The structural chemistry of mercury

Mercury forms two series of compounds, mercurous and mercuric, but the former are not compounds of monovalent Hg in the sense that cuprous compounds, for example, are derivatives of monovalent Cu. A number of elements form compounds in which there are metal-metal bonds but mercury is unique in forming, in addition to Hg and the normal mercuric compounds, a series of compounds based on the grouping (-Hg-Hg-). (Evidence for the formation of the Cdf ion in molten Cd2(AlCl4)2 at 250°C is limited to the observation of one Raman line. ) 

The latest data on mercurous halides (Table 26.1) do not confirm the earlier conclusion that the length of the Hg-Hg bond increases with decreasing electronegativity of the halogen. Organic mercurous compounds containing the system -C-Hg-Hg-C- are not known. Mercurous oxide is apparently a mixture of HgO and Hg.  

In HgCN(N03)( =) linear chains -Hg-CN-Hg-CN- are stacked with their axes parallel, and the coordination group around Hg(ii) is completed by three pairs of 0 atoms of NO J ions. Here the metal atom forms 2 short collinear bonds (2-06 A) to C or N (which were not distinguishable) and 6 Hg-0 bonds (2-73 A) in the equatorial plane (compare uranyl compounds, Chapter 28). 

Molecules (other than the dihalides, for which see later) which have been shown to be linear include H3C-Hg-CH3, F3C-Hg-Cp3, mercaptides RS-Hg-SR, C2H5S—Hg—Cl, Cl—Hg—SCN, and Br—Hg—SCN. From the bond lengths HjC-Hg-Cl and H3C-Hg-Br,( ) namely, Hg-C, 2-07 A, Hg-Cl, 2-28 A, and Hg-Br, 2-41 A, the radius of 2-covalent Hg has been deduced as 1-30 A. (The value 1-48 A, on the Pauling scale, has been suggested for tetrahedral Hg(ii).) 

In several crystalline compounds X-Hg-X or X-Hg-Y in which one or both of the bonds are to C or S, the bonds are not collinear. A combined X-ray and neutron diffraction study of Hg(CN)2 shows that there are two additional weak Hg-N bonds in a plane perpendicular to that of the two Hg—C bonds, (a), the angle between the latter being 171( 2)°. Smaller interbond angles are found in the 

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A. F. Wells, Structural Inorganic Chemistry, 5th edn., Oxford University Press, Oxford, 1984 the structural chemistry of mercury is reviewed on pp. 1156-69. 

Although formally a transition metal the structural chemistry of mercury as a group 12 element in amalgams shows typical features close to those of main group 13 and 14 metals. 

The structural chemistry of mercury] II) is dominated by its tendency to maintain the linear coordination which results from sp hybridization, and to use the non-hybridized vacant orbitals in further bonding. Secondary bonds to mercury will distort this linearity, but not to a great extent. Only stronger bonds (of the dative type) will change the hybridization from sp to sp (in tetrahedral geometries). Distinction between dative and secondary bonds is very difficult for mercury, because the whole range of interatomic distances, between the sum of covalent radii and the expected van der Waals distances, is covered by the known structures. 

As mentioned at the end of Section 6.9.3, reviews of the crystal chemistry of mercury minerals with lower oxidation states and with oxo-centered building blocks have been published72,73 and the structural role of Hg22+ and Hg34+ groups discussed.407... 

The structural chemistry of some metal dithiocarbamates, i.e. systematics, coordination modes, crystal packing, and supramolecular self-assembly patterns of nickel, zinc, cadmium, mercury,363 organotin,364 and tellurium,365 366 complexes has been thoroughly analyzed and discussed in detail. Supramolecular self-assembly frequently occurs in non-transition heavier soft metal dithiocarbamates. Thus, lead(II),367 bismuth(III)368 zinc,369 cadmium,370 and (organo)mercury371 dithiocarbamates are associated through M- S secondary bonds, to form either dimeric supermolecules or chain-like supramolecular arrays. The arsenic(III)372 and antimony(III)373 dithiocarbamates are... 

There are a number of books and articles on general aspects of the coordination compounds of mercury annual surveys are published in Coordination Chemistry Reviews5 and the Annual Reports on the Progress of Chemistry, Section A (Inorganic Chemistry—Mercury).6 McAuliffe s book The Chemistry of Mercury covers the literature up to May 1975.7 The coordination chemistry of mercury(II) halides has been summarized by Dean, covering papers up to 1977.8 A review of dimercury(I) coordination compounds was published by Brodersen in 1981,9 and in the same year Grdeni6 reviewed bonding in the crystal structures of mercury compounds.10... 

The adoption of these different structural types and the observation of polymorpism suggests that there is httle energy difference between them. Cox and Tiekink (1867) studied the structural chemistry of this class of compounds in some detail. They suggest that it may be the need to maximize intermolecular interactions that is important in the structural type adopted. For example, when there is little or no steric barrier, then secondary mercury-sulfiir interactions will give rise to dimeric or polymeric units. 

All four mercury (n) halides are known, and their properties are hsted in Table 5. All can be prepared by the direct combination of the appropriate halogen with mercury metal. Except for the fluoride, which has bonds that are predominantly ionic in character, they are characterized by relatively low-melting and boiling points and display surprising solubility in organic solvents such as acetone. Their coordination chemistry has been reviewed. A summary of the structural features of the crystalline mercury(II) halides is given in Table 6. 

Among the recent reports on metal xanthate chemistry the following are mentioned, dealing with the structural diversity of nickel(II),213 zinc(II),214 mercury(II), 15 and tellurium(II) bis (xanthate) complexes,216 based upon different coordination patterns and supramolecular self-assembly. 

Relativity adds a new dimension to quantum chemistry, which is the choice of the Hamiltonian operator. While the Hamiltonian of a molecule is exactly known in nonrelativistic quantum mechanics (if one focuses on the dominating electrostatic monopole interactions to be considered as being transmitted instantaneously), this is no longer the case for the relativistic formulation. Numerical results obtained by many researchers over the past decades have shown how Hamiltonians which capture most of the (numerical) effect of relativity on physical observables can be derived. Relativistic quantum chemistry therefore comes in various flavors, which are more or less well rooted in fundamental physical theory and whose relation to one another will be described in detail in this book. The new dimension of relativistic Hamiltonians makes the presentation of the relativistic many-electron theory very complicated, and the degree of complexity is far greater than for nonrelativistic quantum chemistry. However, the relativistic theory provides the consistent approach toward the description of nature molecular structures containing heavy atoms can only be treated correctly within a relativistic framework. Prominent examples known to everyone are the color of gold and the liquid state of mercury at room temperature. Moreover, it must be understood that relativistic quantum chemistry provides universal theoretical means that are applicable to any element from the periodic table or to any molecule — not only to heavy-element compounds. 

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