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).
An often studied dihydrogen complex of iron, [HFe(H2)(dppe)2]+.
The usual method for characterization is 1HNMR spectroscopy. 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.[2]
An ideal if nontrivial 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.
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(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:[3]
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).[5] This result rapidly led to the discovery of a variety of related complexes such as Cr(H2)(CO)5[6] and [Fe(H2)(H)(dppe)2]+.[7] 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.
^Kubas, Gregory J. (2001-08-31). Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity (1 ed.). Springer. ISBN0-306-46465-9.
^Crabtree, R. H. (1990). "Dihydrogen complexes: some structural and chemical studies". Accounts of Chemical Research. 23 (4): 95–101. doi:10.1021/ar00172a001.
^Bautista, M. T.; Bynum, L. D.; Schauer, C. K. (1996). "Synthesis of η2-Dihydrogen Complex, trans-{Fe(η2-H2)(H)[1,2-bis(diphenylphosphino)ethane]2}[BF4]: An Experiment for an Advanced Inorganic Chemistry Laboratory Involving Synthesis and NMR Properties of an η2-H2 Complex". Journal of Chemical Education. 73 (10): 988–991. Bibcode:1996JChEd..73..988B. doi:10.1021/ed073p988.
^Morris, R. H. (2008). "Dihydrogen, Dihydride and in Between: Nmr and Structural Properties of Iron Group Complexes". Coord. Chem. Rev. 2252 (21–22): 2381–2394. doi:10.1016/j.ccr.2008.01.010.
^Kubas, G. J.; R. R. Ryan; B. I. Swanson; P. J. Vergamini; H. J. Wasserman (1984-01-01). "Characterization of the first examples of isolable molecular hydrogen complexes, M(CO)3(PR3)2(H2) (M = molybdenum or tungsten; R = Cy or isopropyl). Evidence for a side-on bonded dihydrogen ligand". Journal of the American Chemical Society. 106 (2): 451–452. doi:10.1021/ja00314a049.
^Sweany, Ray L. (1985-04-01). "Photolysis of hexacarbonylchromium in hydrogen-containing matrixes: evidence of simple adducts of molecular hydrogen". Journal of the American Chemical Society. 107 (8): 2374–2379. doi:10.1021/ja00294a030.
^Morris, Robert H.; Jeffery F. Sawyer; Mahmoud Shiralian; Jeffrey Zubkowski (1985). "Two molecular hydrogen complexes: trans-[M(η2-H2)(H)(PPh2CH2CH2PPh2)2]BF4 (M = Fe, Ru). The crystal structure determination of the iron complex". Journal of the American Chemical Society. 107 (19): 5581–5582. doi:10.1021/ja00305a071.
