Zinc Compounds - General Characteristics

General Characteristics

Zinc atoms have an electronic configuration of 3d104s2. When compounds in the +2 oxidation state are formed the s electrons are lost, so the bare zinc ion has the electronic configuration 3d10. Examples of zinc compounds include the oxide, ZnO and sulfide, ZnS, (zinc blende) in which the oxide and sulfide ions are tetrahedrally bound to four zinc ions. Many complexes, such as ZnCl₄2-, are tetrahedral. Tetrahedrally coordinated zinc is found in metallo-enzymes such as carbonic anhydrase. However 6-coordinate octahedral complexes can also be found, such as the ion 2+, which is present when a zinc salt is dissolved in water.

Many zinc(II) salts are isomorphous (have the same type of crystal structure) with the corresponding salts of magnesium(II) which results from the fact that Zn2+ and Mg2+ have almost identical ionic radii. This comes about because of the d-block contraction. Whilst calcium is somewhat larger than magnesium, there is a steady decrease in size as atomic number increases from calcium to zinc. By chance it is the ionic radius of zinc that is almost equal to that of magnesium. In most other respects the chemistry of zinc(II) most closely resembles the chemistry of copper(II), its neighbour in the periodic table, in which there is less electron. However, whereas Cu2+ is classed as a transition metal ion by virtue of its electronic configuration, 3d9, in which there is an incomplete d-shell, Zn2+ is best considered to be an ion of a post-transition main group element. The IUPAC periodic table places zinc in the d-block.

Some compounds with zinc in the oxidation state +1 are known. The compounds have the formula RZn₂R and they contain a Zn — Zn bond analogous to the metal-metal bond in mercury(I) ion, Hg₂2+. In this respect zinc is similar to magnesium where low valent compounds containing a Mg — Mg bond have been characterised.

No compounds of zinc in oxidation states other than +1 or +2 are known. Calculations indicate that a zinc compound with the oxidation state of +4 is unlikely to exist. Although higher oxidation states are more stable with the heavier elements of a group, the compound HgF₄ was only characterized at 4 K in a neon/argon matrix.