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Dihydrogen complexes are coordination complexes containing intact H2 as a
The usual method for characterization is 1H
Dihydrogen complexes are coordination complexes containing intact H2 as a ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
. They are a subset of sigma complexes. The prototypical complex is W(CO)3( PCy3)2(H2). This class of compounds represent intermediates in metal-catalyzed reactions involving hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
. Hundreds of dihydrogen complexes have been reported. Most examples are cation
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 ...
ic transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...
s complexes with octahedral geometry
In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The oc ...
.
Upon complexation, the H−H bond is extended to 0.81–0.82 Å as indicated by neutron diffraction
Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of Neutron temperature, thermal or ...
, about a 10% extension relative to the H−H bond in free H2. Some complexes containing multiple hydrogen ligands, i.e. polyhydrides, also exhibit short H−H contacts. It has been suggested that distances < 1.00 Å indicates significant dihydrogen character, where separations > 1 Å are better described as dihydride complexes (see figure).
Characterization
The usual method for characterization is 1H NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic f ...
. The magnitude of spin–spin coupling, ''J''HD, is a useful indicator of the strength of the bond between the hydrogen and deuterium
Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...
in HD complexes. For example, ''J''HD is 43.2 Hz in HD but 33.5 Hz in W(HD)(CO)3(PiPr3)2. Dihydrogen complexes typically have shorter 1H-spin-lattice relaxation times than the corresponding dihydrides.
An ideal if nontrivial method of characterization of dihydrogen complexes is neutron diffraction
Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of Neutron temperature, thermal or ...
. Neutron
The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s interact strongly with hydrogen atoms, which allows one to infer their location in a crystal. In some cases, hydrogen ligands are usefully characterized by X-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
, but often the presence of metals, which strongly scatter X-rays
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
, complicates the analysis.
The triangular MH2 subunit has six normal modes of vibration, one of which is mainly of νH−H character. In free H2, this very strong bond absorbs at 4300 cm−1, whereas in dihydrogen complexes the frequency drops to around 2800 cm−1.
Synthesis
Two preparation methods involve the direct reactions with H2 gas. The first entails the addition of H2 to an unsaturated metal center, as originally reported for W(CO)3(P-i-Pr3)2(H2). In such cases, the unsaturated complex in fact features an agostic interaction that is displaced by the H2. In other cases, H2 will displace anionic ligands, sometimes even halides. Treatment of chlorobis(dppe)iron hydride with sodium tetrafluorborate under an atmosphere of hydrogen is one example: :HFeCl(dppe)2 + NaBF4 + H2 → Fe(H2)(dppe)2BF4] + NaCl Many metal hydrides can be protonated to give dihydrogen complexes: :H2Fe(dppe)2 + H+ → Fe(H2)(dppe)2sup>+ In such cases, the acid usually is derived from aweakly coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...
, e.g., Brookhart's acid.
History
In 1984, Kubas et al. discovered that the addition of H2 to the purple-colored species M(CO)3(PR3)2 gave a yellow precipitate of mer-trans-M(CO)3(PR3)2(H2) (M = Mo or W; R = cyclohexyl, iso-propyl). This result rapidly led to the discovery of a variety of related complexes such as Cr(H2)(CO)5 and e(H2)(H)(dppe)2">dppe.html" ;"title="e(H2)(H)(dppe">e(H2)(H)(dppe)2sup>+. Kubas et al.'s findings also led to a reevaluation of previously described compounds. For example, the complex "RuH4( PPh3)3" described in 1968 was reformulated as a dihydrogen complex.See also
* Difluorine complex Transition metals Metal hydrides">Transition metals">Difluorine complex Transition metals Metal hydridesReferences
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