Cubic and dodecahedral coordinations

In body-centred cubic coordination, the eight ligands surrounding a transition metal ion lie at the vertices of a cube (cf. fig. 2.6a.). In one type of dodecahedral coordination site found in the ideal perovskite structure (cf. fig. 9.3), the 12 nearest-neighbour anions lie at the vertices of a cuboctahedron illustrated in fig. 2.6b. The relative energies of the eg and t2g orbital groups in these two cen-trosymmetric coordinations are identical to those of the e and t2 orbital groups 

Number of 3d elections Electronic configuration Unpaired e t2 elections CFSE Electronic configuration e t2 Unpaired elections CFSE 

These relative crystal field splittings are portrayed schematically in fig. 2.7. 

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As noted earlier, A depends on the symmetry of the ligands surrounding a transition metal ion. The relationships expressed in eqs (2.7), (2.8) and (2.9) for crystal field splittings in octahedral, tetrahedral, body-centred cubic and dodecahedral coordinations are summarized in eq. (2.26)... 

Figure 3.11 Orgel diagram for transition metal ions possessing rD spectroscopic terms in octahedral crystal fields of increasing intensity. The right-hand side applies to 3d1 (e.g., Ti3+) and 3d6 (e.g., Fe2+) cations and the left-hand side to 3d4 (e.g., Mn3+) and 3d9 (e.g., Cu2+) cations in octahedral coordination. The diagram in reverse also applies to the cations in tetrahedral, cubic and dodecahedral coordinations.

For 3d5 ions, the same diagram applies for octahedral, tetrahedral, cubic and dodecahedral coordination environments. 

Fig. 13. The so-called cubic and dodecahedral modifications of tetrahedral four-coordinate olefin complexes.

Changes of coordination number A guiding principle of crystal chemistry is that the coordination number of a cation depends on the radius ratio, RJR, where Rc and / a are the ionic radii of the cation and anion, respectively. Octahedrally coordinated cations are predicted when 0.414 < 7 c// a < 0.732, while four-fold (tetrahedral) and eight- to twelvefold (cubic to dodecahedral) coordinations are favoured for radius ratios below 0.414 and above 0.732, respectively. The ionic radii summarized in Appendix 3... 

The ten most commonly occurring structure types in order of frequency are NaCl, CsCl, CrB, FeB, NiAs, CuAu, cubic ZnS, MnP, hexagonal ZnS, and FeSi respectively. Structures cF8 (NaCl) and cP2 (CsCl) are ordered with respect to underlying simple cubic and body-centred cubic lattices respectively, as is clear from Figs 1.10(a) and 1.11(a). The Na, G sites and Cs, Cl sites are, therefore, six-fold octahedrally coordinated and fourteen-fold rhombic dodecahedrally coordinated, respectively, as indicated by the Jensen symbols 6/6 and 14/14. 

Figure 2.6 Arrangements of ligands about a transition metal ion in (a) tetrahedral and cubic coordinations, and (b) dodecahedral or cuboctahedral coordination. In tetrahedral coordination, the ligands may be regarded as lying at alternate vertices of a cube. In cubic coordination the ligands are situated on all eight vertices.

Figure 2.7 Crystal field splittings of transition metal 3d orbitals in (a) cubic (8-fold) (b) dodecahedral (12-fold) (c) tetrahedral (4-fold) (d) spherical and (e) octahedral (6-fold) coordinations (from Bums, 1985a).

This complex has tricapped trigonal prismatic nine coordination of Pa, whilst MPaFe has dodecahedral 8 coordination and NasPaFg the very rare cubic 8 coordination. Other halide complexes can be made by appropriate methods ... 

Green rhombohedral crystals of iron(II) nitrate hexahy-drate, Fe(N03)2-6H20 (ferrous nitrate), with a melting point 60.5 °C, are obtained from solutions made by dissolving iron in dilute nitric acid. With more concentrated acid, oxidation takes place, and monoclinic pale violet iron(III) nitrate non-ahydrate, Fe(N03 )s -91120 (ferric nitrate), melting point 47 °C, may be crystallized. A colorless hexahydrate can also separate as cubic crystals. Various basic iron(in) nitrates have been described. The eight-coordinate iron(in) anion [Fe(N03)4] has an essentially dodecahedral symmetry with four almost symmetrical bidentate nitrate groups. ... 

The ideal perovskite structure has cubic (Pm 3m) symmetry, which is composed of a three-dimensional framework of comer-sharing BOg octahedra. As shown in Figure 22.1, the A-site cation is surrounded by 12 oxygen ions in the dodecahedral environment. The B-site cations are coordinated by six oxygen ions, and the oxygen ions are coordinated by four A-site cations and two B-site cations. However, there are several stmctural deviations from the ideal cubic stmcture, both for simple and ordered perovskites. 

In recent years. X-ray methods have also shown a variety of configurations for complexes of coordination number eight. It was long thought that all were either antiprismatic or dodecahedral, but some are cubic e.g. the [U(NCS)g] , [U(bipy)4] and [PaFg] ions) and some are hexagonal pyramidal (e.g, the [U02(MeC02)3] ion). 

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