Chambers, Holliday. Modern inorganic chemistry

436 THE ELEMENTS OF GROUPS IB AND MB

USES

Mercury is extensively used in various pieces of scientific apparatus, such as thermometers, barometers, high vacuum pumps, mercury lamps, standard cells (for example the Weston cell), and so on. The metal is used as the cathode in the Kellner-Solvay cell (p. 130).

Mercury compounds (for example mercury(II) chloride) are used in medicine because of their antiseptic character. The artificial red mercury(II) sulphide is the artist's 'vermilion1. Mercury(II)sulphate is a catalyst in the manufactureof ethanal from ethyne:

C2H2 + H2O ^^ CH3. CHO

COMPOUNDS OF MERCURY

The chemistry of mercury compounds is complicated by the equilibrium

The relevant redox potentials are:

Hg2+(aq) 4- 2e~ -> Hg(I) : E^ = 0.85V + 2e~ -> 2Hg(I) : E^ = 0.79V

Hence mercury is a poor reducing agent; it is unlikely to be attacked by acids unless these have oxidising properties (for example nitric acid), or unless the acid anion has the power to form complexes with one or both mercury cations Hg2+ or Hgf +, so altering the E^ values. Nitric acid attacks mercury, oxidising it to Hg2+(aq) when the acid is concentrated and in excess, and to Hg2+(aq) when mercury is in excess and the acid dilute. Hydriodic acid HI(aq) attacks mercury, because mercury(II)readily forms iodo-complexes (see below, p. 438).

Oxidation state +1

The mercury(I)ion has the structure

so that each mercury atom is losing one electron and sharing one electron, i.e. is 'using' two valency electrons. The existence of Hg|+ has been established by experiments in solution and by X-ray diffraction analysis of crystals of mercury(I)chloride, Hg2Cl2 where

THE ELEMENTS OF GROUPS IB AND MB 437

the mercury ions are in pairs with the chloride ions adjacent, i.e. CP *Hg—Hg+. Cl~. (It is now known that mercury can also form species Hg^ up to Hgg+ ; cadmium also gives Cd^+, and

formation. However, the equilibrium can be moved to the left by using excess of mercury, or by avoiding aqueous solution. Thus, heating a mixture of mercury and solid mercury(II) chloride gives mercury(I) chloride, which sublimes off:

Hg + HgCl2 -> Hg2Cl2

The product, commonly called calomel, is a white solid, insoluble in water; in its reactions (as expected) it shows a tendency to produce mercury(II) and mercury. Thus under the action of light, the substance darkens because mercury is formed; addition of aqueous ammonia produces the substance H2N—Hg—Hg—Cl, but this also darkens on standing, giving H2N—Hg—Cl and a black deposit of mercury.

Mercury(I) ions can be produced in solution by dissolving excess mercury in dilute nitric acid:

6Hg + 8H+ + 2NO3~ -» 3Hg|+ + 2NO + 4H2O

From the acid solution white hydrated mercury(I) nitrate

Hg2(NO3)2.2H2O

can be crystallised out; this contains the ion

[H2O-Hg-Hg-H2O]2 +

which is acidic (due to hydrolysis) in aqueous solution. Addition of chloride ion precipitates mercury(I) chloride.

Oxidation state + 2

solution containing mercury(II) ions, and becomes red on heating. Mercury(II) oxide loses oxygen on heating.

Mercury(II) chloride is obtained in solution by dissolving mercury(II) oxide in hydrochloric acid; the white solid is obtained as a sublimate by heating mercury(II)sulphate and solid sodium chloride:

HgSO4 + 2NaCl -» HgCl2 + Na2SO4

438 THE ELEMENTS OF GROUPS IB AND IIB

The aqueous solution has a low conductivity, indicating that mercury(II) chloride dissolves essentially as molecules Cl—Hg—Cl and these linear molecules are found in the solid and vapour. A solution of mercury(II) chloride is readily reduced, for example by tin(II) chloride, to give first white insoluble mercury(I)chloride and then a black metallic deposit of mercury. The complexes formed from mercury(II) chloride are considered below.

amount of iodide ion to a solution containing mercury(II):

Hg2+ +2r-»HgI2

Addition of excess iodide gives a complex (seebelow).

