Chambers, Holliday. Modern inorganic chemistry

THE TRANSITION ELEMENTS 367

water ligands can be partly or completely displaced by addition of other ligands such as ammonia or chloride ion.The factors which govern the displacement of one ligand by another are rather complicated ; thus, for example, ammonia will often replace water as a ligand, to form ammonia-metal complexes,but this does not happen readily with all transition metal ions (notably not with Fe2+, Fe3+ and Mn2+). However, most complexes of metal ions in oxidation states 2 or 3 are prepared by displacement of the water by other ligands, for example NH3, CN~, halide". Complexes with metal oxidation states 0 are not easily prepared in solution; the metal carbonyls can, however, be prepared by direct reaction, e.g.

Ni + 4CO -> Ni(CO)4 finely

divided metal

For complexes with high metal oxidation states, special methods are used, since these complexes can only exist with certain ligands (seeabove).

Some properties of complex metal ions in aqueous solution

In an aquo-complex, loss of protons from the coordinated water molecules can occur, as with hydrated non-transition metal ions (p. 45). To prevent proton loss by aquo complexes, therefore, acid must usually be added. It is for these conditions that redox potentials in Chapter 4 are usually quoted. Thus, in acid solutions, we have

[Fe(H20)6]3+ + <T -> [Fe(H20)6]2 + : E* =' + 0.77V

In the absence of acid, the half-reaction will approximate to:

[Fe(H2O)5(OH)]2+ + e~ -» [Fe(H2O)5(OH)]+

for which E^ is indeterminate, but certainly less than E^ in acid solution. In presence of alkali, the half-reactionbecomes, effectively,

[Fe(H2O)3(OH)3] + H2O + e~ -> [Fe(H2O)4(OH)2] + OH'

for which E^ = - 0.56 V. Hence theless acidic a solution containing Fe(II) is, the more easily is it oxidised and solutions of iron(II) salts must be acidified to prevent oxidation by air. A more impressive demonstration of the effect of change of ligand on oxidationreduction behaviour is provided by the following scheme:

dissolves in water) cannot be oxidised to the + 3 state (B) in aqueous solution, since B would itself oxidise water to give oxygen. Replacement of the water ligands by chloride alters the shape, colour and redox potential, but again oxidation of C to D is not possible. However, replacement of water ligands by ammonia to give E allows easy oxidation to the stable + 3 complex F. Replacement of water by cyanide would be expected to give G; in fact this is immediately oxidised by the solvent water (with evolution of hydrogen) to the + 3 complex H.

OTHER PROPERTIES

The metals: alloys

Reference has already been made to the high melting point, boiling point and strength of transition metals, and this has been attributed to high valency electron-atom ratios. Transition metals quitereadily form alloys with each other, and with non-transition metals; in some of these alloys, definite intermetallic compounds appear (for example CuZn, CoZn3, Cu31Sn8, Ag5Al3) and in these the formulae correspond to certain definite electron-atom ratios.

The metals: interstitialcompounds

The transition metal structuresconsist of close-packed(p. 26) arrays of relatively large atoms. Between these atoms, in the 'holes', small atoms, notably those of hydrogen, nitrogen and carbon, can be inserted, without very much distortion of the original metal structure, to give interstitial compounds (for example the hydrides, p. 113).

THE TRANSITION ELEMENTS 369

Because the metal structureis 'locked' by these atoms, the resulting compound is often much harder than the original metal, and some of the compounds are therefore of industrial importance (see under iron). Since there is a definite ratio of holes to atoms, filling of all the holes yields compounds with definite small atom-metal atom ratios; in practice, all the holes are not always filled, and compounds of less definite composition (non-stoichiometric compounds) are formed.

The metals: other properties

Adsorption of gases on to transition metal surfaces is important, and transition metals or alloys are often used as heterogeneous catalysts.

The reactivity of the transition metals towards other elements varies widely. In theory, the tendency to form other compounds both in the solid state (for example reactions to form cations) should diminish along the series; in practice, resistance to reaction with oxygen (due to formationofa surface layer of oxide) causeschromium (for example) to behave abnormally; hence regularities in reactivity are not easily observed. It is now appropriate to consider the individual transition metals.

SCANDIUM

Scandium is not an uncommon element, but is difficult to extract. The only oxidation state in its compounds is + 3, where it has formally lost the 3d14s2 electrons, and it showsvirtuallyno transition characteristics. In fact, its chemistry is very similar to that of aluminium (for example hydrous oxide Sc2O3, amphoteric; forms a complex [ScF6]3~ ; chloride ScCl3 hydrolysed bywater).

TITANIUM

THE ELEMENT

Titanium is not a rare element; it is the most abundant transition naetal after iron, and is widely distributed in the earth's surface, mainly as the dioxide TiO2 and ilmenite FeTiO3. It has become of commercial importance since World War II mainly because of its high strength-weight ratio (use in aircraft, especially supersonic), its

370 THE TRANSITION ELEMENTS

resistance to corrosion (use in chemical plant), and its retention of these properties up to about 800K.

