Transition Metals-2081 PDF

Transition Elements Introduction to d-block elements  Definition: The elements in which the last electron enters into the d-sub shell are called d-block elements.  Position in periodic table: The d-block elements lie between s and p block elements in the periodic table. They extend from group 3 t...

Transition Elements Introduction to d-block elements  Definition: The elements in which the last electron enters into the d-sub shell are called d-block elements.  Position in periodic table: The d-block elements lie between s and p block elements in the periodic table. They extend from group 3 to group 12 and from period 4 to period 7. Period wise, they are classified into following four series. Introduction to d-block elements  4th Period: First or 3d series: 21Sc to 30Zn  5th Period: Second or 4d series: 39Y to 48Cd  6th Period: Third or 5d series: 57La to 80Hg  7th Period: Fourth or 6d series: 89Ac to 112Cn Introduction to d-block elements  Electronic Configuration: The general outer electronic configuration of d-block elements is (n-1)d1-10 ns1-2, where n represents the principal quantum number of the valence shell. The electronic configuration of the elements of 3d series are given below:  Introduction to d-block elements Electronic configuration of elements of 3d series 21Sc: [Ar]3d 14s2 26 Fe: [Ar]3d 64s2 22Ti: [Ar]3d 24s2 27 Co: [Ar]3d 74s2 23V: [Ar]3d 34s2 28 Ni: [Ar]3d 84s2 24Cr: [Ar]3d 54s1 29 Cu: [Ar]3d 104s1 25Mn: [Ar]3d 54s2 30 Zn: [Ar]3d 104s2 Introduction to d-block elements Note: 1. 24Cr and 29Cu have unusual configurations due to extra stability of half-filled and completely-filled orbitals. 2. The position of any d-block element in the periodic table can be located as follows:  Period number: n(Principle quantum number of valence shell)  Group number: No. of (n-1)d electrons + No. of ns electrons Example: For Mn (3d54s2), period no. = 4 and group no. = 7 Introduction to transition elements  All d-block elements lie between s and p block elements in the periodic table. They also show transitional (intermediate) behaviour between s and p-block elements. Hence, d-block elements are also called transition elements.  Definition: The transition elements are those elements which have incompletely filled d-subshell in either free state or in any of their oxidation states. So, all d- block elements are not necessarily transition elements. Introduction to transition elements Follow up questions 1. Why are Zn, Cd and Hg not considered as transition elements?  Zn, Cd and Hg have completely filled d-orbitals (d10) in their free state as well as in their most common oxidation state of +2. So, Zn, Cd and Hg do not show typical behaviour of transition elements. They are better called pseudo transition of non- typical transition elements. 2. How can you say that Cu is a transition element even though it has completely filled d-orbitals? Characteristics of transition elements 1. All transition elements are metals and show regular metallic properties such as high density, malleability, ductility, high electrical and thermal conductivity, metallic lustre, high melting and boiling points, alloy formation etc. 2. They show variable valency and oxidation state in their compounds. 3. They show strong tendency to form complex compounds. Characteristics of transition elements 4. Most of the transition elements and their compounds show paramagnetic behaviour due to presence of unpaired d-electrons. 5. Most of the transition elements form coloured ions and compounds due to d-d transition. 6. Most of the transition metals and their compounds show good catalytic properties in various chemical processes. Oxidation states of transition metals  Transition elements show variable oxidation states in their compounds.  Reason: In the case of transition elements, both ns and (n-1)d electrons take part in bonding due to small difference in energy between them.  