Zinc group oxides, stabilities

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. 

The elements of this group (zinc Zn, cadmium Cd, mercury Hg) all exhibit a II oxidation state in aqueous systems, and Hg also shows a I oxidation state as indicated by the unusual cation Hg2. None of the elements shows oxidation states greater than II, which indicates that the d electrons are not involved. Within the group Zn and Cd resemble each other more closely than Cd and Hg. This is especially evident in the nobility of Hg (E° positive for Zn and Cd, negative for Hg), the lack of an Hg hydroxide, the thermal instability of HgO, and the greater stabilities of many Hg complexes as compared to those of Zn and Cd. 

In virtually all of its compounds zinc exhibits the I 2 oxidation state, although compounds of zinc(I) have been reported in the gaseous phase. Zinc is readily oxidized in the presence of hydroxide ions, e.g, by H2O, this behavior being attributed to the stability of the Zn(OH)42- ion. Like other transition group 2 elements zinc has a marked tendency to form covalent structures, e.g, ZnO and ZnS. 

The basic form of ZDDP, Zn4[PS2(R0)2]60, has a structure in which the central oxygen atom is surrounded by four zinc atoms in tetrahedral geometry, and the six 0,0-dialkyl-dithionate groups are attached to the six edges of the tetrahedron (Armstrong et al., 1998). For the basic ZDDP, it was found that the lowest energy corresponds to the attack of the oxide ion on one of the sulfur atoms contained in the additive molecule. The attack induces the cleavage of the three bonds, namely two P-S bonds and one Zn-S bond. Overall, the relative stability of the three forms was found to increase in the order monomeric, dimeric, and basic. 

After inactivation treatment, the catalyst is removed, and phenolic antioxydants and other stabilizers are added. Cross-linking curing is realized on unsaturated pendant groups. Peroxides are avoided because they cause chain scission and therefore systems with sulfur as cross-linker and zinc oxide, 2-mercaptobenzothiazole and tetramethylthiuram monosulfide as accelerators are used. 

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