* By : A P Singh Definition of d-block elements d-block elements: The elements of periodic table belonging to group 3 to 12 are known as d-Block elements. because in these elements last electron enters in d sub shell or d orbital . The d -block elements lies in between s- and p-block elements in the long form of periodic table Transition Elements A transition element is defined as the one which has incompletely filled d orbitals in its ground state or in any one of its oxidation states. i.e. A transition element should have partially filled (n-1) d orbital. Group 7 Presentation How are d - Block Elements & Transition elements different? All d block elements are not transition elements but all transition elements are dblock elements All d block elements are not transition elements because d block elements like Zinc have full d10 configuration in their ground state as well as in their common oxidation state.which is not according to definition of transition elements. 1. Which of the d-block elements may not be regarded as the transition elements? 2. Why Zn, Cd and Hg are not considered as transition elements. 3. Why Scandium is a transition element but Zinc is not. 4. Copper atom has completely filled d orbital (3d10) in its ground state, yet it is transition element. Why 5. Silver atom has completely filled d orbital (4d10) in its ground state, yet it is transition element. Why 6. Why the very name ‘transition’ given to the elements of d-block . Answers 1. Zn, Cd and Hg 2. Because they do not have vacant d-orbitals neither in the atomic state nor in any stable oxidation state. 3. Scandium is a transition because it has incompletely filled d orbitals in its ground state but Zinc have full d10 configuration in their ground state as well as in their common oxidation state 4. Copper (Z = 29) can exhibit +2 oxidation state wherein it will have incompletely filled d-orbitals (3d), hence a transition element. 5. Silver (Z = 47) can exhibit +2 oxidation state wherein it will have incompletely filled d-orbitals (4d), hence a transition element. 6. The very name ‘transition’ given to the elements of d-block is only because of their position between s– and p– block elements. GENERAL & PHYSICAL PROPERTIES OF D-BLOCK ELEMENTS PHYSICAL PROPERTIES ATOMIC & IONIC SIZE IONIZATION ENTHALPY OXIDATION STATES OF D-BLOCK ELEMENTS COLOURED IONS CATALYTIC PROPERTIES MAGNETIC PROPERTIES FORMATION OF COMPLEX COMPOUNDS FORMATION OF INTERSTITIAL COMPOUNDS PHYSICAL PROPERTIES MELTING AND BOILING POINTS (ENTHALPIES OF ATOMISATION) : melting and boiling points are high. A large number of unpaired electrons take part in bonding so they have very strong metallic bonds and hence high m.pt & b.pt They have high enthalpies of atomisation . The maxima is at about the middle of each series. A large number of unpaired electrons take part in bonding so they have very strong metallic bonds and hence high enthalpy of atomization. ATOMIC & IONIC SIZE Along the rows nuclear charge increases but the penultimate d-sub shell has poor shielding effect so atomic and ionic size remain almost same . The radii of the third (5d) series are virtually the same as those of the corresponding members of the second series. This phenomenon is associated with the intervention of the 4f orbital, the filling of 4f before 5d orbital results in a regular decrease in atomic radii called Lanthanoid contraction which essentially compensates for the expected increase in atomic size with increasing atomic number. The net resultof the lanthanoid contraction is that the second and the third d series exhibit similar radii (e.g., Zr 160 pm, Hf 159 pm) IONIZATION ENTHALPIES IE2 :V < Cr > Mn and Ni < Cu > Zn IE3 : Fe << Mn Due to an increase in nuclear charge which accompanies the filling of the inner dorbitals , There is an increase in ionization enthalpy along each series of the transition elements from left to right. However, many small variations occur. oxidation states • Transition elements have variable oxidation states ,due to very small energy difference between (n-1)d & ns sub-shell electrons from both the sub-shell take part in bonding • The elements which give the greatest number of oxidation states occur in or near the middle of the series. Manganese, for example, exhibits all the oxidation states from +2 to +7. • Low oxidation states are found when a complex compound has ligands capable of π-acceptor character in addition to the σ-bonding. *For example, in Ni(CO)4 and