Cd and Hg

The subvalence shell spectra of Zn, Cd and Hg form a group with very similar characteristics. They were first investigated by Beutler [341]. Later researchers pointed out the need for a special coupling scheme (called JCK or pair coupling [342]) and investigated them at much higher resolution. The experiments have been repeated using synchrotron radiation [343]. 

These spectra have already been used in section 2.12 as examples of the extended alkali model. They correspond to the excitation scheme d10 2 1So — d9 2np,nf(J = 1), where 2 are the valence electrons. Double excitations have also been investigated, especially in Zn [344] and are very significantly enhanced as they approach an inner-shell excited transition. This shows that final state mixing is the dominant mechanism for double excitation. 

In short, we can say that these atoms exhibit fairly regular subvalence shell spectra, with properties which are well accounted for within the independent particle basis as a first approximation, and double excitations which do not intrude too heavily in the main inner-shell spectrum. 

In addition to this important effect, the inner-shell excitations from the 6s and 5d subshells also overlap in energy with each other, giving rise to prominent interchannel coupling and examples of the q reversal effect which will be discussed in chapter 8. The 5d spectrum of Tl is thus unexpectedly rich and interesting. 

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In most cases, the activator impurity must be incorporated during crystal growth. An appropriate amount of impurity element is dissolved in the molten Ge and, as crystal growth proceeds, enters the crystal at a concentration that depends on the magnitude of the distribution coefficient. For volatile impurities, eg, Zn, Cd, and Hg, special precautions must be taken to maintain a constant impurity concentration in the melt. Growth occurs either in a sealed tube to prevent escape of the impurity vapor or in a flow system in which loss caused by vaporization from the melt is replenished from an upstream reservoir. 

Group IIB ineludes Zn, Cd and Hg. Zine has some resemblanee to magnesium but the other metals in the group have little in eommon. At room temperature mereury is unaffeeted by air, water or non-oxidizing agents whereas zine is more reaetive, albeit tempered by a proteetive hydroxide film, a property utilized in galvanizing. 

Coordination numbers in this triad are again rarely higher than 6, but the univalent metals provide examples of the coordination number 2 which tends to be uncommon in transition metals proper (i.e. excluding Zn, Cd and Hg). 

It is a remarkable contrast that, whereas Zn is biologically one of the most important metals and is apparently necessary to all forms of life,( Cd and Hg have no known beneficial biological role and are amongst the most toxic of elements. 

With only s- and p-functions present, the two-centre two-electron integrals can be modelled by multipoles up to order 2 (quadrupoles), however, with d-functions present multipoles up to order 4 must be included. In MNDO/d all multipoles beyond order 2 are neglected. The resulting MNDO/d method typically employs 15 parameters per atom, and it currently contains parameters for the following elements (beyond those already present in MNDO) Na, Mg, Al, Si, P, S, Cl, Br, 1, Zn, Cd and Hg. 

The above results for ZnS, CdS, and HgS indicate that an additional electron in a transition-metal atom increases its radius by 0.03 A. 

Because transition-metal carbonyls react with Zn, Cd and Hg to form compounds containing metal-mctal bonds (see 8.3.3.4), the derivatives can be prepared by combining reactants under carbonyl-forming conditions ... 

The other elements in Group 12, Cd and Hg, also form dimethyl compounds. Dimethylmercury, which can be synthesized by bacteria from industrial wastes, is fatal in very small quantities. 

Metallothioneins are a group of non-enzymatic, low-molecular mass (6-7 kDa) metal-binding proteins. They play an important role in the detoxification of a number (Zn, Cu, Cd, and Hg) of trace metals (Chassaigne and Lobinski 1998). 

Interesting compounds are [Cr(R4rinfrared spectra indicate that these complexes are not Cr(V) dithiocarbamato complexes but rather Cr(III) compounds with coordinated thiuram disulfide. As will be shown, thiuram disulfide can oxidise Cu, Ag and Au to M(II) and M(III) dithiocarbamato complexes. The Cr(III)-thiuram disulfide combination seems to be stable, just like the thiuram disulfide combination with Zn, Cd, and Hg. 

Higher Oxidation States of Cd and Hg Quantum-chemical Studies 1257... 

Experimental methods used for studies of Cd and Hg complexes in solution and in the solid state are reviewed briefly, with examples for the application of the method under discussion in recent work. In a separate section quantum-chemical studies, including consideration of relativistic effects, on existing and not-yet-existing species with Cd and/or Hg, are also surveyed. 

