Coordination number eight

The effect of small valence and large coordination number is further shown by the observation that silicon tetrahedra, which share comers only with aluminum octahedra, share edges with magnesium octahedra (in olivine, chondrodite, humite, clinohumite) and with zirconium polyhedra with coordination number eight (in zircon). 

From Adams (193). In all cases, the radii apply to a coordination number of six (radii for coordination number eight are generally about 0.12 A larger). 

The coordination chemistry of zirconium and hafnium is dominated by oxidation state IV and by higher coordination numbers, especially coordination number eight. Oxidation states and coordination geometries are summarized in Table 1. 

It is improbable that a compound K3H[PbF8] is correctly formulated in this way probably there are HF groups in it and the coordination number is lower. The coordination number eight is, however, certain for the volatile OsF8 (B.P. 47.3°) which compound at the same time most clearly justifies placing the platinum metals in the 8th column. 

Metals are significantly different from non-metallic solids both in structure and physical properties. They usually crystallize with dense structures (see Section 1.5) with large coordination numbers (eight to twelve). Typically they consist of positively charged ions in a sea of electrons, in which the outer electrons of the metal atoms are free to move, hence giving high electrical and thermal conductivity. The properties of metallic crystals cannot be accounted for in terms of localized bonds or electron clouds, and are best described by a delocalized electron approach. 

The tetrahydrate of the normal sulfate Zr(S04)2 41120 is obtained as a precipitate upon addition of sulfuric acid to an aqueous solution of zirconium(IV). Its structure consists of sheets of coordination number eight antiprisms in which zirconium is bonded by the oxygen atoms of the four water molecules and the oxygen atoms of four sulfate groups. The... 

Coordination number eight. The common coordination polyhedra for coordination number eight " are the bisdisphenoid or dodecahedron corresponding to hybridization and the square antiprism corresponding to... 

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). 

Figure 11. The crystal structure of Pb°-EDTA contains both a monomeric form and a dimeric form of the compound (170, 203). The monomeric form (a) has coordination number seven with oxygen from a coordinated water providing the seventh donor ligand. The dimeric form (h) has a coordination number eight with the eighth ligand provided by an oxygen from a carboxylate group from an adjoining molecule. Coordinates downloaded from the CSD.

The crystal structure of an ammonium cerium condensed phosphate, whose composition corresponds either to NH4Ce(P03)5 (Ce(IV)) or to NH4HCe(P03)5, has been determined by Palkina et al. (1981e). Complex infinite PO4 tetrahedra chains are held together by the cations and cerium has the coordination number eight. 

The involvement of d orbitals leading particularly to sp d" hybrids (1 < n < 5) for coordination numbers five through nine (Table 2). This typically occurs in the coordination compounds of the d-block transition metals. The polyhedra for coordination numbers five and six can be generated by addition of one or two d orbitals, respectively, to a four-orbital spherical sp manifold. Similarly the polyhedra for coordination numbers eight and seven can be generated by subtraction of one or two d orbitals, respectively, from a nine-orbital spherical sp d manifold. 

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