Mercury(II) sulphate and nitrate are each obtained by dissolving mercury in the appropriate hot concentrated acid; the sulphate is used as a catalyst (p. 436).

MercuryiH) sulphide, HgS, again appears in two forms, red (found naturally as cinnabar) and black, as precipitated by hydrogen sulphide from a solution containing Hg(II) ions.

Complexes

Mercury(I) forms few complexes, one example is the linear [H2O- Hg-Hg—H2O]2+ found in the mercury(I) nitrate dihydrate (above, p. 437). In contrast, mercury(II) forms a wide variety of complexes, with some peculiarities: (a) octahedral complexes are rare, (b) complexes with nitrogen as the donor atom are common, (c) complexes are more readily formed with iodine than with other halogen ligands.

Mercury(II) halides, HgX2, can be regarded as neutral, 2- co-ordinate linear complexes X—Hg- X. X is readily replaced; addition of ammonia to a solution of mercury(II) chloride gives a white precipitate NH2—Hg—Cl; in the presence of concentrated ammonium chloride, the same reagents yield the diamminomercury(II) cation, [NH3—Hg—NH3]2+, which precipitates as [Hg(NH3)2]Cl2. In presence of excess chloride ion, mercury(II) chloride gives complexes [HgCl3]~ and [HgCl4]2~, but the corresponding iodo-complex [HgI4]2", from mercury(II) iodide and excess iodide, is more stable. (It is rare for iodo-complexes to form at all and very rare to find them with stabilities greater than those of

THE LANTHANIDES AND ACTINIDES 441

Reference has been made already to the existence of a set of "inner transition' elements, following lanthanum, in which the quantum level being filled is neither the outer quantum level nor the penultimate level, but the next inner. These elements, together with yttrium (a transition metal), were called the 'rare earths', since they occurred in uncommon mixtures of what were believed to be "earths' or oxides. With the recognition of their special structure, the elements from lanthanum to lutetium were re-named the 4lanthanons' or lanthanides. They resemble one another very closely, so much so that their separation presented a major problem, since all their compounds are very much alike. They exhibit oxidation state + 3 and show in this state predominantly ionic characteristics—the ions, L3+ (L = lanthanide), are indeed similar to the ions of the alkaline earth metals, except that they are tripositive, not dipositive.

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of +3 and +4; hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -I-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides.

When the products of nuclear fission reactions came to be investigated, it was found that the lanthanides frequently occurred among the products. (The lanthanide of atomic number 61, promethium, for instance, probably does not occur naturally and was not discovered until nuclear fission produced it.) Hence it became necessary to devise more effective procedures to separate lanthanides, both from the fission products and from one another. One method used with great success is that of ion exchange chromatography; a mixture of (say)lanthanide salts in solution is run into a cation-exchange resin, which takes up the lanthanide ions by exchange. A solution containing negative ions which form complexes with the lanthanide ions (ammonium citrate is used) is then passed into the column and the column is washed Celuted') with this solution until complexes of the lanthanides begin to emerge. It is found that those of the highest atomic number emerge first, and that the kzone' of concentration of each lanthanide is separated from that of its neighbour.Some examples are shown in Figure15.1.

442 THE LANTHANIDES AND ACTINIDES

The appearance of a peak between those for neodymium (60) and samarium (62) was then strong evidence for the existence of promethium (61).

The reason why lanthanides of high atomic number emerge first is that the stability of a lanthanide ion-citrate ion complex increases with the atomic number. Since these complexes are formed by ions, this must mean that the ion-ligand attraction also increases with atomic number, i.e. that the ionic radius decreases (inverse square

Time or volume of eluting solution passed through

Figure 15,1. Ion-exchangegraph for lanthanides

does decrease slightly as the atomic number increases. This effect, called the lanthanide contraction, occurs because the nuclear charge rises with rise of atomic number, whereas the two outer electron levels (which largely determine the ionic radius) remain unchanged; hence the ionic radius decreases as the increasing nuclear charge "pulls in' the outer electrons to an increasing extent.

