Zinc compounds

Zinc compounds are noteworthy for their nondescript appearance and behavior: they are generally colorless (unlike compounds of other elements with oxidation number +2, which are colored), do not readily engage in redox reactions, and generally adopt symmetrical structures.

This parallel results from the fact that Zn2+ and Mg2+ have almost identical ionic radii as well as filled electron shells.

Zn(II) complexes are kinetically labile, i.e. the Zn-ligand bonds exchange with other ligands rapidly.

Zinc oxide turns yellow when heated due to the loss of some oxygen atoms and formation of a defect structure.

Compounds containing zinc are typically diamagnetic, except in cases where the ligand is a radical.

[15] The most common structure of zinc complexes is tetrahedral which is clearly connected with the fact that the octet rule is obeyed in these cases.

Zn2+ is a class A acceptor in the classification of Ahrland, Chatt and Davies,[16] and so forms stronger complexes with the first-row donor atoms oxygen or nitrogen than with second-row sulfur or phosphorus.

[17] Aqueous solutions of zinc salts are mildly acidic because the aqua-ion is subject to hydrolysis with a pKa of around 9, depending on conditions.

The polarizing effect of Zn2+ is part of the reason why zinc is found in enzymes such as carbonic anhydrase.

The case of the thiocyanate complex illustrates the class A character of the zinc ion as it is the N-bonded isomer, [Zn(NCS)4]2−in contrast to [Cd(SCN)4]2− which is S-bonded.

However, the compound is in fact a trimer, Zn3(acac)6 in which each Zn ion is coordinated by five oxygen atoms in a distorted trigonal bipyramidal structure.

It adopts a polymeric structure consisting of tetrahedral zinc centres linked by bridging cyanide ligands.

A coordination number of 2 occurs in zinc amide Zn(NR1R2)2 (R1=CMe3, R2=SiMe3); the ligand is so bulky that there is not enough space for more than two of them.

In the active site of resting carbonic anhydrase a zinc ion is coordinated by three histidine residues.

When carbon dioxide enters the active site, it subject to nucleophilic attack by the oxygen atom which carries a partial negative charge, or indeed a full negative charge if the water molecule is dissociated.

[23] Some peptidases, such as glutamate carboxypeptidase II are thought to act in a similar way, with the zinc ion promoting the formation of a nucleophilic reagent.

The magnesium ion, which has a higher concentration in biological fluids, cannot perform these functions because its complexes are much weaker than those of zinc.

It was made by reaction of zinc and ethyl iodide and is the first compound known to contain a metal—carbon sigma bond.

In fact the low-molecular weight compounds will ignite spontaneously on contact with air and are immediately destroyed by reaction with water molecules.

The use of zinc alkyls has been largely superseded by the use of the more easily handled Grignard reagents.