Dihydrogen complexes are
coordination complexes
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 ...
containing intact H
2 as a
ligand
In coordination chemistry, a ligand is an ion or molecule ( 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 ele ...
. They are a subset of
sigma complexes. The prototypical complex is W(CO)
3(
PCy3)
2(H
2). This class of
compounds represent intermediates in metal-catalyzed reactions involving
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxi ...
. 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. They are the elements that can ...
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 thermal or cold neutrons to ob ...
, about a 10% extension relative to the H−H bond in free H
2. 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 to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fi ...
. The magnitude of
spin-spin coupling
In quantum mechanics, the procedure of constructing eigenstates of total angular momentum out of eigenstates of separate angular momenta is called angular momentum coupling. For instance, the orbit and spin of a single particle can interact t ...
, ''J''
HD, is a useful indicator of the strength of the bond between the hydrogen and
deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...
in HD complexes. For example, ''J''
HD is 43.2 Hz in HD but 33.5 Hz in W(HD)(CO)
3(P
iPr
3)
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 thermal or cold neutrons to ob ...
.
Neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beha ...
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 determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
, but often the presence of metals, which strongly scatter
X-rays
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30  ...
, complicates the analysis.
The triangular MH
2 subunit has six normal modes of vibration, one of which is mainly of ν
H−H character. In free H
2, 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 H
2 gas. The first entails the addition of H
2 to an unsaturated metal center, as originally reported for W(CO)
3(P-i-Pr
3)
2(H
2). In such cases, the unsaturated complex in fact features an
agostic interaction that is displaced by the H
2.
In other cases, H
2 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 + NaBF
4 + H
2 →
2)(dppe)2">Fe(H2)(dppe)2F
4 + NaCl
Many metal hydrides can be protonated to give dihydrogen complexes:
:H
2Fe(dppe)
2 + H
+ →
2)(dppe)2">Fe(H2)(dppe)2sup>+
In such cases, the acid usually is derived from a
weakly 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 H
2 to the purple-colored species M(CO)
3(PR
3)
2 gave a yellow precipitate of mer-trans-M(CO)
3(PR
3)
2(H
2) (M = Mo or W; R = cyclohexyl, iso-propyl). This result rapidly led to the discovery of a variety of related complexes such as Cr(H
2)(CO)
5 and
2)(H)(dppe">e(H2)(H)( 2)(H)(dppe">e(H2)(H)(dppe)
2">dppe.html"_;"title="e(H
2)(H)(dppe">e(H
2)(H)(dppe)
2sup>+.
_Kubas_et_al.'s_findings_also_led_to_a_reevaluation_of_previously_described_compounds._For_example,_the_complex_"RuH
4(triphenylphosphine.html" ;"title="dppe">e(H
2)(H)(dppe)
2sup>+.
Kubas et al.'s findings also led to a reevaluation of previously described compounds. For example, the complex "RuH
4(
PPh3)
3"_described_in_1968_was_reformulated_as_a_dihydrogen_complex.
_See_also
*_Difluorine_complex
Transition_metals.html" ;"title="triphenylphosphine">PPh
3)
3" described in 1968 was reformulated as a dihydrogen complex.
See also
* Difluorine complex
Transition metals">triphenylphosphine">PPh
3)
3" described in 1968 was reformulated as a dihydrogen complex.
See also
* Difluorine complex
Transition metals
Metal hydrides
References
{{Coordination complexes