The extraction of titanium is still relatively costly; first the dioxide TiO2 is converted to the tetrachloride TiCl4 by heating with carbon in a stream of chlorine; the tetrachloride is a volatile liqwd which can be rendered pure by fractional distillation. The next stage is costly; the reduction of the tetrachloride to the metal, with magnesium, must be carried out in a molybdenum-coated iron crucible in an atmospheric of argon at about 1100 K:

TiCl4 -h 2Mg -» Ti 4- 2MgCl2: Aff = - 540 kJ moPl

The precautions stated are to avoid uptake of oxygen, nitrogen and other impurities which render the metal brittle; the excess magnesium and magnesium chloride can be removed by volatilisation above 1300 K.

PROPERTIES

Titanium is a silver-grey metal, density 4.5 g cm~3, m.p. about 1950 K. When pure it is soft; presence of small amounts of impurity make it hard and brittle, and heating with the non-metals boron, carbon, nitrogen and oxygen gives solids which approach the compositions TiB2, TiC, TiN and TiO2; some of these are interstitial compounds, which tend to be non-stoichiometric and harder than the pure metal (p. 369). With hydrogen, heating gives a nonstoichiometric hydride, which loses hydrogen at higher temperatures. The metal resists attack by chlorine except at elevated temperatures.

COMPOUNDS OF TITANIUM

Oxidation state + 4

In this oxidation state the titanium atom has formally lost its 3d2 and 4s2 electrons; as expected, therefore, it forms compounds which do not have the characteristics of transition metal compounds, and which indeed show strong resemblances to the corresponding compounds of the lower elements (Si, Ge, Sn, Pb) of Group IV—the group into which Mendeleef put titanium in his original form of the periodic table.

THE TRANSITION ELEMENTS 371

CHLORIDES

The important halide is the tetrachloride, TiCl4, made from the dioxide as already stated (p. 370). It is a colourless volatile liquid, b.p. 409 K, readily hydrolysed by water (see below); the Ti—Cl bonds are covalent, and the molecule is monomeric and tetrahedral (cf. the halides of Group IV). It dissolves in concentrated hydrochloric acid to give the hexachlorotitanate(IV) anion, [TiCl6]2~ ; salts of this anion can be precipitated from the solution by addition of an alkali metal chloride, for example KC1gives K2TiCl6 (compare again the behaviour of the Group IV halides). The [TiCl6]2~ ion has an octahedral configurationand is the simplest representative of a large number of titanium(IV) complexes, of general formula [TiX6]"~, where X represents a number of possible ligands and n = 0, 1 or 2. This ability of TiX4 compounds to increase their coordination to TiX6 has an important practical use. If trimethylaluminium, (CH3)3A1, is added to a solution of titaniumtetrachloride and an olefin such as ethylene passed into the mixture, the olefin is readily polymerised.This is the basis of the Ziegler-Natta process for making polyolefins, for example 'polypropylene', and the mechan ism is believed to involve the coordination of the olefin to molecules ofthetypeCH3TiCl3.

Titanium tetrachloride is hydrolysed by water, to give a mixture of anions, for example [Ti(OH)Cl5]~ and [TiCl6]2~, together with some hydrated titaniumdioxide (TiO2,4H2O is one possible hydrate, being equivalent to [Ti(OH)4(H2O)2]). This suggests that titanium dioxide is amphoteric (see below).

TITANIUM DIOXIDE

This occurs naturally as a white solid in various crystalline forms, in ail of which six oxygen atoms surround each titanium atom. Titanium dioxide is important as a white pigment, because it is nontoxic, chemicallyinert and highly opaque, and can be finely ground: for paint purposes it is often prepared pure by dissolving the natural form in sulphuric acid, hydrolysing to the hydrated dioxide and heating the latter to make the anhydrous form.

Anhydrous titanium dioxide is only soluble with difficulty in hot concentrated sulphuric acid; dilution allows the crystallisation of a sulphate of formula TiOSO4 .H2O, but it is doubtful if the ktitanyF cation TiO2 + actually exists, either in solution or the solid. Certainly [Ti(H2O)n]4+ does not exist, and solutions of "titanyl' salts may best be considered to contain ions [Ti(OH)2(H2O4)]2 + . Titanium

372 THE TRANSITION ELEMENTS

dioxide is not soluble in aqueous alkali, but with fused alkali gives a titanate, for example

2KOH + TiO2 -> K2 TiO3 4- H2O

Hence titanium dioxide is clearly amphoteric.

Oxidation state + 3

In this oxidation state the outer electronic configuration is 3d1, so the compounds are necessarily paramagnetic (p. 229) and are coloured.

