Group 12 Metals - Zinc, Cadmium, Mercury

In this triad only mercury seems to form definite supramolecular assemblies by secondary bonding interactions. The supramolecular chemistry of zinc and cadmium (based upon dative bond self-assembly) has been discussed in Sections 3.1.1 and 3.1.2. 

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Owing to some similarities among their properties and alloying behaviour, beryllium and magnesium, metals of the 2nd group, will be presented in this chapter together with the last transition metals zinc, cadmium and mercury (see a few more remarks in 5.4). 

In this section, we will discuss organometallic derivatives of zinc, cadmium, mercury, and indium. The group IIB and IIIB metals have the d10 electronic configuration in the 2+ and 3+ oxidation states, respectively. Because of the filled d level, the 2+ or 3+ oxidation states are quite stable, and reactions of the organometallics usually do not involve changes in oxidation level. This property makes the reactivity patterns of these organometallics more similar to those of derivatives of the group IA and IIA metals than to those of derivatives of transition metals with vacancies in the d levels. The IIB metals, however, are... 

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. 

Dialkyl derivatives of elements of the Second sub-Group, namely, zinc, cadmium, and mercury, contain an almost completely covalent metal-carbon bond. These compounds are normal, unassociated liquids with low boiling points e.g., the dimethyl derivative of zinc boils at 46°, that of cadmium at 105.5°, and that of mercury at 92°. 

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. 

Tetracarbonylcobaltate(l —) forms ionic complexes with group 1 elements. However, compounds of the type M[Co(CO)4] , where 2 and M = zinc, cadmium, mercury, indium, etc., are covalent, possessing M —Co bonds in which the main group metal has normal coordination number. These compounds are monomeric in the solid state. Ag[Co(CO)4] and Cu[Co(CO)4] are tetrameric clusters in which the metal atoms form planar, eight-membered rings. Each of the distorted [Co(CO)4] tetrahedrons is bonded to two atoms of silver or copper. 

Two distinct classes of promoters have been identified for the reaction simple iodide complexes of zinc, cadmium, mercury, indium and gallium, and carbonyl complexes of tungsten, rhenium, ruthenium and osmium. The promoters exhibit a unique synergy with iodide salts, such as hthium iodide, under low water conditions. Both main group and transition metal salts can influence the equilibria of the iodide species involved. A rate maximum exists under low water conditions and optimization of the process parameters gives acetic acid with a selectivity in excess of 99% based upon methanol. IR spectroscopic studies have shown that the salts abstract iodide from the ionic methyl iridium species and that in the resulting neutral species the migration is 800 times faster [127]. 

Unlike cadmium and mercury and, in fact, all metals of Group II, zinc dissolves readily in alkalis forming zincates. in which the zinc atom is contained in a complex hydroxo-anion, for example ... 

These elements formed Group IIB of Mendeleef s original periodic table. As we have seen in Chapter 13, zinc does not show very marked transition-metaf characteristics. The other two elements in this group, cadmium and mercury, lie at the ends of the second and third transition series (Y-Cd, La-Hg) and, although they resemble zinc in some respects in showing a predominantly - - 2 oxidation state, they also show rather more transition-metal characteristics. Additionally, mercury has characteristics, some of which relate it quite closely to its immediate predecessors in the third transition series, platinum and gold, and some of which are decidedly peculiar to mercury. 

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... 

Metallothioneins are a group of small proteins (about 6.5 kDa), found in the cytosol of cells, particularly of liver, kidney, and intestine. They have a high content of cysteine and can bind copper, zinc, cadmium, and mercury. The SH groups of cysteine are involved in binding the metals. Acute intake (eg, by injection) of copper and of certain other metals increases the amount (induction) of these proteins in tissues, as does administration of certain hormones or cytokines. These proteins may function to store the above metals in a nontoxic form and are involved in their overall metaboHsm in the body. Sequestration of copper also diminishes the amount of this metal available to generate free radicals. 

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