Dihydrogen complex

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.

In free H2, this very strong bond absorbs at 4300 cm−1, whereas in dihydrogen complexes the frequency drops to around 2800 cm−1.

Treatment of chlorobis(dppe)iron hydride with sodium tetrafluorborate under an atmosphere of hydrogen is one example:[3] Many metal hydrides can be protonated to give dihydrogen complexes:[4] In such cases, the acid usually is derived from a weakly coordinating anion, e.g., Brookhart's acid.

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]+.

Formation and equilibrium structures of metal dihydrogen and dihydride complexes (L = ligand)
An often studied dihydrogen complex of iron, [HFe(H 2 )(dppe) 2 ] + .