Charge distributions and bonding in compounds of Cd and Hg in the solid and gaseous states can be studied by the well-established X-ray photoelectron spectrometry (XPS) and ultraviolet photoelectron spectrometry (UPS), respectively. With XPS, inner-shell electrons are removed which are indirectly influenced by the bonding, i.e., distribution of the valence electrons. UPS sees this electron distribution directly, since it measures the residual kinetic energies of electrons removed from the valence shells of the atoms, or, better, from the outer occupied orbitals of the molecules. The most detailed information accessible by UPS is obtained on gases, and it is thus applied here to volatile compounds, i.e., to the halides mainly of Hg and to organometallic compounds. 

Investigations of cyanides and cyano complexes of Cd and Hg have augmented tremendously since about 1990, after detection of inclusion compounds of Cd(CN)2. A thorough review on transition-metal cyanides especially emphasizes the chemistry of inclusion compounds of both the Hofmann type (frameworks dominated by planar Ni(CN)4 building blocks) and the cyanocad-mate type (frameworks with tetrahedral Cd(CN)4 units).87 The structures of these inclusion compounds, but also of cyanides themselves, often topologically resemble the structures of minerals this aspect ( mineralomimetic chemistry ) is dealt with in a simultaneous survey.88 A more generic review of framework structures, with a section on cyanide inclusion compounds, is also to be mentioned.90... 

Homo- and heteroleptic complexes of Cd alone and of Cd and Hg with the ligand dicyanamide (dca) N(CN)2-, homologous to cyanamide NCN2-, have been studied in various solvents (formation constants of the complexes [M(dca) ](" 2> (M = Cd, Hg l < n < 4)), with the result that the complexes of Hg are more stable than those of Cd. Otherwise, obviously no studies on the isolated compounds M(dca)2 or on homoleptic complexes derived therefrom have been published. 

Simple amides of this type are the bis(trimethylsilyl)amides M[N(SiMe3)2]2 (M = Cd and Hg) the essential thermodynamic data of which have been determined in calorimetric measurements of the heats of hydrolysis in dilute H2S04.146 Evaluation of the measured data yielded the standard enthalpies of formation AH° = —854(21)kJmoU1 and —834(9)kJmol-1 for M =Cd and Hg, respectively. Using subsidiary data, the average thermochemical bond energies E—(Cd—N) 144 and E(Hg—N) 108 kJ mol-1 were also obtained, i.e., the Cd—N bonds are considerably stronger than the Hg—N bonds. 

Both trimethylsilylamides M[N(SiMe3)2]2 (M = Cd and Hg) have been studied in the gas phase by electron diffraction. The diffraction data are best interpreted assuming monomers with linear N—M—N cores and two planar M—NSi2 groups in staggered conformation.147,148... 

Various substituted sulfonylurea derivatives, partly in use as diabetes drugs, have been coordinated to Cd and Hg. Besides the IR and [H NMR spectra of several species, the structures of one each of the Cd and Hg compounds have been solved.155 In the Cd complex three ligands act as... 

A survey of transition-metal polyazacycloalkane complexes in general, with data also for Cd and Hg species in particular, has been published.180 Structural, 3H and 13C NMR, UV/vis, and conductivity data of several transition-metal complexes, including Cd and Hg complexes, with derivatives of the 16-membered ligand l,9-dithia-5,13-diazacyclohexadecane have been compiled and compared. In particular, conversion between configurational isomers and exchange processes in solution have been discussed.181... 

Cd (and Hg) complexes with bis- and tris(l-pyrazolyl)methanes and -ethanes, as well as with bis(l-triazolyl)methane, have been prepared and studied by conductance measurements (H20, dmso, and dimethylformamide (dmf) solutions), IR, far-IR, H, and 13C NMR spectrometries.203 The Cd halides form complexes of the type [CdLX2] complexes of the types [CdL2](C104)2 and [CdL3](BF4)2 are obtained from the respective Cd salts. 

In many closely related compounds of Cd and Hg with all-oxygen or oxygen dominated coordination spheres, the metals adopt different coordination numbers and geometries—even independent metals in the same structure. As a rule, in the rare isostructural Cd and Hg compounds, Hg possesses the more distorted coordination. 

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