Low‐Barrier Hydrogen Bonds in Negative Thermal Expansion Material H 3 [Co(CN) 6 ]

• Published in: Chemistry : a European journal Vol. 25; no. 27; pp. 6814 - 6822
• Main Authors: Tolborg, Kasper, Jørgensen, Mads R. V., Sist, Mattia, Mamakhel, Aref, Overgaard, Jacob, Iversen, Bo B.
• Format: Journal Article
• Language: English
• Published: 10.05.2019
• Subjects: chemical bonding; Chemical Sciences; electron density; hydrogen bonds; Kemi; Natural Sciences; Naturvetenskap; neutron diffraction; X-ray diffraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201900358

Abstract The covalent nature of the low‐barrier N−H−N hydrogen bonds in the negative thermal expansion material H 3 [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.