Through ten years physical investigation on the geometrically frustrated coordination compound series basic (hydroxyl and deuteroxyl) transition-metal halides M2(OH/D)3X (M=Mn, Fe, Co, Ni and Cu; X=Cl, Br and I), we have determined the existence of four crystal structures, whose space groups belong to Nos. 11, 14, 62 and 166, respectively, in this series till now. After studying their Raman spectra, we firmly believe that a nonnegligible hydrogen bond (H-bond) exists in every material. Considering carefully their local lattice environments of the basic groups, we find a new kind, to the best of our knowledge, of H-bond [(OH)3···M] and nominate it as the trimeric H-bond, and further confirm three types of symmetric structures, which have totally four subtypes of trimeric H-bonds so far. More two subtypes are predicted to exist in nature. This result may provide a new interesting subject for quantum chemists and material physicists.
Cover Feature: Alkali Transition‐Metal Molybdates: A Stepwise Approach to Geometrically Frustrated Systems (Chem. Eur. J. 3/2020)