Group 12 cadmium and mercury

Cadmium is chemically very like Zn, and any differences are attributable to the larger sizes of the Cd atom and Cd + ion. Among the group 12 metals, Hg is distinct. It does bear some resemblance to Cd, but in many respects is very hke Au and Tl. It has been suggested that the relative inertness of Hg towards oxidation is a manifestation of the thermod5mamic 6s inert pair effect (see Box 12.3). 

Cadmium is a reactive metal and dissolves in nonoxidizing and oxidizing acids, but unhke Zn, it does not dissolve in aqueous alkali. In moist air, Cd slowly oxidizes, and when heated in air, it forms CdO. When heated, Cd reacts with the halogens and sulfur. 

Mercury dissolves many metals to give amalgams (see Box 22.3) in the Na—Hg system, for example, Na3Hg2, NaHg and NaHg2 have been characterized. Solid Na3Hg2 contains square [Hg4] units, the structure and stability of which have been rationalized in terms of aromatic character. 

Ionization energies decrease from Zn to Cd but increase from Cd to Hg (Table 22.4). Whatever the origin of the high ionization energies for Hg, it is clear that they far outweigh the small change in and make Hg a noble 

Since much of the chemistries of Cd and Hg are distinct, we shall deal with the two metals separately. In making this decision, we are effectively saying that the consequences of the lanthanoid contraction are of minor significance for the heavier metals of the last group of the J-block. 

For cadmium, the - -2 oxidation state is of most importance, but compounds of Hg(I) and Hg(II) are both well known. Mercury is unique among the group 12 metals in forming a stable [M2] ion. Although there is evidence for 

Their solid state structures suffer from disorder problems, but total neutron diffraction has been used to give the accurate structural data shown in diagram 22.88. The fact that the Au—C/N distance is smaller than the Ag—C/N bond length is attributed to relativistic effects. The same phenomenon is observed in the discrete, linear [Au(CN)2] and [Ag(CN)2]  

In contrast to the bridging mode of [CN] in [Ag(CN)2] , the complex [Ag(CN)(NH3)] crystallizes with discrete linear molecules. At room temperature, [Ag(CN)(NH3)] rapidly loses NH3, decomposing to AgCN. 

Dissolution of AgX in aqueous halide solutions produces [AgX2] and [AgX3] . In aqueous solutions, the ions [AuX2] (X = Cl, Br, I) are unstable with respect to disproportionation but can be stabilized by adding excess X (eq. 22.158). 

Routes to Au(I) complexes often involve reduction of Au(III) as illustrated by the formation of R3PAUQ and R2SAUCI species (eqs. 22.159 and 22.160). 

Relativistic effects (see Box 13.3) have a profound influence on the ability of gold to exist in the 1 oxidatirm state. The 

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The elements in Groups 3 through 11 are called the transition metals because they represent a transition from the highly reactive metals of the s block to the much less reactive metals of Group 12 and the p block (Fig. 16.1). Note that the transition metals do not extend all the way across the d block the Group 12 elements (zinc, cadmium, and mercury) are not normally considered to be transition elements. Because their d-orbitals are full, the Group 12 elements have properties that are more like those of main-group metals than those of transition metals. Just after... 

This volume is concerned with fundamental developments in the coordination chemistry of the elements of Groups 9-12 since 1982. The individual chapters cover the coordination chemistry of cobalt, iridium, nickel, palladium, platinum, copper, silver and gold, zinc and cadmium, and mercury. Unfortunately, because of factors beyond the Editors control, the manuscript for the proposed chapter on rhodium was not available in time for publication. 

Of the three group 12 metals, only mercury has a well-developed chemistry with the metal in the +1 oxidation state, while cadmium and zinc, respectively, exhibit this oxidation state either exceedingly seldom or not at all. This increase in the stability of the lower oxidation state as one descends the group is characteristic of main group metals, but not of transition metals. 

Of the Group 12 elements, zinc, cadmium and mercury, only Hg has a water-stable -I-1 state, and all three elements have + 2 states that are water-stable. Their reduction potentials are summarized in the Latimer diagram ... 

Group 12 In order of increasing atomic number, ihese are zinc, cadmium, and mercury, The eleiiieuls of this group are characterized by the presence ol two electrons in an outer shell Although mercury also has a valence of I -l, all of the elements in this, group have a 7+ valence in common. 

The (/-block elements tend to lose their valence s-electrons when they form compounds. Most of them can also lose a variable number of d-electrons and show variable valence. The only elements of the block that do not use their (/-electrons in compound formation are the members of Group 12 (zinc, cadmium, and mercury). The ability to exist in different oxidation states is responsible for many of the special properties of these elements and plays a role in the action of many vital biomolecules (Box 16.1). 

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. 

The Group 12 metals, zinc, cadmium, and mercury, have valence electron configurations of n - )d Zinc and, to a lesser extent, cadmium resemble beryllium or magnesium in their chemistry. 

Group 12 zinc, cadmium and mercury (G4) Complexes structure and isomerism (H6)... 

In fact, the classification of chemical elements is valuable only in so far as it illustrates chemical behaviour, and it is conventional to use the term transition elements in a mote restricted sense. The elements in the irmer transition series from cerium (58) to lutetium (71) are called the lanthanoids those in the series from thorium (90) to lawrencium (103) are the actl-noids. These two series together make up the /block in the periodic table. It is also common to include scandium, yttrium, and lanthanum with the lanthanoids (because of chemical similarity) and to include actinium with the actinoids. Of the remaining transition elements, it is usual to speak of three main transition series from titanium to copper from zirconium to silver and from hafnium to gold. All these elements have similar chemical properties that result from the presence of unfilled d-orbltals in the element or (in the case of copper, silver, and gold) in the ions. The elements from 104 to 109 and the undiscovered elements 110 and 111 make up a fourth transition series. The elements zinc, cadmium, and mercury have filled d-orbltals both in the elements and in compounds, and are usually regarded as nontransition elements forming group 12 of the periodic table. 

The equilibrium structures of the dihalides of the Group 12 metals zinc, cadmium and mercury all appear to be linear or nearly so in no case has an angular structure been proven. The difference between the dihalides of Ca and Zn, between Sr and Cd and between Ba and Hg suggests an explanation of the bent equilibrium stmctures in terms of the vacant valence shell d orbitals of the Group 2 metals. 

Metal-sulfur bonds are comparable to those in related disulfur ligands, and generally decrease across the periodic table and increase on going down a group, in line with the expected decrease and increase, respectively, of the metal ion size. Values for the groups 11 (IB) and 12 (11B) elements seem to be unexpectedly high, however, it is here where anisobidentate binding becomes common (especially for cadmium and mercury) and low oxidation states dominate (silver). 

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