Cubic, eight coordination

Fig. 7.14. Part of body-centred cubic (bcc) metallic lattice, showing its relationship to the CsCl ionic lattice with cubic eight-coordination of each atom.

Mercury(II) fluoride, Hgp2, is obtained by fluorination of metallic mercury or HgCl2 it is a white crystalline solid, which darkens in colour on exposure to air due to hydrolysis. Cubic eight coordination of mercury atoms is found in Hgp2, which possesses the fluorite structure it is one of the few mercury(II) compounds in which the bonding is beUeved to be mainly ionic. 

Fig. 12-38 Energy level diagrams for eight-coordination (a) cubic 1C ) (b) square antiprismattc (Du) (c) dodecahedral (D J. (From Burden. J. K- Hoffmann. R.. Fay. R C. Inorg. Chem. 1978, 17. 2253 2566. Reproduced with permission.]...

Transition metal difluorides are known mainly for first transition series elements, with palladium and silver difluorides from the second series, and no examples from the third. All these compounds have either the rutile structure, or, for chromium, copper, and silver, a distorted variant, which can be correlated with a Jahn-Teller distortion of the octahedral coordination of the ions. This rutile structure type is associated with smaller cations and, for comparison, although zinc difluoride has the same rutile structure, cadmium and mercury difluorides have the cubic fluorite structure with eight coordination of the cations (12). 

Finally, eight-coordinate H atoms are theoretically possible since a cubic cavity is inherently much larger than an octahedral one. Although several cubic metal clusters are known258), none of them is, as far as we know, suspected to contain interstitial H atoms. 

For the +3 oxidation state, where there is much resemblance to the lanthanides, octahedral coordination is often found, but so also is coordination number 9. Eight-coordination is very common for the +4 oxidation state. In the +5 oxidation state the AnFg" ions afford rare examples of discrete cubic coordination. In oxidation state +6, the hexafluorides are strictly octahedral. 

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. ... 

Fig, 12.38 Energy level diagrams for eight-coordination (a) cubic (0 ,) (b) square antiprismatic ( Ojj) (c) dodecahedral... 

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