Titanium Group

Introduction

This group consists of three elements viz., titanium, zirconium and hafnium. Titanium is comercially important as:

• (?) It gives strength, hardness and corrosion resistance when alloyed with other metals.
• (?'?') TiCl₃ is used as Zeigler-Natta catalyst for the polymerisation of ethylene.

Zirconium and hafnium are used in nuclear reactors.

Physical Characteristics

Occurrence

Titanium ore is called as ilmenite and ТЮ₂ is most abundant on earth. Zirconium is found mainly in the form of zircon ZrSi0₄ and small amounts as baddeleyite ZrO,. Hf and Zr show similar properties and size due to the interception of lanthanide contraction. Separation of Zr and Hf is very difficult.

Electronic Configuration

The valence shell electronic configuration is given as:

Titanium (Ar) 3d²4s^l

Zirconium (Kr) 4d²5s²

Hafnium (Xe) 4f^M5d²6s²

Oxidation States

The common oxidation states of these three elements are listed ahead: Oxidation states Ti —1,0, +2, +3, +4 Zr +2, +3, +4

Hf +2, +3, +4

(The more important, abundant and stable oxidation states are shown in bold ink.)

Extraction of Metals

Ti is accepted as the modern “wonder metal” of twentieth century. It is harder, stronger, higher melting and lighter than steel. Traces of non-meta!lic impurities like H, C, N and О make Ti, Zr and Hf brittle. Ti and A1 alloy is

used in su

rersonic aeroplanes.

Physical Constants

When alloyed with steel it makes it harder and tougher. Due to high melting point Ti metal is quite difficult to be extracted. The main reason is its high reactivity with air, 0-,, N, and hydrogen. In the reaction, the formed oxide is not reduced by carbon or CO because carbides are formed.

In the first step, TiO₀ is heated with C and Cl₂ at 900°C to form TiCl₄.

TiCl₄ is a colourless liquid which is removed by distillation. Ti is obtained from the following methods:

(?) Kroll Process: TiCl₄ is reduced by Mg or Ca.

MgCl, is removed by leaching with H₂0 or dilute HC1. With Na metal > reduced in the atmosphere of Argon.

NaCl is easily removed by leaching with water. Zirconium is also obtained by Kroll process.

(ii) Van Arkel-de Boer Process

This method is quite useful in order to obtain very pure metal and is used for both metals viz. Ti and Zr. With iodine, they are heated in an evacuated vessel and both the metals form their respective iodides. The iodides of these metals at 1400°C using tungsten filament get volatilise and separate metal from the impurities.

Zirconium is also extracted by the similar method on a smaller scale. Zr is more immune to corrosion than Ti. Zr and Hf have very similar properties and can be separated by solvent extraction of their compounds. An alloy made by Zr and Nb is frequently used in superconductors. Hf has an application in nuclear reactors for regulating neutrons.

Atomic Size

When we move down in Group IV from Ti to Hf, the covalent and ionic radii increase, also noticed an identical size in Zr and Hf due to filling of 4/level with 14 lanthanide elements. The electronic configuration of Hf and La are quite similar therefore, they show almost identical properties.

Chemical Characteristics

Reactivity and Passivity

The compressed form of metal structures renders them unreactive at low or moderate temperatures. Sometimes formation of their oxide film enables us to check further attack At room temperature, the metals do not react with acid or alkali.

It has been noticed that Ti can be dissolved in hot concentrated HC1 resulting in the formation of Ti³⁺ and H_v Also, hot HN0₃ reacts to give Ti0₂. (H-,0)_n. Regarding Zr, it can be dissolved in concentrated H,S0₄ and aquaregia. The HF has been found to be an outstanding solvent in order to dissolve all these three metals.

At 400°C they begin to react and above 600°C they are highly reactive. They form oxides MO,, halides MX₄, interstitial nitrides MN and interstitial carbides MC by direct combination. Their hydrides MH, are not affected by air and water.

Their reduction potential with respect to acid solution is tabulated below: Oxidation state

Behaviour of Ti Group Compounds in Different Oxidation States

The most stable and common oxidation state for all the elements is +4. Anhydrous TiCl₄ is covalent and has tetrahedral structure in the gaseous state. TiF₄ is ionic and its crystal structure is polymeric F-bridged structure where Ti is surrounded by 6F atoms.

The +4 state compounds are white or colourless and diamagnetic, they have d° configuration with all paired electrons. The ТЮ²⁺ titanyl ion is formed in solution, but it polymerises (TiO)²⁺.

Ti⁺³ ions are stronger reducing agents than Sn²⁺ ions. They exist as solids and are stable.

Ti^34- compounds have d^l configuration and are paramagnetic. Their magnetic moment value is 1.73 BM. There is one d-d transition and one band in visible spectrum. In Ti^34- solution, the colour is violet.

Ti²⁺ compounds are unstable and strong reducing agents, and exist in solid state. Ti(O), (-1) and (-2) states exist in the dipyradyl complexes.

