Dodecahedral coordination

Mn(N03)4] , [Fe(N03)4] and [Sn(N03)4], which feature dodecahedral coordination about the metal [Ce(N03)5] in which the 5 bidentate nitrate groups define a trigonal bipyramid leading to tenfold coordination of cerium (Fig. 11.17b) [Ce(N03)6] and [Th(N03)6] , which feature nearly regular icosahedral (p. 141) coordination of the metal by 12 O atoms and many lanthanide and uranyl [U02] complexes. It seems, therefore, that the size of the metal centre is not necessarily a dominant factor. 

The dodecahedral coordination is produced by two interpenetrating tetrahe-dra (slightly distorted) of chlorine and arsenic atoms. 

In class I compounds (or complexes) the two sites are very different from each other and the valences are strongly localized. The properties of the complex are the sum of the properties of the constituting ions. The optical MMCT transitions are at high energy. The compounds are insulators. Here are some examples [60, 97]. In GaCl2, or Ga(I)[Ga(III)Cl4] there are dodecahedrally coordinated Ga(I) ions with Ga-Cl distances of 3.2-3.3 A and tetrahedrally coordinated Ga(III) ions with Ga-Cl distance 2.2 A. In [Co(III)(NH3)6]2- Co(II)Cl4)3 there are low-spin, octahedrally coordinated Co(III) ions and high-spin, tetrahedrally coordinated Co(II) ions. For our purpose this class is not the most interesting one. 

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. 

The reaction between [MoCl4(NCPr)2] and dithiocarboxylic acids is a general route to the preparation of eight-coordinate [Mo(S2CR)4] complexes.168 The crystal structure of [Mo(S2CPh)4] reveals these compounds to be isostructural with the dithiocarbamates, with a dodecahedral coordination around the molybdenum and average Mo—S distances of 2.475(1) and 2.543(1) A to the two different sulfur sites.170 Cyclic voltammetry has shown that in [Mo(acda)4] (Hacda = 2-aminocyclopent-l-ene-l-dithiocarboxylic acid) the Mo can be reversibly oxidized and reversibly reduced in one-electron processes.171... 

The crystal structure of Te(Et2Dtc)4 shows a distorted dodecahedral coordination around the eight-coordinate Te(IV) ion. The lone pair (ns2) is... 

The structure of the V(PhDta)4 complex (50) is very similar to that of the corresponding V(BzDta)4 (45) complex, and a dodecahedral coordination geometry with chelation along the m edges of the dodecahedron was reported. 

In the case of eight-coordination, the cube is fairly rare the compounds Na3[MFg] (M = Pa, U, Np) constitute the best-established examples. As for the trigonal prism versus the octahedron, interligand repulsion can be reduced by distortion of the cube to yield the square antiprism or dodecahedron (this is best demonstrated by playing with models). These two are about equally common. For example, in the case of Mo(CN)g-both square antiprismatic and dodecahedral coordination are found in the solid state in solution, the ion is stereochemically nonrigid (or fluxional) and appears to convert rapidly from one geometry to the other. 

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... 

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. 

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... 

Tin(IV) nitrate is obtained as a colorless volatile solid by interaction of N2Os and SnCh it contains bidentate N03 groups giving dodecahedral coordination. The compound reacts with organic matter. 

Chromate(V) was mentioned earlier. Treatment of basic [Cr04] solutions with 30% H2O2 yields the moderately stable peroxide ion [Cr(02)4]. The crystal structure of the representative K3[Cr(02)4] has been determined the Cr -containing anion features side-on bound peroxide ligands (O, 149 pm) and a distorted dodecahedral coordination geometry. The red brown salt is paramagnetic with /leff = 2.94/iB. 

The most common coordination numbers shown by RE + /3-diketonate complexes is eight, where the two most frequent chemical formulae are [RE(/3-diketonate)4] and [RE(/3-diketonate)3(unidentate)2] corresponding to dodecahedron and square antiprism polyhedra, respectively. It is noted that the number of crystal structures presented for the tetrakis complexes is very low compared with the tris complexes. The majority of the tetrakis complexes have square antiprism polyhedron structure [Ce(acac)4]. On the other hand, a number of rare earth /3-diketonate complexes has the dodecahedral coordination polyhedron. 

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