The elements of group 12 - zinc, cadmium and mercury - differ from the transition metal elements (groups 3 to 11) in that they form compounds in which their oxidation states are no higher than +2. Though some beryllium and magnesium compounds show similar structures to many zinc compounds, leading us to expect to see many parallels between group 12 and the alkaline earth metals, the two groups do not have a great deal in common. The reason for the difference in chemistry between the alkaline earth metals and the elements of group 12 is the arrangement of the electrons just below the valence shell. For the M²⁺ ions of group 2 this is a closely held full shell; for the ions of group 12 the next level down from the valence shell is a full set of d orbitals which are less tightly held and much more polarisable. Mercury has many unique properties within its group, and some of its chemistry is very different to zinc and cadmium.

Isolation, Properties and Uses of the Elements

The metals have quite a low natural abundance in the Earth's crust but can be readily extracted from their ores. Zinc is most commonly found in the minerals sphalerite, (ZnFe)S, and zinc blende, ZnS. Cadmium has very similar properties to zinc and so occurs in most of its ores, not having many of its own. The metals are extracted from the ores by roasting with air to give the oxides, ZnO and CdO. These are then subjected to high temperatures in the presence of carbon to yield carbon dioxide and the metal. The cadmium and zinc can then be separated by distillation.

Zinc and cadmium are both light silvery metals that easily tarnish. The elements all have very low melting and boiling points for their reasonably heavy atomic masses (especially mercury as it is a liquid at room temperature, see table 1). Indeed, mercury is the only liquid metal at room temperature, although caesium and gallium have melting points very close to room temperature.

Zinc has many uses, the most important of which is in protective coatings for other metals and galvanisation. Zinc is also used in a variety of alloys, a common example being the copper-zinc alloy, brass. Zinc is of enormous biological importance, being one of the most important metals employed by nature. Zinc ions play an important role in a class of proteins, called zinc-finger proteins, where the zinc ions coordinate to group on the peptides chain thereby holding it in the required conformation. Zinc ions are also found to mediate catalytic reaction in the active sites of some enzymes, the classic example being the enzyme carbonic anhydrase which catalysis the conversion of CO₂ to HCO₃^-.

Cadmium is also used in alloys, and, along with zinc, finds applications in electrical batteries.

Mercury is extracted from the ore 'cinnabar', HgS. The metal is easily obtained by roasting the ore in air at about 500°C, forming the oxide HgO. This is unstable at these temperatures and breaks down to give elemental mercury vapour which is separated and condensed.

Mercury dissolves and combines with many other metals to form amalgams. This ability has historically been used in the extraction of precious metals such as gold and silver. Amalgams have been used for dental fillings for over 150 years. Typically, they contain up to 45% of mercury and variable amounts of other metals such as silver, copper, tin and zinc. There has been recent concern over the safety of dental amalgams due to the volatility and toxicity of mercury.

Amalgams of sodium, potassium and zinc are used as reducing agents in the laboratory.

Mercury also has more familiar uses in thermometers and barometers and in mercury lamps.

According to legend, the First Emperor of the Qin Dynasty, who was buried protected by an army of terracotta figures¹ in 210-209BC, was surrounded by a scale replica of the universe complete with gemmed ceilings representing the cosmos, and flowing mercury which represented the great earthly bodies of water. Recent scientific work at the site has shown high levels of mercury in the soil, which tentatively indicates an accurate description of the site’s contents given by Sima Qian.

Mercury (II) nitrate was once used in the curing of felt, used in hat-making. It is thought that this may have provided the inspiration for Lewis Carroll's story of the Mad Hatter, as prolonged exposure to mercury vapour causes mercury poisoning. The symptoms of this include severe and uncontrollable muscular tremors and twitching limbs, called 'hatter's shakes'. Other symptoms include distorted vision and confused speech. In advanced cases, hatters developed hallucinations and other psychotic symptoms.

Table 1: Some properties of the group 12 elements.

Zinc and cadmium both react with various acids to give H₂ gas and M²⁺ salts; mercury, however, is inert to these acids. Zinc also dissolves in strong bases to give H₂ and a 'zincate' ion - ZnO₂^2-.

Zn + 2 OH^- → ZnO₂^2- + H₂

Zinc and cadmium both react readily with oxygen on heating to form oxides. The corresponding reaction of mercury, though thermodynamically stable, is very slow. All three element of group 12 react with the halogens, and elements such as sulphur, selenium and phosphorus.

