First-row transition metal sulfides

The results of a rerefinement of the FeS2 data and of more recent accurate data on CoS2 and NiS2 (Nowack et al. 1991) are listed in Table 10.6. Anharmonic 

J From population parameters in Stevens et al (1980) total Fe population fixed at 6 electrons with anharmonic cumulant temperature factors. 

Gram-Charlier temperature factors (chapter 2) were applied in this analysis, which made use of the XD programming package (Su and Coppens, to be published). In accordance with the results on H3P04, discussed in chapter 3, the values of n, were chosen as 6, 6, 7, and 7 for / = 1, 2, 3, and 4, respectively. However, the differences between the results from this choice and those of two alternative selections (4, 4, 4, and 4 and 6, 6, 6, and 6) are within one standard deviation. 

The model deformation densities for all three compounds show a peak between the metal atom and the bonded sulfur, located closer to the sulfur atoms, in a region not covered by Fig. 10.7. A local density functional calculation of pyrite by Zeng and Holzwart (1994) gives a theoretical deformation density which closely reproduces the features of the experimental densities, including the Fe—S bond peak. No orbital populations are as yet available from this calculation. 

Unlike the metal atoms in the chalcogenides discussed in the previous section, the iron atoms in iron(II) tetraphenyl porphyrins and the related iron(II) phthalocyanine generally occupy crystallographic sites with symmetry T or 1 only. The exception is the iron site in iron(II) tetraphenyl porphyrin (FeTPP), space group I42d, which has 4 symmetry. 

---

Earlier work has been reviewed by Weisser and Landa (1973) and certain more recent studies reviewed by Chianelli (1983), who points to the primary catalytic effects being electronic in nature, related to the position occupied by the metal concerned in the periodic table, with first-row transition-metal sulfides being relatively inactive and second- and third-row transition metals (Ru and Os) showing maximum activity (by three orders of magnitude). A secondary effect appears to be geometric in origin for example, MoSj catalysts seem to exhibit activity associated particularly with edges rather than basal planes. 

---