Dihydrogen complexes are coordination complexes containing intact H2 as a ligand. The prototypical complex is W(CO)3(PCy3)2(H2). This class of compounds represent intermediates in metal-catalyzed reactions involving hydrogen. Hundreds of dihydrogen complexes have been reported. Most examples are cationic transition metals 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).
The preferred method of characterization of dihydrogen complexes is neutron diffraction. Neutrons 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.
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(HD)(CO)3(PiPr3)2. 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.
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(PPh3)3" described in 1968 was reformulated as a dihydrogen complex.
1. ^ Kubas, Gregory J. (2001-08-31). Metal Dihydrogen and σ-Bond Complexes - Structure, Theory, and Reactivity (1 ed.). Springer. ISBN 0306464659.
1. Burdett, J. K.; O. Eisenstein, S. A. Jackson (1991-12-06). "Transition Metal Didydrogen Complexes: Theoretical Studies". in A. Dedieu (ed.). Transition Metal Hydrides. Wiley-VCH. pp. 149–184. ISBN 0471187682.