Vanadium Group

Introduction

Vanadium is the parent metal along with niobium and tantalum metals. Vanadium enjoys commercial importance specially by forming alloy as ferrovanadium and its significant role as a catalyst in chemical reactions in the form of V metal and V₂0₅. The role of oxoanion of vanadium called vanadates occupy importance in solution chemistry. The other two metals of vanadium group viz. niobium and tantalum are useful though in little requirement. Vanadium forms a number of complexes of interest in its lower oxidation states.

Physical Characteristics

Occurrence and Extraction

An odd atomic numbers are observed in V, Nb and Та, and in comparison to the elements of neighbouring group therefore, they are not much abundant. Regarding abundancy of metals, vanadium occupies 19th position by weight in the earth’s crust. Vanadium can be found in some ores like vanadite PbCl₇. 3Pb₃(P0₄)₇ with lead ore, carnotite K₂(U0-,),(V0₄)₇.3H₂0 with uranium ore and also in crude oil. When vanadates are heated with Na₂C0₃ or NaCl at 800°C it gives NaV0₄ which forms a red precipitate of sodium poly vanadate with H₂S0₄. Sodium polyvanadate on heating at 700°C gives black V₂O_s (black colour is due to impurities).

In order to get pure vanadium metal VC1₅ is reduced with H or Mg, or by reducing V,0₅ by Ca. Vanadium can also be obtained by the process of electrolysis of fused halide complex. V₇O_s is used as a catalyst in the manufacture of H-,S0₄ by Contact process. In the Contact process for making sulphuric acid the catalyst Pt has been replaced by V₂O_s as it is cheaper and also reduces the poisoning level created by impurities due to arsenic. V₂O_s is used in the oxidation reactions for the conversion of naphthalene to phthalic acid and toluene to benzaldehyde. V₂0₅ plays its role as an essential catalyst.

The other two elements viz. niobium and tantalum occur together. The common ores for Nb are pyrochlorite CaNaNb-,0₆F and columbite (Fe, Mu) Nb-,0₆. For tantalum, the ore is tantalite (Fe, Мп)Та-,О_б. Nb is used in stainless steel (Nb/steel is used in nuclear reactors). Та has its role in electronic industry for making capacitors. Та rod is also used to repair fractured bones by using it in form of wires or screws. TaC has highest melting point of 3800°C.

Electronic Configuration and Oxidation States

The important oxidation states are underlined and the less important in normal type. Unstable or doubtful oxidation states are given in brackets.

Electronic structure Oxidation states

Vanadium V (Ar) 3dHs² (-1), (0), (I), (II), IV V

Niobium Nb (Kr) Ad^l5s² (-1) (0) (I) (II) m (IV) V

Tantalum Та (Xe) 4/^uV/³6.s² (-1) (0) (I) (II) III (IV) V

Behaviour of Compounds in Different Oxidation States: For all the

three elements in the group, the oxidation states vary between -1 to +5. In case of vanadium, +11 and +III states are reducing agents, +IV is stable and +V is slightly oxidising. Regarding Nb and Та, +V is the most stable state and most common although, these two elements also exist in lower oxidation states. Zinc and acid can reduce vanadium +V to vanadium +11. Nb +V can be reduced to Nb³⁺ while. Та +V cannot be reduced. This shows stability of +V state and instability of lower oxidation states.

Atomic Size

Atoms are smaller than the titanium group due to poor shielding by (/-electrons. The atomic size of Nb and Та are identical due to lanthanide contraction. Therefore, Nb and Та occur together, show similar properties and are difficult to separate.

Graded General Properties

The three metals of vanadium group viz. V, Nb and Та are silvery metals having high melting points. Vanadium has the highest melting point in 3(/-series of metals. This is due to maximum d electrons participation in metallic bonding, Nb and Та also show high melting points. Impurities make these metals hard and brittle, otherwise they are soft and ductile. They are non-corrosive owing to the oxide film on the surface and with HF they form complexes. These metals are not affected by air, water and acids at room temperature. V dissolves in concentrated H-,S0₄, HN0₃ and aquaregia (3 volume of concentrated HC1 and one volume of concentrated HN0₃), but unaffected by alkali, whereas Nb and Та dissolve in NaOH.

At high temperature, these three metals form non-stoichiometric interstitial compounds with non-metals С, H, N, О etc. Comparatively, vanadium is stable in different oxidation states than Nb and Та which are stable in +5 oxidation state. V^2- and V³⁺ are reducing agents. In +4 oxidation state V forms V0²⁺ ion which is very stable in solids and solutions. Vanadium +5 state is covalent such as, VF₅, V0₂⁺ and V0₄^3_ (vanadates). The basic property of +5 oxides increases when we move down the group. V,0₅ is amphoteric. V dissolves in water giving pale yellow solution and with NaOH, it forms vanadates which are colourless solutions and wide ranging vanadates that are formed depend upon the pH. At high pH, orthovanadates (V0₄⁺) are obtained. Niobates and tantalates are also formed like Nb_:0₅ and Та₀О₄ which can be dissolved in NaOH. V²⁺ and V³⁺ are well known ionic compounds whereas, Nb(II) (III) Ta(II) and Ta(lll) are not ionic, but are complexes.

Colour: Colour in transition metal compounds may be due to:

(0 d-d transitions (if) Crystal defects (ш) Charge transfer spectra

Below +5 oxidation state the colour is attributed to d-d transition due to incomplete (/-orbital electrons. In +5 oxidation state, the configuration is d° therefore, the compounds are expected to be white or colourless e.g. VF₅, NbF₅ etc. However, V,0₅ is red or orange, NbCl₅ is yellow, NbBr₅ is orange and Nbl₅ is brass coloured owing to factors other than d-d transition.