Fe(CO)5, the oxidation state of nickel and iron is zero. Trends in the M2+/M Standard Electrode Potentials COLOURED IONS Most of the transition metal compounds (ionic as well as covalent) are coloured both in solid state & in aqueous state. Generally the elements/ions having unpaired electrons produce coloured compound. Titanium oxide Scandium oxide Vanadyl Sulphate dihydrate sodium chromate Potassium ferricyanide Mangnaese(II) chloride tetrahydrate Nickel(II) nitrate hexahydrate Cobalt(II) chloride Zinc sulfate Heptahydrate Copper(II) sulfate pentahydrate Splitting of d-orbital energies by an octahedral field of ligands ES ES eg GS eg hv t2g complex in electronic Ground State (GS) t2g d-d transition Do GS complex in electronic excited state (ES) An artist’s wheel Questions: Q1.Of the ions Ag+, Co2+ & Ti4+ which one will be coloured in aqueous soln. ? Q2. Why hydrated copper sulphate is blue while anhydrous copper sulphate is white? Q3.[Ti(H2O)6]3+ is coloured while [Sc(H2O)6]3+ is colourless . Explain? Q4. Why transtion metals & their compounds act as good catalyst? Q5. Why transtion metals generally forms coloured compounds CATALYTIC PROPERTIES Vanadium(V) oxide,V2O5 (in Contact Process) Finely divided iron (in Haber’s Process) Nickel (in Catalytic Hydrogenation) Cobalt (Synthesis of gasoline) This property is due to Presence of unpaired electrons in their incomplete d orbitals. Variable oxidation state of transition metals. In most cases , provide large surface area with free valencies. For example iron(III) catalyses the reaction between iodide and per sulphate ions Explanation MAGNETIC PROPERTIES When a magnetic field is applied to substances, mainly two types of magnetic behaviour are observed: diamagnetism and paramagnetism. Diamagnetic substances are repelled by the applied field while the paramagnetic substances are attracted. Substances which are attracted very strongly are said to be ferromagnetic. In fact, ferromagnetism is an extreme form of paramagnetism. Most of the transition elements and their compounds show paramagnetism. Paramagnetism arises from the presence of unpaired electrons, each such electron have a magnetic moment. The magnetic moment of any transition element or its compound/ion is given by (assuming no contribution from the orbital magnetic moment). μs = √n(n+2) BM Here n is the number of unpaired electrons The paramagnetism first increases in any transition element series, and then decreases. The maximum paramagnetism is seen around the middle of the series. QUESTIONSQ. 1: Which ion has maximum magnetic moment (a) V3+ (b) Mn3+ (c) Fe3+ (d) Cu2+ Ans: c Q.2. What is the magnetic moment of Mn2+ ion (Z= 25) in aqueous solution ? Ans.- With atomic number 25, the divalent Mn2+ ion in aqueous solution will have d5 configuration (five unpaired electrons).Hence, The magnetic moment, μ is μ = √5(5 + 2) = 5.92BM FORMATION OF COMPLEX COMPOUNDS Complex compounds are those in which the metal ions bind a number of anions or neutral molecules giving complex species with characteristic properties. The transition metals form a large number of complex compounds. A few examples are: [Fe(CN)6]3–, [Fe(CN)6]4–, [Cu(NH3)4]2+ and [PtCl4]2–. This property is due to the comparatively smaller sizes of the metal ions their high ionic charges and the availability of d orbitals for bond formation. FORMATION OF INTERSTITIAL COMPOUNDS The transition elements form a large number of interstitial compounds in which small atoms such as hydrogen, carbon, boron and nitrogen occupy the empty spaces in their lattices. They are usually non stoichiometric and are neither typically ionic nor covalent, for example, TiC, Mn4N, Fe3H, VH0.56 and TiH1.7, etc. C Fe The principal physical and chemical characteristics of these compounds are -: a) high melting points, higher than pure metals. b) very hard. c) retain metallic conductivity. d) chemically inert. those of Oxides and Oxoanions of Metals All the metals except scandium form MxOy oxides which are ionic. As the oxidation number of a metal increases, ionic character decreases. In higher oxides, the acidic character is predominant V2O2 Less basic < V2O4 more basic < V2O5. amphoteric Potassium dichromate K2Cr2O7 Preparation : Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr2O4) with sodium or potassium carbonate in free access of air. 4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2 2Na2CrO4 + 2 H+ → Na2Cr2O7 + 2 Na+ + H2O Na2Cr2O7 + 2 KCl → K2Cr2O7 + 2 NaCl Structure : The chromate ion is tetrahedral whereas the dichromate ion consists of two tetrahedra sharing one corner with Cr–O–Cr bond angle of 126°. Properties Oxidising Properties In acidic solution,its oxidising action can be represented as follows: Cr2O72– + 14H+ + 6e– → 2Cr3+ + 7H2O ; e.g. a) 6 I– → 3I2 + 6 e– c) 3 Sn2+ → 3Sn4+ + 6 e– b) 3 H2S → 6H+ + 3S + 6e– d) 6 Fe2+ → 6Fe3+ + 6 e– Potassium Permanganate, KMnO4 Preparation : prepared by fusion of MnO2 with an alkali metal hydroxide and an oxidising agent(O2 or KNO3) this produces the dark green K2MnO4 which disproportionates in a neutral or acidic solution to give permanganate. 