Gadolinium(III) chloride, also known as gadolinium trichloride, is GdCl
3. It is a colorless, hygroscopic, water-soluble solid. The
hexahydrate
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
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 5">dCl5 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 5">dCl5 + 6 NH
3 + 3 H
2O
from hydrated gadolinium chloride:
::4 NH
4Cl + 2 GdCl
3∙6H
2O → 2 (NH
4)
2 5">dCl5 + 12 H
2O
from
gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. Gadolinium is a silvery-white metal when oxidation is removed. It is only slightly malleable and is a ductile rare-earth element. Gadolinium reacts with atmospheric oxygen ...
metal:
::10 NH
4Cl + 2 Gd → 2 (NH
4)
2 5">dCl5 + 6 NH
3 + 3 H
2
In the second step the pentachloride is decomposed at 300 °C:
:: (NH
4)
2 5">dCl5 → GdCl
3 + 2 NH
4Cl
This pyrolysis reaction proceeds via the intermediacy of NH
4 2Cl7">d2Cl7
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 HCL may refer to:
Science and medicine
* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia
* Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development
* Hollow-cathode lamp, a spe ...
.
::Gd + 3 HCl → GdCl
3 + 3/2 H
2
Gadolinium(III) chloride also forms a
hexahydrate
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
, GdCl
3∙6H
2O. The hexahydrate is prepared by gadolinium(III) oxide (or chloride) in concentrated
HCl HCL may refer to:
Science and medicine
* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia
* Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development
* Hollow-cathode lamp, a spe ...
followed by evaporation.
Structure
GdCl
3 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 theYCl
3 motif:
DyCl3, HoCl
3,
ErCl3, TmCl
3, YdCl
3, LuCl
3, YCl
3). The UCl
3 motif features 9-coordinate metal with a tricapped trigonal prismatic
coordination sphere
In coordination chemistry, the first coordination sphere refers to the array of molecules and ions (the ligands) directly attached to the central metal atom. The second coordination sphere consists of molecules and ions that attached in various ...
. In the hexahydrate of gadolinium(III) chloride and other smaller 4f trichlorides and tribromides, six H
2O
molecules
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
and 2 Cl
− ions
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
coordinate to the
cations
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
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
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...
). 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 complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es. GdCl
3∙6H
2O can not be used as an
MRI
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...
contrasting agent due to its low solubility in water at the body's near neutral pH.
"Free" gadolinium(III), e.g.
2(H2O)6">dCl2(H2O)6sup>+, is
toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
, so chelating agents are essential for biomedical applications. Simple
monodentate
In coordination chemistry, denticity () refers to the number of donor groups in a given ligand that bind to the central metal atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals ...
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
Ethylenediaminetetraacetic acid (EDTA) is an aminopolycarboxylic acid with the formula H2N(CH2CO2H)2sub>2. This white, water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-soluble complexes eve ...
4−, ethylenediaminetetraacetate, which is a commonly employed
hexadentate ligand
A hexadentate ligand in coordination chemistry is a ligand that combines with a central metal atom with six bonds. One example of a hexadentate ligand that can form complexes with soft metal ions is TPEN. A commercially important hexadentate liga ...
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
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubil ...
of the Gd
3+ at the body's neutral pH ''and'' still allows for the
paramagnetic
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, d ...
effect required for an
MRI
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...
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
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
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
2O)">d(DTPA)(H2O)sup>2− is a distorted tricapped trigonal prism.
The following is the reaction for the formation of Gd-DTPA:
References
*
*
{{DEFAULTSORT:Gadolinium(Iii) Chloride
Gadolinium compounds
Chlorides
Lanthanide halides
Coordination complexes