Dihydrogen complex
Encyclopedia
Dihydrogen complexes are coordination complexes containing intact H2 as a ligand
. The prototypical complex is W(CO)3(PCy3
)2(H2). This class of compound
s represent intermediates in metal-catalyzed reactions involving hydrogen
. Hundreds of dihydrogen complexes have been reported. Most examples are cationic transition metal
s complexes with octahedral geometry.
Upon complexation, the H−H bond is extended to 0.81–0.82 Å as indicated by neutron diffraction
, 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 dihydrido complexes (see figure).
. Neutron
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
, but often the presence of metals, which strongly scatter X-rays, complicates the analysis. NMR
techniques are also widely used. The magnitude of spin-spin coupling, JHD, is a useful indicator of the strength of the bond between the hydrogen and deuterium
in HD complexes. For example, JHD 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.
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.
Many metal hydrides can be protonated to give dihydrogen complexes:
In such cases the acid usually is derived from a weakly coordinating anion.
)3" described in 1968 was reformulated as a dihydrogen complex.
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
. The prototypical complex is W(CO)3(PCy3
Tricyclohexylphosphine
Tricyclohexylphosphine is the tertiary phosphine with the formula P3. Commonly used as a ligand in organometallic chemistry, it is often abbreviated to PCy3, where Cy stands for cyclohexyl...
)2(H2). This class of compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s represent intermediates in metal-catalyzed reactions involving hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. Hundreds of dihydrogen complexes have been reported. Most examples are cationic transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
s complexes with octahedral geometry.
Upon complexation, the H−H bond is extended to 0.81–0.82 Å as indicated by neutron diffraction
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 obtain a diffraction pattern that provides information of...
, 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 dihydrido complexes (see figure).
Characterization
The preferred method of characterization of dihydrogen complexes is neutron diffractionNeutron 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 obtain a diffraction pattern that provides information of...
. Neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
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
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
, but often the presence of metals, which strongly scatter X-rays, complicates the analysis. NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
techniques are also widely used. The magnitude of spin-spin coupling, JHD, is a useful indicator of the strength of the bond between the hydrogen and deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
in HD complexes. For example, JHD 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
Relaxation (NMR)
In nuclear magnetic resonance spectroscopy and magnetic resonance imaging the term relaxation describes several processes by which nuclear magnetization prepared in a non-equilibrium state return to the equilibrium distribution. In other words, relaxation describes how fast spins "forget" the...
times than the corresponding dihydrides.
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 some cases, H2 will displace weakly bound ligands, sometimes even halides in favorable cases:- LnMX + H2 → [LnM(H2)]+ + X−
Many metal hydrides can be protonated to give dihydrogen complexes:
- LnM−H + H+ → [LnM(H2)]+
In such cases the acid usually is derived from a weakly coordinating anion.
History
In 1984, Kubas et al. discovered that the addition of H2 to 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 [Fe(H2)(H)(dppe)2]+. Kubas et al.'s findings also led to a reevaluation of previously described compounds. For example, the complex "RuH4(PPh3Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...
)3" described in 1968 was reformulated as a dihydrogen complex.