College Park, Maryland June 6 - 10 , 2004
T2-B5 (2:45 PM): Neutron Diffraction Studies on Metal Pseudohalide between 300 K and 50 K
D. J. Williams, L. Daemen, S. Vogel (Los Alamos Neutron Science Center, Los Alamos National Laboratory)
The structural investigation of metal pseudohalide compounds (e.g. cyanides, cyanates, etc.) has been the object of numerous studies over more than a century. Several unresolved problems remain. Those have been addressed inconclusively with x-ray scattering and a variety of other techniques providing indirect structural information. One such outstanding structural question in binary pseudohalide compounds is whether only carbon or only nitrogen atoms in the polyhedron coordination sphere surround the metal cation. This structural question regarding the coordination sphere of the metal cation in cyanide and cyanate compounds is still debated among chemists.
The use of powder and single crystal x-ray diffraction techniques has been limited because the scattering contrast between neighboring elements, carbon, nitrogen, and oxygen, is weak. The possibility of disorder further complicates the matter. Fourier-Transform Infrared Spectroscopy (FT-IR) has been used to shed light on the nature of the metal to C or N bonding. However most of the information that is obtained is n(C-N), n(C-O) stretches, and broad n(M-(C, N, O)) stretches at low wave numbers. (When the latter can be observed at all, it usually does not reveal any information about the metal cation preferred bonding orientation to the cyanide ligand.) Other spectroscopic techniques like Raman and NMR have had equally limited success. Because of the difference in neutron scattering length among C, N, and O, powder neutron diffraction should prove useful in resolving controversial structural issues in binary pseudo-halides. Similarly, incoherent inelastic neutron scattering is an attractive, alternative to conventional spectroscopies to provide chemical bonding information on the metal to pseudo-halide ligand. With LANSCE's newly commissioned powder diffractometer HIPPO (High Pressure Preferred Orientation powder diffractometer) and FDS (Filter Difference Spectroscopy) a complete structural investigation of AgCN and AuCN was performed at 300 K, 200 K, 150 K, 100 K, and 50 K. The results confirm that cyanide ligands in these simple binary compounds are disordered throughout the crystal structure. The ordering/disordering of cyanate ligands has also been investigated (e.g., NaOCN, KOCN, AgOCN…) using neutron scattering to determine whether the metal cation (e.g. Na, K…) is completely surrounded by the nitrogen or the oxygen end of a cyanate ion (e.g. OCN-M-NCO, NCO-M-NCO, NCO-M-OCN, etc…). These results will be discussed in detail.
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