The Group 12 Elements Zn, Cd, Hg

The elements Zn, Cd, and Hg follow Cu, Ag, and Au, respectively. Each has a filled ( -l)d shell plus two ns electrons. While Cu, Ag, and Au all give rise to ions or complexes in which one or even two d electrons are lost, that is to compounds in oxidation states II and III, no such compounds have ever been isolated for the Group 12 metals. Thus, while Cu, Ag, and Au are classified as transition elements, Zn, Cd, and Hg are not. For mercury it has been claimed that at -78°C [Hg cyclam](BF4)2 can be oxidized electrochemically in acetonitrile to give the [Hg cyclam]3+ ion, with a half-life of ca. 5s, and there have also been theoretical arguments that HgIV might exist, for example, in HgF4.  

The only deviations from strict divalence are (a) the M2+ ions, of which only Hg + is stable under normal conditions, and (b) compounds containing Hg2+ and Hg2+ ions. 

Outer configuration Ionization enthalpies (kJ mol 1) 3d °4s2 4d105s2 5dw6s2 

The Group 12 elements differ markedly from those in Group 2 in nearly all respects except having II as their only important oxidation state. Thus, while the Zn2+ and Mg2+ ions are very similar in their 6-coordinate radii (0.88 A and 0.86 A, respectively), Zn2+ has a relatively polarizable 3d10 shell whereas the neon core of Mg2+ is very hard. This special combination of softness and a high charge-to-radius ratio appears to be responsible for the unique role played by zinc in biochemistry (see Section 15-17). 

While only a little useful nmr work can be done on compounds of either Zn or Hg, 113Cd has proven to be very practical for nmr studies.3 

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It thus remains to discuss the inclusion of groups 11 and 12. The group 11 elements Cu, Ag, Au, and Rg clearly have a too pronounced involvement of their (n —1) d-orbitals in bonding, even in their lower oxidation states, to be safely considered main-group elements. The group 12 elements Zn, Cd, Hg, and Cn are usually considered to be main-group or post-transition elements. Yet recently quantum-chemical predictions [5] of oxidation-state Hg(-l-IV) in the form of the... 

Whether the group 12 elements (Zn, Cd, Hg) can be calculated like main group metals that is, whether the ECPS may include the (n - l)s and (n -l)p electrons in the core. 

Fig. 39 Sublimation enthalpies of the group 12 elements Zn, Cd, and Hg from the literature and extrapolated sublimation enthalpy of Cn (Z = 112) using the correlation depicted in Fig. 38...

The alkaline-earth metals and group 12 metals (Zn, Cd, Hg) have the right number of electrons to completely All an s band. However, s-p mixing occurs and the resulting combined band structure remains incompletely filled, as shown in Fig. 4.3.3(b). Hence these elements are also good metallic conductors. 

The three rows of block metals are shown in the schematic periodic table at the beginning of the chapter. The term transition elements (metals) is also widely used. However, the group 12 metals (Zn, Cd and Hg) are not always classified as transition metals. The elements in the /-block (see Chapter 25) are sometimes called inner transition elements. Throughout our discussions, we shall use the terms rf-block and /-block metals, so being consistent with the use of the terms s-block and p-block elements in earlier chapters. Three further points should be noted ... 

Many chemistry textbooks include Group 2B (12) elements Zn, Cd, and Hg as transition elements, basing the definition on position within the periodic table rather than on electron configuration. The narrower definition used in this text is the one adopted by the International Union of Pure and Applied Chemists (lUPAC). 

Class 2, with the metals, is roughly speaking, further divided, into main group elements (class 2.2) and transition metals (2.1). Aluminum is missing, from class 2.2, but is quite near the border of the class. This set is also in the other classification of elements of group 12 (IIB) (Zn, Cd, and Hg), which is reasonable from the point of view of electron configuration, as well as elements of group 3 (IIIB) (Sc, Y, La), the lanthanides, and thorium. 

Peterson, K.A. and Puzzarini, C. (2005) Systematically convergent basis sets for transition metals. II. Pseudopotential-based correlation consistent basis sets for the group 11 (Cu, Ag, Au) and 12 (Zn, Cd, Hg) elements. Theoretical Chemistry Accounts, 114, 283-296. 

The effects are observable by a comparison of the metallic radii, the first three ionization energies and the first electron attachment energy of the Group 11 elements as shown in Table 7.3, and the metallic radii and the first two ionization energies of the elements of Group 12 (Zn, Cd and Hg) as given in Table 7.4. 

The elements zinc, cadmium, and mercury, which have two electrons outside filled penultimate d shells, are classed in Group 12. Although the difference between the calcium and zinc subgroups is marked, zinc, and to a lesser extent cadmium, show some resemblance to beryllium or magnesium in their chemistry. We discuss these elements separately (Chapter 15), but note here that zinc, which has the lowest second ionization enthalpy in the Zn, Cd, Hg group, still has a value (1726 kJ mol-1) similar to that of beryllium (1757 kJ mol"1), and its standard potential (-0.76 V) is considerably less negative than that of magnesium. 

In the next sections we will show how our approach works for the more sophisticated cases of group 12 elements where no binding has been found at the HF level. Applying the method of increments to Zn, Cd, and Hg the calculated ground-state properties are shown to be in excellent agreement with experiment. Moreover, due to the possibility to analyze individual... 

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