Compounds of Zinc and Cadmium

The zinc ion, Zn(H20)4 , is a colorless ion formed by solution of zinc in acid. It is poisonous to bacteria, and is used as a disinfectant. It is one of the metal ions required in small amounts by human beings (as well as animals) for good health. The normal intake by an adult, obtained from meat and, in smaller amounts, from fruits and vegetables, is 10 to 15 mg per day. The zinc ion is a coenzyme for a number of enzymes, including human carbonic anhydrase (in erythrocytes) and human alcohol dehydrogenase (in the liver). 

Zinc ion forms tetraligated complexes readily, such as Zn(NH3)4 , Zn(CN)4, and Zn(OH)4. The white precipitate of zinc hydroxide, Zn(OH)2, that forms when ammonium hydroxide is added to a solution containing zinc ion dissolves in excess ammonium hydroxide, forming the zinc ammonia complex. The zinc hydroxide complex, Zn(OH)4, which is called zincate ion, is similarly formed on solution of zinc hydroxide in an excess of strong base zinc hydroxide is amphiprotic. 

Zinc sulfate, ZnS04-7H20, is used as a disinfectant and in dyeing calico, and in making litliopone, which is a mixture of barium sulfate and zinc sulfide used as a white pigment in paints  

Zinc oxide, ZnO, is a white powder (yellow when hot) made by burning zinc vapor or by roasting zinc ores. It is used as a pigment (zinc white), as a filler in automobile tires, adhesive tape, and other articles, and as an antiseptic (zinc oxide ointment). 

Zinc sulfide, ZnS, is the only white sulfide among the sulfides of the common transition metals. 

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Formation of the Group-IB or -IIB-Group-IB or -MB Metal Bonds 8.2.3. In Univalent Compounds of Zinc and Cadmium... 

Goldberg, R. N., (1980b). "Evaluated Activity and Osmotic Coefficients for Aqueous Solutions Bi-univalent Compounds of Zinc and Cadmium, and Ethylene bis(trimethylammonium) Chloride and Iodide," manuscript in review. 

The mercuric compounds, in which mercury is bipositive, differ somewhat in their properties from the corresponding compounds of zinc and cadmium. The differences are due in part to the very strong tendency of the mercuric ion, Hg + +, to form covalent bonds. Thus the covalent nvstal mercuric sulfide, HgS, is far less soluble than cadmium sulfide or zinc sulfide (Chap. 22). 

A35. N. I. Sheverdina and K. A. Kocheshkov, The Organic Compounds of Zinc and Cadmium. North-Holland Puhl., Amsterdam, 1967. 252 pp. For Russian edition,... 

Complexing of zinc and cadmium by amino acids, both natural and otherwise, and of polypeptides, has been the subject of study for many years because of the importance of zinc-protein systems and the physiological effect of cadmium compounds (see Section 56.1.14). 

The apparent anomaly between mercury and the lighter elements of transition group 2. in that mercury regularly forms both univalent and divalent compounds, while zinc and cadmium do so very rarely, is partly under mm id from the observation that mercury III salts ionize even in the gaseous late to Hg.. rather than Hg Evidence for this double ion is provided by its Hainan spectral line, by the lineal CI-Hg-Hg-CI units in crystals or mercury It chloride, and by the cml of incrciirytll nitrate concentration cells The anomaly is fuitlicr removed by the obsetv.ttioii that cadmium also forms a (much less stable) diatomic ton Cdj T eg., ill Cd.-lAICL) . 

Despite the fact that the compounds of the type 802 are formally tridentate ligands, a substitution of the proton of the NH group to the metal does not take place even under electrosynthesis conditions. This was confirmed by H NMR data of zinc and cadmium complexes and x-ray diffraction data of 803 (M = Zn, R = 2,4-dimethyl) [593],... 

In addition to the oxides, the other six chalcogenides are also known. Table 15-3 shows the structures of the eight compounds. Clearly, with the sole exception of CdO, the chalcogenides of zinc and cadmium prefer tetrahedral coordination, though preference for the cubic zinc blende structure or the hexagonal wurtzite structure varies irregularly. 

Table A-53 Measured enthalpies of dissolution of zinc and cadmium selenites in H2S04(aq, 1 50) and mercury selenites in HCl(aq, 1 35). The enthalpies of formation have been estimated from these data. The uncertainties stem from the original publication. The data for the compounds provided with an asterisk are given for information only as they do not seem to form under normal aqueous conditions cf. phase diagram studies reported in Chapter V).

The values of the bond ionicily can be used to predict the forbidden band width of zinc and cadmium diphosphides this width should be 1.5-2 eV. This prediction is supported by the recent [19, 20] measurements of the optical properties of these compounds which yield the optical width of the forbidden band AE = 2 eV for ZnP2 [19] and CdP2 [20]. 

Another method that provides good results involves adding a fluorescent indicator to the adsorbent used to coat the plates. A mixture of zinc and cadmium sulfides is often used. When treated in this way and held under UV light, the entire plate fluoresces. However, dark spots appear on the plate where the separated compounds are seen to quench this fluorescence. 

The chemistry of zinc and cadmium is especially simple, in that these elements form compounds representing only the oxidation state +2. This oxidation state is closely correlated with the electronic structures shown in Table 19-4 it represents the loss or the sharing of the two outermost electrons. The ions Zn" and Cd" " are eighteen-shell ions. 

Mabrouk, H.E., Tuck, D.G. and Khan, M.A. (1987) The direct electrochemical synthesis of zinc and cadmium catecholates and related compounds , Inorg. Chim. Acta, 129, 75-80. 

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