Mn „ polyhedra

Compound Phase, Mineral Mn Polyhedra and Bond Lengths (av. in A)... 

The arsenate polyhedra, on the other hand, show distinctly more variability of the > 60 species described,270 many of which are minerals, structural analyses are available for one-third and, of these, one-third appear to have non-octahedral Mn polyhedra (Table 25). (The proviso results from a report268 that the earlier analysis, describing arsenoclasite as containing [Mn04], [MnOs] and [MnOs] polyhedra, was in error, and that there are only [MnOs] polyhedra—albeit distorted.)... 

The sulfate MnS04 and its hydrates (7, 5, 4, 2 and 1) represent the stable oxidation state under ambient conditions. Compounds are colourless in finely-divided form and pale pink or rose coloured when crystalline. All appear to be examples of high spin octahedral Mn11. Aqueous solutions have been much studied, to define the physical parameters, because of their importance in Mn production from its ores. Stability constants for 1 1, 1 2 and 1 3 species have been defined but detail (structurally) of the labile Mn polyhedra is unclear. Examples of unidentate and bridging sulfato are seen in the known crystal structures, but not bidentate or terdentate to the same metal atom. (However, bidentate function may occur in some of the mixed-ligand compounds described earlier.)... 

Mn " is fonnally d, and therefore the high spin configuration in an octahedral field is one of those, like the of Cu", which the Jahn-Teller theorem predicts should distort. Such distortions are indeed observed in the great majority of, but not all, known structures and they often serve to identify the Mn" polyhedra in mixed oxidation state compounds. There are few examples of other spin states (Table 43) [Mn(CN)6] is an example of a low spin (S =, 1) compound, and there are other examples in the macrocyclic compounds (Section 41.4.8). 

All of these coordination polyhedra are quite labile, and the structures of the Mn" polyhedra in any solution depend only on the nature of the solvent and not the starting material. Stability constants K, to X3, for aqueous fluoro species in perchlorate media, have recently been reported. ... 

R n Mn Polyhedra Bridge Type Comments (Mn—0 Bond Lengths in X)... 

The oxyanions of phosphorus and arsenic take up the major portion of a recent large volume of Gmelin (C9), which includes more than 100 compounds, hydrates and phases containing phosphate groups.263 To pursue the detail of the latter is an exercise in phosphorus chemistry, so we concentrate on a survey of the available data on the Mn" coordination polyhedra. 

Figure 42. The crystal structures of triplite and triploidite (a) triplite projected onto (010) (b) triplite projected onto (100) (c) triploidite projected onto (001) (d) triploidite projected onto (001). (Mn( )6) octahedra are shadow-shaded, (Mn(f5) polyhedra are line-shaded.

The Fe—O distances in hematite are 1.99 and 2.06 A. The (Mn,Fe)—O distances in bixbyite are expected to be the same in case that (Mn, Fe) has the coordination number 6, and slightly smaller, perhaps 1.90 A, for coordination number 4. The radius of 0= is 1.40 A, and the average O—O distance in oxide crystals has about twice this value. When coordinated polyhedra share edges the O—O distance is decreased to a minimum value of 2.50 A, shown by shared edges in rutile, anatase, brookite, corundum, hydrargillite, mica, chlorite, and other crystals. Our experience with complex ionic crystals leads us to believe that we may... 

Other single-crystal x-ray diffraction studies of transition element dopants in jS-rh boron are based on the results of a refinement of the /3-rh boron structure that establishes the occurrence of four new low-occupancy (3.7, 6.6, 6.8 and 8.5%) B positions in addition to the earlier known ones. The dopant elements studied, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf and Ta, do not enter B positions in the framework, but they enter the Al, A2, D and E positions. In some cases the doping elements have been studied at several concentrations for each element and for different cooling rates. The percentage occupancies of certain positions are eorrelated with the atomie sizes of the dopants. The bond distances between the polyhedra are shorter than those within the polyhedra. The mechanism of doping for some cases is denoted displacive, rather than interstitial or substitutional, because of competing interactions between the six different partially occupied B positions and dopant atoms. 

Anomalous structures due to magnetism 231 Table 8.1 Coordination polyhedra of the a-Mn structure type... 

Despite the importance of this area of Mn" chemistry, we still have much to learn about basic structural preferences the variety of polyhedra adopted their relative importance, distortions of... 

There is little doubt that the basic species formed when almost any compound MnX2 is dissolved in water204,205 is [Mn(H20)6]2+. It is seen in various crystalline solids, and the solution data give no real hint that we need to suspect the other coordination polyhedra, either higher or lower, are present in the aqueous solutions. So, [Mn(H20)6]2+ it is—mainly on the basis205 of the optical spectra and correlation with the crystalline solids. In this respect, we note that XAFS studies on aqueous solutions derive an Mn—OH2 distance of 2.18 A, in good agreement with the observed distances in solids (Table 1). 

Table 22 The Coordination Polyhedra of Crystalline Mn Borates and Carbonates...

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