ABSTRACT
It is thought that hydrogen bonding is responsible for the ferroelectricity in ammonium tetrafluoroberyllate, (NH(4))(2)BeF(4). In the past X-ray data have been collected, but these did not permit accurate determination of the H-atom positions. In order to obtain more accurate information the neutron structures have now been determined for the paraelectric and ferroelectric phases. Going from the paraelectric to the ferroelectric phase, both the BeF(4)(2-) and the NH(4)(+) ions rotate and shift from the mirror planes of the paraelectric phase. This results in removal of the mirror-plane symmetry and formation of a superlattice with the a axis doubled. Along the polar c axis, the NH(4)(+) ions move towards the BeF(4)(2-) ions within chains of molecules and the chains move slightly relative to one another. The rotations and translations give rise to stronger hydrogen-bonding interactions.
ABSTRACT
Pentacoordinate hydrogen atoms were identified by single-crystal neutron diffraction analysis of [N(CH3)4]3[H2Rh13(CO)24]. The hydrogen atoms are located in square pyramidal cavities of the Rh13 cluster, in positions almost coplanar with the Rh4 faces on the surface of the cluster. They are slightly displaced inward, toward the central rhodium atom of the cluster, with average H-Rh(central) and H-Rh(surface) distances of 1.84(2) and 1.97(2) angstroms, respectively. This result shows that hydrogen, which normally forms only one bond, can be attached to five other atoms simultaneously in a large metal cluster.
