Beryllium monohydride (BeH) is an example of a molecule with a half-bond order according to molecular orbital theory. It is a metastable monoradical species which has only been observed in the gas phase. In beryllium monohydride, beryllium has a valency of one, and hydrogen has a valency of one. BeH has been studied spectroscopically since 1928 and in over 80 theoretical studies (see for a review). The bond length is 134.2396(3) pm and the dissociation energy is 17702(200) cm−1. The dimeric molecule Be2H2 has also been observed in an argon matrix at 10 K