Example: Electronic configuration of Ti is [Ar]3d24s2. It shows +2 oxidation state if only 4s electrons are involved in bonding. Similarly, it shows +3 and +4 oxidation states if it also uses its one and two 3d electrons respectively. Element Outer EC Oxidation states Sc 3d14s2 +2, +3 (Predominantly only +3) Ti 3d24s2 +2, +3, +4 V 3d34s2 +2, +3, +4, +5 Cr 3d54s1 +1, +2, +3, +4, +5, +6 Mn 3d54s2 +2, +3, +4, +5, +6, +7 Fe 3d64s2 +2, +3, +4, +5, +6 Co 3d74s2 +2, +3, +4 Ni 3d84s2 +2, +3, +4 Cu 3d104s1 +1, +2 (+2 state stable due to extensive hydration) Zn 3d104s2 +2 Oxidation states of transition metals Note: 1. The most common oxidation state shown by the transition elements is +2 (except Sc) 2. The number of oxidation states increases from Sc to Mn and then decreases to Zn. (do you know why?) Oxidation states of transition metals Follow up questions 1. Why does Mn show the highest O.S. of +7? Ans: Mn has electronic configuration [Ar] 3d5 4s2. Since, 4s and 3d electrons have similar energies, they both can participate in bonding. Hence, the maximum O.S. shown by Mn is given by (2+5), i.e. 7 as in KMnO4. Oxidation states of transition metals 2. Why does Zn show only +2 O.S.? Ans: Zn: [Ar]3d104s2 and Zn2+: [Ar]3d10 After losing two 4s electrons, it acquires 3d10 configuration which is highly stable as all electrons are paired. 3. Which one is more stable Fe2+ or Fe3+? 4. Find the oxidation state of Ni in Ni(CO)4. Complex formation  Transition metals show strong tendency to form complexes. Reasons: 1. Transition metal atoms or ions are small in size with high effective nuclear charge so that they can easily accept the lone pairs of electrons from certain electron rich species (ions or molecules) called ligands. 2. They have vacant d-orbitals with appropriate energy to accommodate the electron pairs donated by ligands. Complex Ions Definition: An electrically charged species in which a metal atom or ion is coordinated to a number of electron rich species (ions or molecules) is called a complex ion. Types: 1. Cationic complex ions: They have +ve charge, e.g., [Cu(NH3)4]2+, [Ag(NH3)2]+ etc. 2. Anionic complex ions: They have -ve charge, e.g., [Fe(CN)6]3-, [CuCl4]2- etc. Complex Ions Note: 1. A complex ion is represented by writing its formula within a square bracket [] 2. A complex ion remains intact and hence retains its identity in solution. For example, the complex [Cu(NH3)4]2+ does not dissociate and hence does not give the test for Cu++ ion. Complex compounds Definition: A compound containing at least one complex ion and which retains its identity in solid as well as in solution is called a complex compound or coordination compound or metal complex. Examples: [Cu(NH3)4]SO4, K4[Fe(CN)6], [Pt(NH3)4][PtCl4] etc. A complex compound usually ionizes. For2-example: [Cu(NH3)4]SO4 → [Cu(NH3)4]2+ + SO4 Neutral complex: A complex which has no charge and does not ionize is called neutral or non-ionic complex. Examples: [Ni(CO)4], [Co(NH3)3Cl3] etc. Complex compounds Follow up questions 1. Identify whether the complex ions present in the given compounds are cationic or anionic. a. [Cu(NH3)4](OH)2 b. K3[Fe(CN)6] c. [Ag(NH3)2]Cl d. Fe4[Fe(CN)6]3 Complex compounds 2. Find the number of moles of AgCl produced when 1 mole of each of the following complexes is treated with excess of AgNO3 solution: a. [Co(NH3)6]Cl3 b. [Co(NH3)5Cl]Cl2 c. [Co(NH3)4Cl2]Cl d. [Co(NH3)3Cl3] Definition of some terms related to complex  Central metal atom or ion: The metal atom or ion to which a group of electron rich species (ions or molecules) are attached through coordinate bonds in a complex is called central metal atom or ion. For example, in [Cu(NH3)4]2+ , Cu2+ is the central metal ion.  Ligands or coordinating groups: The electron rich species (ions or molecules) which are directly attached to the central metal atom or ion through coordinate bonds in a complex are called ligands or coordinating groups. For example, in [Cu(NH3)4]2+ , the four NH3 molecules act as ligands. Definition of some terms related to complex Donor atom or coordinating atom: The atom in the ligand which actually donates the electron pair to the central meta l is called donor atom or coordinating atom. For example, in NH3, N-atom is the donor atom Types of ligands: A ligand can have one, two or more donor atoms and is accordingly called monodentate, bidentate or polydentate ligand. Examples: Definition of some terms related to complex 1. Monodentate ligands: Cl-, Br-, OH-, CN-, H2O, NH3, CO etc. 2. Bidentate ligands: Ethylene diamine (en): H2N-CH2-CH2-NH2 Oxalate ion (ox): - OOC-COO- 3. Polydentate ligands Diethylene triamine (dien): H2N-CH2-CH2-NH-CH2-CH2- NH2 Ethylene diamine tetraacetate ion (EDTA) Ethylene diamine tetraacetate ion (EDTA) Definition of some terms related to complex  Coordination number: The total number of ligands (more correctly their donor atoms) attached directly to the central metal atom or ion in a complex is called coordination number. For example, in [Cu(NH3)4]2+ , the coordination number of Cu is 4. Follow up question What is the coordination number of Co in [Co(en)3]Cl3 Definition of some terms related to complex  Coordination sphere: The central metal atom or ion along with the ligands directly attached to it is called coordination sphere. It is always shown in a square bracket []. It is not ionizable.  