Characterization
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.
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