Low-Barrier Hydrogen Bonds in Negative Thermal Expansion Material H₃ [Co(CN)₆ ]

The covalent nature of the low-barrier N-H-N hydrogen bonds in the negative thermal expansion material H₃ [Co(CN)₆ ] has been established by using a combination of X-ray and neutron diffraction electron density analysis and theoretical calculations. This finding explains why negative thermal expansion can occur in a material not commonly considered to be built from rigid linkers. The pertinent hydrogen atom is located symmetrically between two nitrogen atoms in a double-well potential with hydrogen above the barrier for proton transfer, thus forming a low-barrier hydrogen bond. Hydrogen is covalently bonded to the two nitrogen atoms, which is the first experimentally confirmed covalent hydrogen bond in a network structure. Source function calculations established that the present N-H-N hydrogen bond follows the trends observed for negatively charge-assisted hydrogen bonds and low-barrier hydrogen bonds previously established for O-H-O hydrogen bonds. The bonding between the cobalt and cyanide ligands was found to be a typical donor-acceptor bond involving a high-field ligand and a transition metal in a low-spin configuration.