Borderline hydrides

[1] ZnH2 is also a solid at room temperature that breaks down at 90 °C, but even left alone decomposes over several days to zinc metal and hydrogen gas.

The classic form is the dihydride M(H)2 configuration, where the metal is bound to two free hydrogen atoms.

[6] Those that are thermally unstable exhibit stretching frequencies νHH greater than 2150 cm−1 as a result of poor electron donation from the metal center.

An electron dense metal center will yield hydride with a νHH less than 2060 cm−1, while anything between is considered to be in the borderline region.

Kubas, et al. state that a stretching frequency of 2090 cm−1 is within the bounds of stable H2 complexes while 2060 cm−1 is right on the borderline between dihydrogen and dihydrides.