M⁺ Compounds of Group 12

The group 12 metals will form M⁺ ions which exist as dimers, having a metal-metal bond (⁺M-M⁺). The strength of the bond increases down the group and the ions Zn₂²⁺ and Cd₂²⁺ are highly unstable. They can only be observed when trapped in solid materials. If ZnCl₂ is heated to above its melting point and metallic zinc is added, on cooling a yellow glass like solid is formed that contains Zn₂²⁺ ions, which can be analysed by spectroscopy.

The +1 state of mercury is much more stable and the Hg₂²⁺ can be obtained by reducing Hg²⁺ salts in solution. It is only marginally more stable however. The ions can be made to undergo a disproportionation reaction⁴ by addition of many different reagents which will include the accompanying negatively-charged counter ion. This happens if the reagent or potential counter ion will form a strong complex with Hg²⁺ or form a Hg²⁺ compound which is insoluble and will precipitate out of solution.

Hg₂²⁺ → Hg + Hg²⁺

For example:

Hg₂²⁺ + S^2- → Hg + HgS _(s)

Hg₂²⁺ + 2 CN^- → Hg + Hg(CN)_2(aq)

Amongst the stable mercury(I) compounds are the halides (apart from the fluoride), for example Hg₂Cl₂, which are insoluble. Other examples are mercury(I) nitrate, Hg₂(NO₃)₂, and the perchlorate, Hg₂(ClO₄)₂, both of which are soluble in water, and the sulphate, Hg₂SO₄, which is not.

Zinc(II) and Cadmium(II) Compounds

Oxides and Hydroxides

The oxides ZnO and CdO can be formed by burning the metals in air and also from the combustion of certain M²⁺ compounds, such as the carbonates and nitrates. ZnO will dissolve in both acids, forming slats, and bases, from which the hydroxide Zn(OH)₂ is precipitated. Zinc hydroxide will easily dissolve in strongly basic solutions to give ions, termed 'zincates', such as [Zn(OH)₃]^- and [Zn(OH)₄]^2-. It will also dissolve in concentrated ammonia to give the amine complex [Zn(NH₃)₄]²⁺.

CdO will also dissolve in bases and precipitate Cd(OH)₂ but this is insoluble in further strong base solution. It will, however, dissolve in concentrated ammonia to give the analogous amine complex [Cd(NH₃)₄]²⁺.

Halides

The fluorides of zinc and cadmium, MF₂, are both ionic and are insoluble in water due to having very high lattice energies. The other halides of the metals are much more soluble and will dissolve in other solvents, such as alcohols, and are much more covalent in nature. In aqueous solutions of cadmium halides, there is an equilibrium between different cadmium species, so Cd²⁺, CdX⁺, CdX₂ and CdX₃^- are all present in varying proportions.

Oxo Salts

Zn²⁺ and Cd²⁺ both form water soluble oxo salts with nitrate, sulphate and perchlorate ions. The metal ions are similar to Mg²⁺ and so these salts have similar properties to the analogous magnesium salts. When dissolved in water, aqua ions are formed, which are acidic, liberating H⁺ with solutions containing M(OH)⁺ ions.

Zn²⁺_(aq) + H₂O ↔ Zn(OH)⁺_(aq) + H⁺

Sulphides

Zinc sulphide (ZnS) is of interest as it produces luminescence in the presence of tritium (³H) and hence is used on luminous dials in clocks and watches, replacing radium and promethium.

Cadmium sulphide (CdS) exists naturally as the mineral Greenockite. It is a canary yellow salt, known as 'cadmium yellow', which is used as a pigment by artists, and is perhaps the most important yellow pigment in the artist's palette, although there are now concerns over its toxicity. It is also used as a pigment to give a bright yellow colour to plastic products. It is made by precipitation from acid solution of a soluble cadmium salt (ie, the chloride or the sulphate) with hydrogen sulphide or an alkali metal sulphide. The colour of cadmium sulphide ranges from lemon yellow to deep orange, depending on how it is prepared.

Mercury(II) Compounds

Mercury(II) Oxide and hydroxide

As well as being formed by heating in air, mercury oxide, HgO, can be produced by the combustion of mercury(I) and mercury(II) nitrates and can be precipitated by the addition of a hydroxide salt to a solution of a Hg²⁺ salt in water. The colour of HgO varies from red to yellow depending on the particle size of the solid. It is soluble in water and forms what is thought to be the hydroxide, Hg(OH)₂, although this has never been isolated. The hydroxide is amphoteric - it can act as either an acid or a base, depending on the conditions - but it is more basic than it is acidic.

HgO + H₂ ↔ Hg(OH)₂ ?

