Mercury anionic partial structures

Extended anionic partial structures of mercury occur in some high melting amalgams (MHg2 and related examples) with medium Hg content. Significant ionic bonding contributions between M and Hg in addition to covalent Hg-Hg contributions can be assumed to be responsible for the properties of these solids. 

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

Fig. 2.4-7. Anionic mercury partial structures for (a) y-NaHg (NaTI type), (b) NaHg2 (AIB2 type), (c) a-KHg2 (KHg2 type) and (d) l<5Hg7 (unique structure type).

Fig. 2.4-8. More complex anionic and polyhedral mercury partial structures in amalgams with intermediate compositions (a) Rb5Hg,9 (Cs5Hg,9), (b) Rb2Hg7 and (c) K7Hg3, (Rb7Hg3n).

Hg UPD on Au(lll) electrodes in the presence of bisulfate anions has been studied by Abruna et al. [27] in order to illustrate the effects of the partial charge, retained by the metal, on the interactions between the adsorbed metal and the anion. In order to obtain structural information on the adsorbed species, the authors have carried out grazing incident X-ray diffraction measurements at several potentials. Three ordered structures were observed depending on the applied potentials, which were adjusted from cyclic voltammograms. At the early stages of Hg UPD, when mercury was still partially charged, an ordered mercurous-sulfate bilayer structure was formed at the electrode... 

Compounds of less electropositive metals also show structural effects that can be attributed to partial covalent bonding. CuCl and ZnO have structures with tetrahedral coordination although from radii the (octahedral) rocksalt structure would seem more likely. Partial covalent bonding involves some transfer of electrons back from the anions, into the empty 45 and 4p orbitals on Cu+ and Zn2+. Tetrahedral coordination is the normal bonding geometry when a complete set of 5 and p orbitals is used in this way (see Topics C2 and C6). Mercury forms an... 

Introduction Anions have a strong tendency to adsorb specifically at metal surfaces, for example, to estabKsh a direct bond with the electrode by partial loss of their hydration shell. As a consequence of the contact with the electrode, the ionic character of the anions is markedly reduced, resulting in a higher surface concentration than in case of nonspecific adsorption. This effect was first observed in double-layer studies on mercury [229, 230] and later confirmed and studied in detail on single-crystal solid electrodes [231-234]. Specifically adsorbed anions can form various types of ordered structures, either more open (cf. sulfate on Au(hld) [235, 236]) or close-packed as reported for halides on different solid electrodes [21]. Cyclic current-potential curves often reveal sharp current peaks, indicative of phase transitions within the anionic adlayers and hence of the existence of ordered phases [21, 237]. Thermodynamic data of specific anion adsorption was obtained in surface tension studies (on mercury only [229,238-240]), capacitance measurements [231-233], cyclic voltammetry, and chronocoulometry [234]. As an... 

The mercury cuprates are diflScult to synthesize as pure phases, due to their low stability which originates form the dumbbell coordination of mercury. The difficulty in isolating pure phases increases with the number of copper layers. Moreover, only the barium layered cuprates HgBa2Ca jCu, 02m+2 are obtained, whereas the strontium homologous series could not be synthesized. The introduction of a rare earth into the Hg sites allows the anionic sites at the level of the Hg sites to be partially filled, and consequently the structure can be stabilized (for a review see Raveau et al. 1995a). 

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