.
GdCl3 is monoclinic with a hexagonal UCl3 structure, as seen for other 4f trichlorides including those of La, Ce, Pr, Nd, Pm, Sm, Eu (in contrast GdCl3 crystallizes in the PuBr3 motif and 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.
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 Gd(DTPA)(H2O)]2- is a distorted tricapped trigonal prism.
The following is the reaction for the formation of Gd-DTPA:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gadolinium(III)_chloride". A list of authors is available in Wikipedia.|