Ionization sphere: The portion of the complex molecule outside the coordination sphere is called ionization sphere. It is ionizable. Definition of some terms related to complex  Example Coordination sphere Ionization sphere [Cu(NH3)4]SO4 Ligands Coordination Central metal (NH3) Ion (Cu2+) number (4) Definition of some terms related to complex Note: 1. The oxidation state of the central metal in a complex is also called primary valency and the coordination number of the central metal is also called secondary valency. For example, in [Cu(NH3)4]SO4, Primary valency = 2 Secondary valency = 4 Definition of some terms related to complex 2. The total number of electrons that the central metal atom or ion possesses including those gained from ligands is called effective atomic number (EAN). Thus, EAN = At. no. of metal – No. of electrons lost in ion formation + No. of electrons gained from ligands For example, in [Cu(NH3)4]SO4, EAN = 29 – 2 + 4×2 = 35 Generally, EAN matches the atomic number of the next noble gas element. This is known as EAN rule. Shape of complex ions  The geometry or shape of a complex ion can be predicted from the coordination number (C.N.) of the central atom or ion. The most common coordination numbers exhibited by metal atoms or ions are 2, 4 and 6.  Coordination number 2 gives linear geometry  Coordination number 4 gives tetrahedral or square planar geometry  Coordination number 6 gives octahedral geometry Geometry C. N. Hybridization Examples Linear 2 sp [Ag(NH3)2]+, [CuCl2]- Tetrahedral 4 sp3 [Zn(NH3)4]2+, [CuCl4]2-, [Ni(CO)4] Square Planar 4 dsp2 [Cu(NH3)4]2+, [Ni(CN)4]2-, [Pt(NH3)2Cl2] Octahedral 6 d2sp3 [Cr(NH3)6]3+, [Fe(CN)6]4- (Inner orbital) Octahedral 6 sp3d2 [Fe(H2O)6]3+, [CoF6]3- (Outer orbital)  Shape of complex ions  Note: i. The inner orbital octahedral complexes are also called low spin or spin paired complexes as they have relatively less number of unpaired electrons. ii. The outer orbital octahedral complexes are also called high spin or spin free complexes as they have relatively more number of unpaired electrons. Follow up question: Draw the shape of following copper complexes: [CuCl4]2-, [Cu(NH3)4]2+ and [Cu(H2O)6]2+. Shape of d-orbitals eg set dx2-y2 dz2 𝐝𝐳𝟐 𝐝𝐱𝟐−𝐲𝟐 t2g set Crystal field splitting of d-orbitals  According to crystal field theory (CFT)  Ligands are treated as point charges.  The attraction between the central metal and the ligands is purely electrostatic, there being no orbital overlapping.  In case of free atom or ion, all the d-orbitals of the metal have same energy, i.e., degenerate. However, in case of a complex, the approaching ligands destroy the degeneracy, i.e., d-orbitals will have different energies. It results in splitting of d-orbitals called crystal field splitting. Crystal field splitting in octahedral complexes  In an octahedral complex, the metal is at the centre of the octahedron and the six ligands are at its six corners. Crystal field splitting in octahedral complexes  As the six ligands approach the central metal all along x, y, z, -x, -y and -z directions, the 𝑑 𝑥 2 −𝑦 2 and 𝑑 𝑧 2 orbitals lying along the axes experience more repulsion than the 𝑑 𝑥𝑦 , 𝑑𝑦𝑧 and 𝑑𝑧𝑥 orbitals lying in between the axes. As a result, the energy of 𝑑 𝑥 2 −𝑦 2 and 𝑑 𝑧 2 will be raised more than the energy of 𝑑 𝑥𝑦 , 𝑑𝑦𝑧 and 𝑑 𝑧𝑥. This removes the degeneracy of the d-orbitals which now splits into two sets t2g and eg.  + 0.6 ∆𝟎 - 0.4 ∆𝟎 Fig. Splitting of d-orbitals in octahedral complex Crystal field splitting in octahedral complexes  Crystal field splitting energy (∆𝒐): The energy difference between the two sets of d-orbitals (i.e. t2g and eg) in an octahedral field is called crystal field splitting energy and is denoted by ∆o (o for octahedral). The value of ∆o depends upon the field strength of ligands.  Spectrochemical series: The ligands in increasing field strength is given by experimentally determined series called spectrochemical series. The series is: − − − − − − − I < Br < Cl < F < OH < H2O < NH3 < en < NO2 < CN < CO Crystal field splitting in octahedral complexes  Note: The actual configuration of the split d-orbitals in an octahedral field depends on the relative value of crystal field splitting (∆𝑜) and pairing energy (𝑃).  If ∆𝑜> 𝑃, the electrons tend to pair in lower 𝑡2𝑔 level rather than to occupy higher 𝑒𝑔 level. This effect is shown by strong field ligands and it results in the formation of low spin or spin paired complexes.  