Mercury high coordination number

Tab. 2.4-1. Summary of alkali metal (M) amalgams containing anionic" mercury clusters, extended mercury partial structures and/or high coordination number polyhedra.

KyHg3i, which is not too far from K2Hg7 in chemical composition, is better characterized by an interpenetrating complex arrangement of K-centered high coordination number polyhedra [d(Hg-Hg) > 300 pm, Figure 2.4-8(c)] than by its mercury partial structure alone [4], thus leading into the essential aspects of Section 2.4.2.5. 

Details of Cs3Hg2o, which is a beautiful new example of an amalgam with high coordination number polyhedra, have been published recently. It can be described simply as being built up by linked octa-capped icosahedra of mercury or as a framework structure related to the clathrate-I type [6]. 

In mercury rich alloys such as M Hg12 (M = K, Rb), a particular situation occurs due to high coordination number (CN) polyhedra (up to CN 22) in their structures. In these compounds the electropositive atoms are located in the centers of these polyhedra and are thus spatially separated from each other. The covalent Hg-Hg interaction, as discussed above, is of minor importance in these amalgams. 

Protonation of a carbonyl oxygen rather than the metal may be encouraged in this case by the high coordination number of vanadium. This would then promote halide attack on the carbonyl carbon to yield an intermediate hydroxyhalocarbene, which reacts further to yield the indicated products. This system represents a potential photoassisted water-gas shift catalyst system since H3V(CO)3(diars) upon photolysis with a mercury vapor lamp yields H2, and in the presence of CO regenerates the starting complex HV(CO)4(diars). The feasibility of coupling these two reactions in the same reaction solution remains to be demonstrated. 

High-temperature modifications generally have lower coordination numbers, just as low-pressure modifications do. Structures of certain metals such as Zn, Cd and Hg do not conform to this pattern. Mercury has a rhombohedral structure in which each atom is six-coordinated following the (8 — N) rule indicating the presence of the covalent... 

The reactivity of mercury salts is a fimction of both the solvent and the counterion in the mercury salt. Mercuric chloride, for example, is unreactive, and mercuric acetate is usually used. When higher reactivity is required, salts of electronegatively substituted carboxylic acids such as mercuric trifiuoroacetate can be used. Mercuric nitrate and mercuric perchlorate are also highly reactive. Soft anions reduce the reactivity of the Hg " son by coordination, which reduces the electrophilicity of the cation. The harder oxygen anions leave the mercuric ion in a more reactive state. Organomercury compounds have a number of valuable synthetic applications, and these will be discussed in Chapter 8 of Part B. 

Metallothioneins are a unique and widely distributed group of proteins. They are characterized by their low molecular weight (—6000), high cysteinyl content, and the ability to bind substantial numbers of metal ions (43). The proteins bind copper and zinc, thereby providing a mobile pool as part of the normal metabolism of these elements, and offer protection from the invasion of inorganic forms of the toxic elements cadmium, lead, and mercury. In addition, other metals, such as iron and cobalt, can be induced to bind. XAS is ideally suited to probe the environment of these different metal atoms (see Fig. 1), and the structural interpretations obtained from an analysis of the EXAFS data obtained in several such studies are summarized in Table 1(44). Thus, in each case, the data are consistent with the primary coordination of the metal deriving from the cysteinyl residues. 

The number of species involved here (MeHg, Me2Hg, Hg and Hg(0)) is small relative to those of some other elements (e.g., arsenic), with only the counterion varying. In the aquatic environment, methylmercury may be present coordinated to chloride, hydroxide, alkanethiol, or various other forms of organic material, either dissolved or in the solid phase. Coordination depends on competition between chemical, biochemical, pH or Eh (oxidation levels), and salinity. Reduction to inorganic mercury or further slow methylation to dimethylmercury may occur (this latter may lead to transport to the atmosphere). Similarly, coordinated methylmercury may enter the sediment layer. This latter may be considered as the main zone for microbial methylation of Hg in view of the high amount of biological activity there. Similar coordination may exist as in the aquatic zone, but with more likelihood of sulfide or alkanethiol complexation. 

---