Oxides

Ti0₂ shows +4 oxidation state and is used as a white pigment in paints. It is non-toxic and does not turn black when exposed to H-,S. It is used for whitening paper and nylon. It is used as a filler in plastics and rubber. Due to impurities Ti0₂ becomes coloured and in order to obtain pure TiO,, rutile is heated with chlorine and coke at 900°C. Then formed TiCl₄ is heated at 1200°C with oxygen.

Ilmenite or titanium iron oxide (FeTi0₃) is reacted with concentrated H₂S0₄ to form sulphate cake as (TiO) S0₄ (Titanyl sulphate). (TiO)S0₄ is hydrolysed by boiling with H,0.

The crystal structures of oxides indicate that they are ionic. In TiO, structure, Ti is surrounded by 6 atoms octahedrally. Oxides are insoluble in H₂0. Ti⁴⁺ exists as Ti0²⁺ titanyl ions in solution which is polymeric.

Similarly, Zr0²⁺ exists in solution and form polymeric crystals. Zr0(N0₃), is soluble in water. Phosphate of Ti, Zr and Hf are insoluble. Ti0₂, ZrO, and HfO, are white solids and non-volatile. The basic property increases with increase in atomic number. ТЮ, is amphoteric and ZrO, and HfO, are basic in nature.

Mixed oxides are formed when TiO„ ZrO, or HfO, are fused at 2500°C with other metal oxides to form titanates, zirconates and hafnates.

Similar to ilmenite, perovskite (СаТЮ₃) also occurs naturally. Perovskite structure has a cubic close-packed with О and Ca, with Ti occupying one third of octahedral sites. Ca has a coordination number as 12, also Ca and Ti are as far apart as possible.

Perovskite Structure

Also, BaTi0₃ has a perovskite structure, Ba²⁺ ions are very big in size to fit into close-packed oxide without expanding it. This results in enhancement of the size of octahedral holes in which Ti can rattle. In an electric field, Ti atoms go on one side creating polarisation and making the crystal ferroelectric. It is used in gramophone, pick-ups microphones and ceramic capacitors.

Ti(IV) solution give yellow-orange colour on addition of H,0,. The colour is due to the formation of peroxo complex [TiO,. 0H.(H,0)J⁺ below pH 1.0.

Halides

Ti and Zr form all the halides like fluorides, chlorides, bromides and iodides TiCl₄ is prepared by passing Cl, over TiO, and C.

Fluorides can be prepared by the reaction of

The iodides can be prepared by heating I, and Ti. Til₄ is used in van Arkel- de Boer process for purifying the metals. The compound titanium tetrachloride (TiCl₄) is found to be colourless, diamagnetic, fuming liquid whereas, ZrCl₄ is a white solid. All halides are tetrahedral and can be hydrolysed vigorously by

H_:0 with the formation of fumes in moist air giving polymeric Ti0-,.(H-,0)_n. When hydrolysed with aqueous HC1 they form oxychlorides.

TiF₄ is most stable compared to other halides.

Complexes: Octahedral complexes are formed with a variety of ligands.

Complexes of Zr and Hf have a higher coordination number e.g., Na₃ [ZrF_?]. Na₃[HfF₇], Na₄[ZrF₈] and Na₄[HfF_s],

Ti³⁺ compounds have d^l configuration and are more basic than Ti⁴~ compounds. Ti^34- used as its halide TiCl₃ which is violet powder and has its role in Zeigler-Natta catalyst. Ti⁴+ forms purple aqua ion [Ti(H₁0)₆]³⁺ when TiCl₄ aqueous solution is reduced by Zn which is a powerful reducing agent. Ti^(III> Cl₃ shows different colours in aqueous solution due to different number of water molecules and chloride ions surrounding them i.e. [Тл(Н-,0)_б]³⁺ЗСГ and [Ti(H,0)₅Cl]²⁺2Cl^_. These two situations develop different level of crystal field splitting of the d level. To determine Fe³⁺ through titration, Ti³⁺ is frequently used.

On heating disproportionation takes place.

Zr(III) and Hf(III) are unstable in H,0 and exist only as solid.

Tf compounds are highly unstable and so are Zr²+ and Hf²⁺ compounds.

Organometallic Compounds

Cyclopentadienyl compounds of Ti, Zr and Hf are quite stable and their hepta coordinate compounds are also well-known. Several stable compounds of Ti are known such as, [Ti(ri-C₅H₅)₂(CO)₂], [Ti(r|⁵-C₅H₅)₂(NR₂)₂], [Ti(q⁵- C₅H₅)_?(SCN)₁] where cyclopentadienyl molecules are pentahaptic i.e. 5-C atoms in each ring are attached to Ti. Reduction of these compounds yield [Ti(C₅H₅)-,.X)] and [Ti(C₅H₅)-,] resembling ferrocene, but the Ti compound is dimeric. Only a few Ti alkyl and aryls are known e.g., CH₃TiCl₃, C₆H₅Ti(C₃H₇0)₃. Compounds with alkyl group attached to Ti help in the polymerisation of alkenes.