TitaniurnHHI) chloride, TiCl3, is made by reduction of the tetrachloride with, for example, hydrogen. In the anhydrous form it has a covalent polymeric structure and is coloured violet or brown(there are two crystalline forms). In water, it forms a violet/green solution, and from a slightly acid solution a hydrated solid TiCl3.6H2O can be obtained. Hence, clearly, [Ti(H2O)6]3+ can exist (as might be expected since (Ti3 *) would have a lower charge and larger radius than (Ti4+)). The aqueous solution has reducing properties:

TiO2+(aq) + 2H3O+ + e~ -» Ti3+(aq) 4- 3H2O: E^ = +0.1 V

It must be kept under an atmosphere of nitrogen or carbon dioxide; it reduces, for example, Fe(III) to Fe(II) and mtro-organic compounds RNO2 to amines RNH2 (it may be used quantitatively to estimate nitro-compounds). In neutral solution, hydrolysis occurs to give species such as [Ti(OH)(H2O)5]2*, and with alkali an insoluble substance formulated ,as Ti2O3 aq' is produced; this is rapidly oxidised in air.

Complexes of titanium(III) can be made from the trichloride— these are either approximately octahedral,6-coordinate(for example TiCl3.3L (L = ligand) and [TiCl2(H2O)4]*, formed when TiCl3 dissolves in aqueous hydrochloric acid), or 5-coordinate with a trigonal bipyramid structure.

Other oxidation states

Titanium forms dihalides TiX2, for example titanium(II) chloride, formed by heating titanium metal and the tetrachloride to about 1200 K. TiCl2 is a black solid, which disproportionates on standing to TiCl4 + Ti. Since it reduces water to hydrogen, there is no aqueous chemistry for titanium(II). A solid oxide TiO is known.

THE TRANSITION ELEMENTS 373

TESTS FOR TITANIUM

Aqueous solutions containing titanium(IV) give an orange-yellow colour on addition of hydrogen peroxide; the colour is due to the formation of peroxo-titanium complexes, but the exact nature of these is not known.

VANADIUM

THE ELEMENT

Vanadium is by no means as common as titanium, but it occurs in over sixty widely distributed vanadium ores. It is named after Vanadis (a name of the Scandinavian goddess Freia), because it forms compounds having many rich colours. Vanadium is a silvergrey metal; it is not very useful itself, and most of the metal produced is in the form of an alloyferrovanadium, containing between 40 and 90% vanadium. This is adde^ to steel to produce a very tough 'high-speed' steel. Ferrovanac|i|im is obtained by reduction of the oxide V2O5 with "ferrosilicorfl (Fe 4- Si). The pure metal is very difficult to prepare because it combines even more readily with hydrogen, carbon, nitrogen and oxygen than does titanium; as with the latter, the compounds produced are often interstitial and nonstoichiometrie, but with oxygen the pentoxide V2O5 is ultimately obtained. Vanadium dissolves readily in oxidising acids.

With the outer electronic configuration 3d34s2 vanadium can attain an oxidation state of + 5, but it shows all oxidation states between + 5 and + 2 in aqueous solution (cf. titanium).

COMPOUNDS OF VANADIUM

Oxidation state + 5

Although vanadium has formally lost all its outer electrons in this state, the resemblance to the Group V elements is not so marked as that of titaniumdV) to Group IV.

HALIDES

The vanadium(V) state is very strongly oxidising; hence the only stable halide is the fluoride VF5, a white, easily hydrolysed solid

THE TRANSITION ELEMENTS 375

by hydrogensulphite. The VO2+ (aq) cation is probably best represented as [VO(H2O)5]2 + , with the oxygen occupying one coordination position in the octahedral complex. However the kVO' entity is found in many other complexes, both cationic and anionic; an example of the latter is [VOC14]~ where the vanadium(V) is 5-co- ordinate, thus

The V(IV)species are all d1 complexes, hence their colour. Besides the *VO' compounds, some halides VX4 are known, for example VC14, a liquid with a tetrahedral, covalent molecule and properties similar to those of TiCl4, but coloured (red-brown).

Other oxidation states

In the + 3 oxidation state, vanadium forms an oxide V2O3, and the blue [V(H2O)6]3+ cation in acid solution; the latter is obtained by reduction of V(IV) or V(V):

VO2 + (aq) + 2H3O+ + e~ -» V3+ (aq) 4- 3H2O: E^ = + 0.36V The hexaquo-cation occurs in the blue-violet alums, for example

NH4V(SO4)2.12H2O

The + 2 oxidation state is achieved by more drastic reduction (zinc and acid) of the +5, + 4 or + 3 states: thus addition of zinc and acid to a solution of a yellow vanadate(V) gives, successively, blue[VO(H2O)5]2+ ,green [VC12(H2O)4]+and violet [V(H2O)6]2+. The latter is of course easily oxidised, for example, by air. The oxide VO is usually non-stoichiometric, but anhydrous halides VX2 are known.

The O oxidation state is known in vanadium hexacarbonyl. V(CO)6, a blue-green, sublimable solid. In the molecule V(CO)6, if each CO molecule is assumed to donate two electrons to the vanadium atom, the latter is still one electron short of the next noble gas configuration (krypton); the compound is therefore paramagnetic, and is easily reduced to form [V(CO6)]~. giving it the