Dihydrogen complexes are coordination complexes containing intact H₂ 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 elec ...

. They are a subset of sigma complexes. The prototypical complex is W(CO)₃( PCy₃)₂(H₂). This class of

compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive struc ...

s 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-toxic ...

. Hundreds of dihydrogen complexes have been reported. Most examples are cationic

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 ca ...

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, 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 to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fie ...

. 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(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 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 beh ...

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, complicates the analysis. The triangular MH₂ subunit has six normal modes of vibration, one of which is mainly of ν_H−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 In organometallic chemistry, agostic interaction refers to the interaction of a coordinatively-unsaturated transition metal with a C−H bond, when the two electrons involved in the C−H bond enter the empty d-orbital of the transition metal, r ...

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)₂F₄ + 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 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 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)( 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(triphenylphosphine.html" ;"title="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.html" ;"title="triphenylphosphine">PPh₃)₃" described in 1968 was reformulated as a dihydrogen complex.

References