Mossbauer coordination geometries

The coordination chemistry of iron in such melts is a little better understood. Fe " and Fe" can act as network formers and modifiers in both tetrahedral and octahedral coordination geometries. It is also of interest to note in the light of the above results on Ni" that multiple site occupancy is also indicated for Fe" . In a number of Ca0-Al203-Si02-Fe0 glasses two different tetrahedral complexes of this ion can be observed using Mossbauer spectroscopy. 

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Mossbauer spectroscopy is a versatile technique for determining cation locations and coordination geometries in materials where the Mossbauer ions may be found in several sites. Such is the case in zeoHtes where the cations required to balance the negative charge of the aluminosihcate lattice can take up a munber of different positions to produce a distribution with the lowest free energy. Mossbauer studies have been made on a number of zeoHtes, most interest being shown in the catalytically active Y-zeoHtes [2,3]. Because of these catalytic properties the interaction between the cations in the zeoHte and absorbed gases is also of interest and one that is particularly amenable to study by Mossbauer spectroscopy. 

The various iron(ll) complexes of pyridine provide a range of related compounds which illustrate how the Mossbauer parameters respond to changes in coordination environment and coordination geometry as well as structural phase transformations. 

The Mossbauer spectral parameters for the bis pyridine complexes are of interest because they reveal differences in the structural properties of the compounds. The room temperature isomer shifts range from l.(X) to 1.09 mm/s for all these compounds, except Fe(pyridine)2l2, for which the value of d is only 0.76 mm/s. This much smaller isomer shift value is clearly indicative of the reduced coordination number found in this and related pseudo tetrahedral compounds (e.g. Long Coffen, 1974). The tetrahedral coordination geometry, which is further supported by X-ray diffraction and electronic spectral studies, presumably results from the large size of the iodide ions as compared with the other halide and pseudohalide ligands. 

There may, however, be a number of other reasons to pursue a predictive first principles theory of Mossbauer spectroscopy. For example, one may want to elucidate structure/spectroscopy correlations in the cleanest way. To this end one may construct in the computer a number of models with systematic variations in oxidation states, spin states, coordination numbers, and identity of hgands to name only a few chemical degrees of freedom. In such studies it is immaterial whether these molecules have been made or could be made what matters is that one can find out which structural details the Mossbauer parameters are most sensitive to. This can provide insight into the effects of geometry or covalency that are very difficult to obtain by any other means. 

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