Mercury interatomic distance

Mercury, with interatomic distances 2-999(6) and 3-463(6), appears to have valency 3 . With bond numbers and, respectively, these distances lead to Rx = 1-410 and 1-498, the latter being much too large fort = 4(1 = 1-403), whereas bond numbers and lead to I i = 1-410 and 1-408, in approximate agreement with the value 1-418 for v — 3 . The decrease in valency from cadmium to mercury conforms to a general trend toward smaller metallic valencies with increasing atomic number in a group of elements. 

It is known from a variety of crystal structure determinations that the typical interatomic distances d(Hg-Hg) in cationic mercury clusters are significantly smaller ( 250 pm) than in neutral ( 330 pm) and anionic ones ( 300 pm). In a first approximation this is due to a preferred covalent Waals bonding in the neutral (weak) and a preferred Op bonding (medium) in the anionic forms. 

A new, more accurate electron diffraction study of gaseous mercuric chloride has been reported.159 The interatomic distances (Hg—Cl = 2.25 A, Cl—Cl = 4.48 A) are shorter than previously reported values by 0.02 to 0.09 A. A complete normal-co-ordinate analysis of bis(methylthio)mercury has also been reported.160 The Raman spectra of gaseous mercuric chloride, bromide, and iodide have been reported.161 The bond polarizability derivatives calculated from the data increase in the order Cl < Br < I, suggesting an increased degree of covalence in the mercury-halogen bond with increasing size of the halogen atom. 

Mercury is a soft metal and as such it is expected to form secondary bonds, most readily with sulfur, selenium, and other heavy non-metals. The situation is, however, more complex and secondary interactions with other electronegative atoms have also been observed in the solid state. Interatomic distances longer than the expected van der Waals distances are, moreover, sometimes observed between molecules orientated such that weak interactions lead to particular arrangements in the crystal [69]. The are numerous examples of secondary bonds in organomercury chemistry although most are intramolecular there are several examples of inter-molecular secondary bonds leading to supramolecular self assembly. A review has been published on this subject [70] and many new examples have subsequently been reported. 

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. 

The four-membered ring unit Hg2Cl2 can occur as a part of a ladder structure. Thus, (2-pyridylphenyl)mercury(II) chloride, 15a, is a tetramer, with the skeleton shown in 15b (only atoms directly bonded to mercury are shown for clarity). The mercury-chlorine interatomic distances are in the range 3.184-3.442 A [75]. 

Because sulfur is a soft donor it is expected to participate in Hg - S secondary interactions and this is observed quite frequently. The expected van der Waals interatomic distance between sulfur and mercury is 3.5 A, and intermolecular distances below this value are often observed. 

It is necessary to notice that determining N is the most difficult problem for the XRD study of melts since it depends on a number of assumptions and even conventions, hence the reported values vary widely. For example, in the case of mercury, N derived from the area of the peak corresponding to the interatomic distance, is 14.6, whereas other approaches can give half this value. Gaboon [69] proposed to estimate iV in liquids from the geometrical relations between N and the packing density of atoms,... 

Taking into account the short interatomic 0(1)- - -C(4) distance (2.88 A) and the distortion of the C(3)C(4)C(4a) bond angle to 130.3°, an attractive secondary interaction between the 0(1) and C(4) atoms can be assumed. Apparently, these specific structural features of SPNO are responsible for its rapid conversion to compound P rather than to the open quinoid form upon its irradiation in solution with a mercury lamp.57... 

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