Another characteristic change across the lanthanide series is that of the paramagnetism of the ions; this rises to a maximum at neodymium, then falls to samarium, then rises to a second maximum at gadolinium before falling finally to zero at the end of the series.

Before it was known that elements beyond uranium were capable of existence, the heaviest known natural elements, thorium, protactinium and uranium,were placed in a sixth period of the periodic classification, corresponding to the elements hafnium, tantalum and tungsten in the preceding period. It was therefore implied that these elements were the beginning of a new, fourth transition series, with filling of the penultimate n = 6 level (just as the penultimate n = 5

elements) and a study of their properties, show that, in fact, a new inner transition series is being built up, starting after actinium. Hence the elements beyond actinium are now called the actinides.

Initially, the only means of obtaining elements higher than uranium was by a-particle bombardment of uranium in the cyclotron, and it was by this means that the first, exceedingly minute amounts of neptunium and plutonium were obtained. The separation of these elements from other products and from uranium was difficult; methods were devised involving co-precipitation of the minute amounts of their salts on a larger amount of a precipitate with a similar crystal structure (the "carrier1). The properties were studied, using quantities of the order of 10~6 g in volumes of solution of the order of 10"3 cm3. Measurements of concentration could, fortunately, be made by counting the radioactive emissions— a very sensitive method. However, much of the chemistry of plutonium was established on this scale before nuclear fission reactions yielded larger quantities of plutonium, and also yielded the first amounts of americium and curium. It soon became apparent that the ion-exchange chromatography method could be used in the separation of these new elements in just the same way as for the lanthanides. The fact that, when this was done, a series of concentration peaks was obtained exactly similar to those shown in Figure 15 J, is in itself strong evidence that the actinides and lanthanides are similar series of elements.

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ionexchange, and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm~~3 but in 'atoms cm~~3 \ The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope.

Evidence other than that of ion-exchange favours the view of the new elements as an inner transition series. The magnetic properties of their ions are very similar to those of the lanthanides; whatever range of oxidation states the actinides display, they always have 4- 3 as one of them. Moreover, in the lanthanides, the element gado-

444 THE LANTHANIDES AND ACT1NIDES

linium marks the half-way stage when filling of the inner sub-levelis half complete. It is known that this represents a particularly stable electronic configuration—hence gadolinium forms only the ions Gd3+ (by loss of three outer electrons) and shows no tendency to add or lose electrons in the half-filled inner level. This behaviour may be compared with the element before gadolinium, europium, Eu, which exhibits an oxidation state of two as well as three, and the element following, terbium, which exhibits states of -1-3 and +4.

In the actinides, the element curium, Cm, is probably the one which has its inner sub-shell half-filled; and in the great majority of its compounds curium is tripositive, whereas the preceding elements

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of + 3, +4, +5 and -f 6, four being the most stable oxidation state. These states are all known in solution, for example Pum as Pu3+ , and PuIV as PuOf+ . PuO|+ is analogous to UOf + , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuO^+ is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuF3, and a brown fluoride, PuF4; a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PuO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods.

THE ELEMENTS BEYOND THE ACTINIDES

Element 103,lawrencium, completes the actinides. Following this series, the transition elements should continue with the filling of the 6d orbitals. There is evidence for an element 104 (eka-hafnium); it is believed to form a chloride MC14, similar to that of hafnium. Less positive evidence exists for elements 105 and 106; attempts (so far unsuccessful) have been made to synthesise element 114 (eka-lead), because on theoretical grounds the nucleus of this element may be stable to decay by spontaneous fusion (as indeed is lead). "Super-

THE LANTHANIDES AND ACTINIDES 445

heavy' elements, well beyond this range, may also have nuclear stability, but their synthesis remains as a formidable problem.

QUESTION

1. The lanthanides and actinides are two series of fourteen elements, the members of each series having very similar properties. How do you account for these similarities, and for the fact that all the elements are metals?