If ∆𝑜< 𝑃, the electrons tend to remain unpaired and occupy higher 𝑒𝑔 level rather than to pair in lower 𝑡2𝑔 level. This effect is shown by weak field ligands and it results in the formation of high spin or spin free complexes. Colour of transition metal compounds  Most of the transition metal compounds are coloured in solid as well as in solution form. This is mainly due to the d- d transition of the unpaired electrons present in the central metal atom or ion.  d-d transition Definition: The electronic transition from one d-level to another d-level due to absorption of light is called d-d transition. Colour of transition metal compounds  Explanation: When light falls on the compound or its solution, the electron from the lower t2g level gets promoted to the higher eg level. It is accompanied by the absorption of light of a particular wavelength (or colour) in the visible region (𝜆 ≈ 400 – 800 nm) and the rest of the light (i.e., unabsorbed light) gets reflected or transmitted. The reflected or transmitted light has a colour complementary to the absorbed colour. Therefore, the compound or the solution appears to be of the complementary colour. colour chart Colour of transition metal compounds  Example: A solution of Cu2+ ion appears blue. Reason: In solution, the Cu2+ ion exists as the hydrated ion [Cu(H2O)6]2+. The electronic configuration of Cu2+ is Cu2+: [Ar] 3d9 Cu2+ has incompletely filled d9 configuration with one unpaired electron. Due to the presence of unpaired electron, d-d transition takes place. In this process, orange colour is absorbed and hence the solution appears blue which is complementary colour of orange. Colour of transition metal compounds  Note: 1. The colour of transition metal compounds and ions depends on the energy difference between the two d-levels which in turn depends on the nature of ligands and the type of complex formed. 2. The metal ion which have either completely filled or completely empty d-orbitals (d10 or d0) are colourless due to no d-d transition. Thus, Zn2+ (d10) and Ti4+ (d0) ions are colourless. 3. In the absence of ligands, the crystal field splitting does not occur and hence the compound becomes colourless. For example, anhydrous CuSO4 is colourless while CuSO4.5H2O is blue. Colour of transition metal compounds  Charge transfer spectra In some compounds such as KMnO4 (purple), K2Cr2O7 (orange) etc, the colour is the result of charge transfer spectra in visible region. For example, in KMnO4, Mn is in +7 oxidation state with 3d0 configuration. Since there is no d electron, the colour cannot be explained by d-d transition. Here, purple colour arises from the charge transfer between Mn and O atoms. In this case, an electron is momentarily transferred from O to Mn absorbing colour in visible region and the transmitted colour appears purple. Colour of transition metal compounds Follow up questions: 1. ZnSO4.7H2O is white while CuSO4.5H2O is blue. Why? 2. TiO2 is white while TiCl3 is violet, why? 3. CuSO4.5H2O is blue while CuSO4 is colourless. Why? Catalytic properties of transition metals  Many transition metals and their compounds act as catalyst in many chemical processes. Reason: 1. Transition metals have vacant d-orbitals and show variable oxidation states. Therefore, these metals can form unstable intermediate compounds with reactants. These intermediate readily decompose into the final products and the original catalyst. Catalytic properties of transition metals 2. In some cases, finely divided transition metals and their compounds provide a large surface area for the adsorption of reactant molecules which thereby come closer and react faster.  Examples 1. Ni and Pd act as catalysts in hydrogenation. For example 𝑁𝑖/𝑃𝑑 𝐶𝐻2 = 𝐶𝐻2 + 𝐻2 CH3 − CH3 Catalytic properties of transition metals 2. Fe in presence of metal oxides acts as catalyst in Haber’s process 𝐹𝑒+𝑀𝑜 𝑁2 + 3𝐻2 2𝑁𝐻3 3. V2O5 acts as catalyst in contact process 𝑉2𝑂5 2𝑆𝑂2 + 𝑂2 2𝑆𝑂3 4. MnO2 acts as catalyst for the decomposition of KClO3 𝑀𝑛𝑂2 2𝐾𝐶𝑙𝑂3 2𝐾𝐶𝑙 + 3𝑂2 Magnetic properties  Most of the transition elements and their compounds show paramagnetic behaviour due to presence of unpaired d- electrons. The paramagnetic substances are attracted by magnetic field. The magnetic moment is given by the formula: 𝜇 = 𝑛(𝑛 + 2) BM (BM = Bohr Magneton) Where, n = number of unpaired electrons. Exceptions: Sc3+, Ti4+, Cu+ etc. having no unpaired electrons are diamagnetic. Follow up question: Which is more paramagnetic Fe2+ or Fe3+?