Whither High-Temperature Coordination Chemistry

A summary of the symmetries observed for the Group VIII metal ions in melts appears in Table XLVII. 

TABLE XLVII. Symmetries Observed for Group VIII Metal Ions in Melts 

O Wide range of symmetries and equilibria between species 

Tetrahedral symmetry certainly occurs for Fe- and Co + but a weak ligand such as F can lead to octahedral Fe-, and dodecahedral Co + appears to be quite common. The coordination number four is a very close description for Pd + and Pt with central ion size eliminating tetrahedral in favor of square-planar coordination.Finally, Ni- spans the octahedral-tetrahedral equilibrium category and that where the system distorts rather than adopt six-coordination. 

To date all the symmetries adopted by transition metal ions in melts are very simply related to the octahedron and tetrahedron. The T and O centers discussed in detail in Section 4.4 have not had specific geometries assigned to them but they are probably distorted versions of and Oh, respectively. In the same way that the dodecahedral symmetry can arise from double of flattened tetrahedra (Section 4.3 and Ref. 196), so a T Oh transformation through a trigonal pyramid (Cgy) and a trigonal bipyramid ( 3 ) is easily visualized (Fig. 48). One might then expect O-center spectra to resemble those of Ni(II) in D h symmetry and indeed a considerable likeness is apparent between the spectrum of a known complex (Fig. 49D),(27d that of the proposed NiCla (Fig. 34), and the O-center spectra depicted in Fig. 33. Unfortunately, no trigonal pyramidal complexes of Ni(II) appear to be known. 

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