Octahedral coordination site

Figure 14. Fe and A1 in octahedrally coordinated sites of illite, celadonite and glauconites. M-B = muscovite beidellite theoretical compositions Mo = montmorillonite (octahedral charge) Ce = celadonite open circles = illite triangles = glauconites dots = celadonites.

Actually the contribution from upper states may not always be very important. A dominance by a few low-lying states can yield surprising results as the magnetism of rrans-dimesityl bis(diethylphenylphosphine)cobalt(II) shows. This nominally square planar low-spin cobalt(II) molecule shown in Fig. 8a, in which the a-methyl groups of the mms-mesityl ligands effectively block the fifth and sixth octahedral coordination sites around the cobalt, was studied by Bentley et al. (5). Although the symmetry of the first coordination shell is... 

It can be seen from table 2.2 that transition metal ions with 3d5, 3cP and low-spin 3 configurations acquire significantly higher CFSE s in octahedral coordination than other cations. Therefore, ions such as Cr3+, Mn4+, Ni2+, and Co3+ are expected to show strong preferences for octahedral coordination sites. Cations with 3d°, 3d10 and high-spin 3d5 configurations, such as Ca2+, Zn2+, Mn2+ and Fe3+, receive zero CFSE in octahedral coordination. 

In an octahedral coordination site, the electrostatic field produced by the six ligands (represented as point negative charges and interacting with an electron in the vicinity of the central cation) is expressed by the potential... 

A converse situation exists whereby the two oxygen ions along the z axis may move closer to the Mn3+ ion (fig. 2.8 >). This results in the stabilization of the dx2 y2 orbital relative to the dz2 orbital, and shorter Mn-O distances along the z axis compared to the x-y plane. In either of the tetragonally distorted environments shown in fig. 2.8 the Mn3+ ion becomes more stable relative to a regular octahedral coordination site. In most minerals, however, the Mn3+ ion occurs in an axially elongated octahedron (see table 6.1). 

Measurements of absorption spectra of oxides, glasses and hydrates of transition metal ions have enabled crystal field stabilization energies (CFSE s) in tetrahedral and octahedral coordinations to be estimated in oxide structures (see table 2.5). The difference between the octahedral and tetrahedral CFSE is called the octahedral site preference energy (OSPE), and values are summarized in table 6.3. The OSPE s may be regarded as a measure of the affinity of a transition metal ion for an octahedral coordination site in an oxide structure such as spinel. Trivalent cations with high OSPE s are predicted to occupy octahedral sites in spinels and to form normal spinels. Thus, Cr3, Mn3, V3+... 

Bush, W. R., Hafner, S. S. Virgo, D. (1970) Some ordering of iron and magnesium at the octahedrally coordinated sites in a magnesium-rich olivine. Nature, 227, 1339-41. 

Figure 12.21 shows pyridine adsorption at 25°C on an alumina pre-treated at 450 C. Only Lewis sites are visible because the Bronsted sites are too weak to be shown by pyridine. The existence of two Lewis sites L] and L2 should be noted (bands at 1 622 cm and 1617 cm ) corresponding respectively to aluminium atoms in tetrahedral and octahedral coordination sites. Lutidine adsorption on the same alumina support is used to show the Bronsted sites (Fig. 12.22 and Table 12.4). 

The complex W(CO)3(PPr 3)2(H2) is a true electron pair forming the H-H bond. Consequently, it is the midpoint of the H2 ligand which occupies one of the octahedral coordination sites at the tungsten atom. The H-H distance in the complex is 0.82 A. It is only slightly longer than the H-H distance in the hydrogen molecule with 0.74 A, whereas for a classical dihydride an H-H distance of about 1.8 A would be expected. 

Our current knowledge of the stereospecificity of siderophore uptake remains still very fragmentary. Therefore the formulation of a generally accepted rule is not possible. However one trend conforms to all the data at hand so far it seems that the predominantly recognized part of the molecule is the iron octahedral coordination site and its adjacent functionalities. 

All the cited literature references to the above compounds have described solid-state syntheses at temperatures of 700-1200°. Such synthesis conditions will always lead to pyrochlore structure compounds in which all of the octa-hedrally coordinated sites are occupied by the noble metal cation, thus requiring the post-transition metal to noble metal molar ratio always to be 1.0. This paper focuses on solution medium syntheses at quite low temperatures (<75°), thereby stabilizing a new class of pyrochlore compounds in which a variable fraction of the octahedrally coordinated sites are occupied by post-transition element ca-tions.5,6 The specific example here involves the Pb2[Ru2 Pb4+]06 s series. The synthesis conditions may be simply adapted, however, to accommodate preparation of a wider range of pyrochlores which can be described by the formula A2[B2 xAx]07.3> where A is typically Pb or Bi, B is typically Ru or Ir and 0 < 1, and 0 < 1. 

It should be noted that the exact cation stoichiometry of the product is highly sensitive to the exact metal concentration of the ruthenium source solution and temperature and pH of the reaction medium (inadvertent increases in both of these parameters lead to increased solubility of lead in the alkaline reaction medium and consequently yield solid products of lower lead ruthenium ratios). While synthesis of a pure lead ruthenium oxide pyrochlore is relatively easy, the precise cation stoichiometry of the product is a property that is not always easy to control. A relatively quick check on the cation stoichiometry of the lead ruthenium oxide product can be obtained, however, by using the correlation between lattice parameter and composition that is displayed in Fig. 1. When lattice parameter and cation stoichiometry are independently determined, the relationship shown in Fig. 1 also provides an assessment of product purity since data points that show significant departures from the displayed linear correlation indicate the presence of impurity phases. The thermal stability of the lead ruthenium oxides decreases with increasing occupancy of tetravalent lead on the octahedrally coordinated site, but all of the ruthenium oxide pyro-chlores described are stable to at least 350° in oxygen. 

Mn in an octahedrally coordinated site (Fig. 7). Such a change is observed in synthetic Sr and Ba apatite phosphors (Shionoya and Yen 1999). Sr preferentially occupies the Ca2 site (Rakovan and Hughes 2000), and it is probable that Pb and Ba also prefer the Ca2... 

As the crystallographic site occupancy is concerned, in inverse spinels all Li ions reside in octahedral coordination sites, while the transition metal ions (TMI) can occupy both octahedral and tetrahedral sites. The results obtained are reported in Table 1 they suffer a limitation due to the fact that the XRPD technique can discriminate between ion and Co ... 

We have examined (8), as did Brady and coworkers (3), the electronic absorption spectrum of the Saltman- piro ball. The spectrum (Figure 6) (8) shows a pattern of four weak bands with the lowest band at about 900 nm, in very good agreement with [Fe(III)06]oct coordination. The derived LF parameters are A<>ct = 11,260 cm" C/B = 3, and B = 815 cm" We can say with considerable confidence that most of the Fe(III) ions occupy octahedral coordination sites. There is no hint of bands attributable to [Fe(III)04]tet, and as these bands are intrinsically more intense than are those assigned to [Fe(III)06]oct> we can eflFectively rule out tetrahedral coordination in this synthetic model compound. 

The spinel structure was first determined by Bragg (1915) and Nishikawa (1915). The ideal structure is formed by a cubic close-packed (fee) array of O atoms, in which one-eighth of the tetrahedral and one-half of the octahedral interstitial sites are occupied by cations. The tetrahedrally coordinated sites and the octahedrally coordinated sites are referred to as the A and B sites, respectively. 

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