A new type of hydrogen bond
The π electrons of an aromatic ring can sometimes act as an H bond acceptor to form an X-H…π hydrogen bond. Surprisingly in a structure containing the arrangement acceptor X-H…H-Y the H…H distance, typically 1.9Å, is much shorter than the normal H…H contact of 2.5 Å [R.C. Stevens, et al, Chem. Soc. Dalton Trans, 1990 1429, L.S. Van der Shiys et al, JACS 112, 4831 (1990)]. This Y-HT attractive interaction between two hydrogens has come to be called the “dihydrogen bond”. Y is always an electropositive atom such as boron or a transition metal. A neutron-diffraction structure of H3BNH3 (Koetzle, unpublished) unambiguously show the presence of short H…H contacts (2.02 to 2.23 Å), but the most surprising aspect of this work was the demonstration that boron and nitrogen had been misassigned in the previous X-ray work.