Topochemical reactions have led to great progress in the discovery of new metastable compounds with novel chemical and physical properties. With these reactions, the overall crystal structure of the host material is generally maintained. Here we report a topochemical synthesis of a hexagonal nitride hydride, h-Ca₃CrN₃H, by heating an orthorhombic nitride, o-Ca₃CrN₃, under hydrogen at 673 K, accompanied by a rotational structural transformation. The hydrogen intercalation modifies the Ca-N rock-salt-like atomic packing in o-Ca₃CrN₃ to a face-sharing octahedral chain in h- Ca₃CrN₃H, mimicking a ‘hinged tessellation’ movement. In addition, the h-Ca₃CrN₃H exhibited stable ammonia synthesis activity when used as a catalyst.