Halides

Mercury(II) fluoride, HgF₂, is an ionic compound and is hydrolysed by cold water to form hydrofluoric acid, HF. The other halides of mercury are much more covalent in nature and have low melting and boiling points in comparison to the fluoride. They are also reasonably soluble in a variety of organic solvents, and in water it is almost entirely in the form of HgX₂ molecules. There is a small amount of hydrolysis that occurs, however, to give Hg(OH)X. For example:

HgCl₂ + H₂O ↔ Hg(OH)Cl + HCl

Mercury (II) Sulphide

Mercury (II) sulphide (HgS) exists in two forms, red and black. The red form is the naturally occurring 'cinnabar', used as a pigment under the name of 'vermilion'. Occurring in light red earthy or granular masses, it is particularly abundant in Spain, California and China, and is the chief source of mercury and its compounds. The black form is precipitated when hydrogen sulphide gas is passed through a solution of a soluble mercury(II) salt:

Complexes

There are many examples of strong complexes of Hg²⁺. Commonly, these contain two ligands (linear) or four ligands (tetrahedral), but examples of three and five coordinate complexes are also known. The complexes, especially the two coordinate complexes, are very highly covalent. Mercury forms simple coordination complexes, such as the anions [HgCl₄]^2- and [Hg(CN)₄]^2- and cations such as [Hg(NH₃)₄]²⁺.

Compounds can also be formed with ligands such as phosphines, PR₃, where R is some organic group. These, compounds such as HgX₂(PR₃) and HgX₂(PR₃)₂, tend to be dimeric or polymeric forming Hg-X-Hg halide bridges between mercury centres. There is also a wide variety of complexes with thiolate ligands derived from thiols, RSH, which are analogous to alcohols, ROH. Thiols are also called mercaptans because of their ability to strongly bind to mercury.

There are neutral dithiolates, which are linear, for example Hg(SCH₃)₂ and Hg( SCH₂CH₃)₂, although the compound Hg{SC(CH₃)₃}₂ is a polymer with Hg-S-Hg thiolate bridges. There are additionally anionic tri- and tetrathiolates complexes such as [Hg(SC₆H₅)₃]^- or the dimeric complex [Hg₂(SCH₃)₆]^2- (see figure 1)

Figure 1: Structure of the complex [Hg₂(SCH₃)₆]^2- (the sulphur atoms about each Hg atom are approximately tetrahedral). H₃C CH₃ CH₃ S S S \ / \ /  Hg Hg / \ / \  S S S H₃C CH₃ CH₃

Mercury will also form some complexes with metal-metal bonds to transition metals, such as in the reaction of mercury(II) chloride with the anionic cobalt carbonyl complex Co(CO)₄^-.

2 NaCo(CO)₄ + HgCl₂ → 2 NaCl + (CO)₄Co-Hg-Co(CO)₄

Organometallic Chemistry of Group 12

The first organometallic compounds of zinc, the zinc alkyls ZnR₂ and RZnX, were discovered by Frankland in 1849 and led to their use in synthetic organic chemistry as sources of R^-, although these were later superseded by Grignard reagents. Heating zinc in boiling alkyl halides, RX, under an inert and dry atmosphere, leads to the formation of the zinc(II) species, RZnX. On further heating and distillation, this gives ZnR₂ and ZnX₂.

2 Zn + 2 RX → 2 RZnX → ZnR₂ + ZnX₂

The reaction work best if the halide X is iodide, but the bromides can also be used. ZnR₂ compounds are covalent in nature and are non-polar volatile liquids with very low melting and boiling points (for Zn(CH₃)₂, MP = -29.2 °C, BP = 46 °C). They are sensitive to air and moisture (lower molecular weight zinc alkyl can be spontaneously flammable in air) and react with water and alcohols, in the same way as the magnesium Grignard reagents, to form hydroxides and alkoxides respectively. The analogous organocadmium compounds, CdR₂, are prepared by the reaction of cadmium halides with Grignard reagents. The lighter cadmium alkyl compounds are also quite volatile and liquid, but are less thermally stable and less reactive than their zinc analogues, for instance they don't spontaneously inflame in air. Organomercury compounds are made in a similar way to those of cadmium by the reaction of mercury halides with Grignard reagents.

HgCl₂ + RMgX → RHgCl + MgXCl
RHgCl + RMgX → MgXCl + HgR₂

The compounds RHgX are crystalline solids, whereas the compounds HgR₂ are liquids or low melting solids. These dialkyl mercury compounds are extremely toxic. Due to their extremely non-polar character, they are highly lipophilic, and, if accidentally brought into contact with skin, can be absorbed through it. This leads to a slow release of the compound into the blood stream, and, eventually, death.