2MnO2 + 4KOH + O2 → 2K2MnO4 + 2H2O 3MnO42– + 4H+ → 2MnO4– + MnO2 + 2H2O In the laboratory, a manganese (II) ion salt is oxidised by peroxodisulphate to permanganate. 2Mn2+ + 5S2O82– + 8H2O → 2MnO4– + 10SO42– + 16H+ Properties Oxidising Properties : Strong oxidising agent in acidic as well as in neutral & basic medium : Structure : Questions for Practice 1. Complete the following reactions : a) MnO4- (aq.) + C2O42-(aq) + H+(aq) -------> b) Cr2O7-- + H2S + H+ -----> c) d) e) f) g) h) i) j) Fe3+ + I- --------> CrO4 2- + H+ ------> MnO4- (aq.)+S2O3--(aq.) + H2O(l) Cr2O7-- (aq.)+ Fe2+ (aq.)+ H+ (aq.) KMnO4 ----> Cu++ (aq) + I- (aq) --------> Cr2O7-- (aq.)+ I- (aq.)+ H+ (aq.) -----> MnO4- (aq.) + NO2-(aq) + H+(aq) ----> 2. What is meant by lanthanoid contraction ? 3. Describe the preparation of following compounds – i. Potassium dichromate from Sodium chromate. ii. KMnO4 from K2MnO4 A. Explain the following :1. Cu(l) is not stable in an aq. solution. 2. With same (d4) configuration Cr(ll) is reducing whereas Mn (lll) is oxidising. 3. Transition metals are in general act as good catalyst. 4. Metal- metal bonding is more extensive in 4d & 5d series of transition metals than the 3d series. 5. Mn(lll) undergoes disproportionation reaction easily. 6. Co(ll) is easily oxidised in presence of strong ligands. 7. In a transition series of metals , the metals which exhibits the greatest No. of oxdn. occurs in the middle of the series . 8. Unlike Cr3+, Mn2+, Fe3+ & subsequent other M2+ ions of the 3d series of the elements , the 4d and 5d series metals generally do not form stable cationic species. 9. Transition metals and their compounds generally exhibits paramagnetic behaviour. 10. Actinoids exhibits greater range of oxdn states than lanthenoids. 11. Transition metals generally forms coloured compounds . 12. Mn exhibits the highest oxdn state of +7 among the 3d series of transition elements. 13. The enthalpy of atomisation of transition metals ar quite high. 14. There is a close similarity in physical & chemical properties of the 4d & 5d series of the transition metals , much more than the expected on the basis of usual family relationship. 15. The oxidising power of oxoanions are in the order – VO2+ < Cr2O7-- < MnO416. The third ionisation enthalpy of Mn is exceptionally high. 17. Cr 2+ is a stonger reducing agent than Fe 2+ 18. La 3+ (Z= 57) and Lu3+(Z= 71) do not show any colour in solution 19. Among the divalent cations of the first transition series elements, 20. Mn exhibits the maximum paramagnetism . 21. Generally there is an increase in density of elements from Ti (Z= 22) to Cu (Z= 29). in the 3d series of metals. 22. The atomic radii of the metals of third (5d) series of transition metals are virtually the same as those of the corresponding members of the second(4d) series. 23. The Eo value for Mn3+/ Mn2+ couple is much more positive than that for Cr3+/Cr2+ couple or Fe3+/Fe2+ couple. 24. The highest oxdn state of a metal is exhibited in its oxides or fluorides. 25. Zn is not regarded as a transition metal. 26. Explain why Ce4+ is a strong oxidising agent. 27. Many of the transition elements form interstitial compound 28. Lanthanoid form primarily +3 ions , while the actinoids have higher oxdn statesin their compounds, +4 or even +6 being typical. 29. Co2+ is easily oxidised to Co3+ in the presence of a strong ligand. 30. CO is stronger complexing ligand than NH3 . 31. Mn2+ is much more resistant than Fe2+ towards oxdn. 32. The enthalpies of atomisation of transition metals are quite high. 33.There occur much more frequent metal-metal bonding in compounds of 3rd (5d) transition series of d-block elements. 34. The Eo value for Cu2+/ Cu couple is positive in the 1st series (rest have negative). 35. With the same d-orbital configuration (d4) Cr 2+ is a reducing agent while M4+ is an oxidising agent.