Chapter 24 Transition elements Introduction of transition elements • Definition of transition elements: D-block elements forming one or more stable ions with partially filled (incomplete) d-sub shells. All the transition elements are metals, they form strong metallic bonds Higher melting, boiling points and densities than s-block metals K m. pt / °C density / g cm-3 63 0.86 Ca Sc Ti 850 1400 1677 1917 3 4.5 6.1 1.55 V Cr Mn Fe Co 1903 1244 1539 1495 7.9 8.9 7.2 7.4 Electronic configuration of the first row transition elements INCREASING ENERGY / DISTANCE FROM NUCLEUS 4f 4 4d 4p 3d 4s 3 Chromium: 1s2 2s2 2p6 3s2 3p6 4s1 3d5 3p Copper: 1s2 2s2 2p6 3s2 3p6 4s1 3d10 Electronic configuration of the first row transition elements Strictly speaking, Scandium and Zinc are not defined as transition elements because… Scandium: 1s2 2s2 2p6 3s2 3p6 4s2 3d1 Sc3+: 1s2 2s2 2p6 3s2 3p6 Zinc: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 Zn2+: 1s2 2s2 2p6 3s2 3p6 3d10 Neither of these two ions has an incomplete d subshell Ions of transition elements All transition elements are metals, they tends to lose electrons to form positive ions Electrons are first removed from the outermost electron shell (4s subshell) K 1s2 2s2 2p6 3s2 3p6 4s1 Ca 1s2 2s2 2p6 3s2 3p6 4s2 Sc 1s2 2s2 2p6 3s2 3p6 4s2 3d1 +7 Ti 1s2 2s2 2p6 3s2 3p6 4s2 3d2 +6 +6 +6 V 1s2 2s2 2p6 3s2 3p6 4s2 3d3 Cr 1s2 2s2 2p6 3s2 3p6 4s1 3d5 Mn 1s2 Fe 1s2 2s2 2p6 3s2 3p6 4s2 3d6 Co 1s2 2s2 2p6 3s2 3p6 4s2 3d7 Ni 1s2 2s2 2p6 3s2 3p6 4s2 3d8 Cu 1s2 2s2 2p6 3s2 3p6 4s1 3d10 Zn 1s2 2s2 2p6 3s2 3p6 4s2 3d10 2s2 2p6 3s2 3p6 4s2 Sc 3d5 Ti V Cr Mn Fe Co Ni +5 +5 +5 +5 +5 +4 +4 +4 +4 +4 +3 +3 +3 +3 +3 +3 +3 +2 +2 Cu Zn +4 +4 +2 +2 +2 +2 +3 +2 +2 +2 +1 Ions of transition elements Electrons are first removed from the outermost electron shell (4s subshell) Transition metals can for one or more ions with different oxidation states The most common oxidation state is +2, when 2 electrons from 4s are lost maximum O.N rises across row to manganese, involving removal electrons from both 4s and 3d subshell The 3d electrons become increasing hard to remove since the nuclear charge increases, the maximum O.N falls Ions of transition elements Transition metals have variable O.N, the resulting ions are of different colors ions with partially filled d-orbitals tend to be coloured ions with d10 (full) or d0 (empty) configuration are colourless it is caused by the ease of transition of electrons between energy levels the frequency of light is proportional to the energy difference colour depends on ... transition element oxidation state ligand coordination number General physical properties of transition element High melting points giant metallic structure High densities high relative atomic mass, small atomic radius Hard and rigid, used as construction materials strong metallic bonding Good conductor of electricity and heat delocalised electrons are free to move Physical properties compare with s-block elements Melting point is higher than s-block elements greater charge density, stronger metallic bonding Densities is higher than s-block elements greater the Ar, smaller the atomic radius Atomic radius/ionic radius is smaller than s-block elements; the first ionisation energy is higher Increasing in the nuclear charge holding the outmost electrons more tightly. Similar shielding effect. Poorer conductor of electricity and heat than s-block elements, except Cu electrons are harder to become delocalised Redox titration Potassium manganate(VII) with iron(II) MnO4 – + 8H+ + 5e– → Mn2+ + 4H2O EӨ= +1.52V Fe3+ + 2e– → Fe2+ EӨ= +0.77V Overall equation: MnO4 – + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O EӨ= +0.75V purple colorless Pale green at high concentration Yellow at high concentration What is the amount/ purity of iron (Fe2+ ) in an sample? Redox titration Potassium manganate(VII) with iron(II) MnO4 – + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O colorless purple Pale green at high concentration Yellow at high concentration What is the amount/ purity of iron (Fe2+ ) in an sample? Standard solution KMnO4 Unknown solution Fe2+ Indicator End-point MnO4 – purple color Mn2+ colorless All the iron(II) has been oxidized to iron(iii). The one more drop of manganate(VII) gives a pink color Ligand (配合基) and complex ions Ligand • Atoms, or ions, which possess lone pairs of electrons and form coordinate bonds to the central metal ion/atom • ONE lone pair of electrons is donate by an atom into vacant orbitals on the central species Complex ion : the resulting ion when metal atom or ion is bonded to one or more ligands Ligand and complex ions Co-ordination number • The number of the co-ordinate bonds to the central metal ion • Monodentate ligands • Bidentate ligands Each ligand forms one co-ordinate bond with central metal ion Each ligand forms two co-ordinate bond with central metal ion Ligand and complex ions Isomerism in complexes Geometrical (cis-/trans-) isomerism • The complex are usually in square planar shape (coordiantion number = 4) • There are two types of ligands. The complex formula must be [MA2B2]n+ trans platin cis platin Cis- : the same type of ligand are next to each other Trans- : the same type of ligand are opposite to each other • An important anti-cancer drug. • It can bind with DNA which prevents cell division Isomerism in complexes stereoisomerism (optical isomer) • The complex must be in octahedral shape (coordination number = 6) • Exist of bidentate ligands • For example, the complex ion Ni(NH2CH2CH2NH2)32+ The ligand NH2CH2CH2NH2 can be written as ‘en’ Mirror image to each other, cannot be superimposed Isomerism in complexes [Co(en)2Cl2]+ OPTICAL ISOMERISM GEOMETRICAL ISOMERISM CIS TRANS A complex ion can exhibit both types of isomerism Ligands substitution • It happens when the new complex formed is more stable Yellow solution Blue solution Deep blue solution Ligands substitution What is the observation when NH3 (aq) is gradually added to Cu2+, until the NH3 solution is in excess? • Stage 1: [Cu(H2O)6]2+ blue solution • Stage 2: [Cu(H2O)6]2+ + 2OH− → Cu(OH)2(H2O)4 + 2H2O blue ppt • Stage 3: Cu(OH)2(H2O)4 + 4NH3 → [Cu(H2O)2(NH3)4]2+ + 2H2O + 2OH− Deep blue solution Ligands substitution Pink solution Pink solution Blue ppt blue solution Orange/brown solution Stability constant, Kstab • Different ligands form different stabilities Blue solution Yellow solution • The position of equilibrium lies in the direction of more stable complex Stability constant, Kstab, is the equilibrium constant for the formation of the complex ion from its ions or molecules Stability constant, Kstab Key points when writing Kstab • Unit of Kstab depends on the equation of equilibrium • H2O is not included in the expression because it is in a large excess Stability constant, Kstab • Kstab are given on a log10 scale • The higher the Kstab value, the greater the stability of the complex ion • The equilibrium is likely to shift to the direction to form more stable complex Stability constant, Kstab An iron(III) ion, Fe3+, in aqueous solution has six water molecules bonded to it as ligands. i Draw the structure of this ion. ii When thiocyanate ions, SCN–, are added to an aqueous solution of iron(III) ions, the solution turns red and one water molecule is replaced by a thiocyanate ion. Use the concept of stability constants to explain why the reaction occurs. SCN– has a higher value of Kstab than H2O. Stability constant, Kstab An iron(III) ion, Fe3+, in aqueous solution has six water molecules bonded to it as ligands. iii Deduce the formula of the ion forming the red solution. iv The stability constant for aqueous Fe3+ ions with SCN– as a ligand is 891dm3 mol–1. The stability constant for aqueous Fe3+ ions with fluoride ions, F–, as a ligand is 2 × 105 dm3 mol–1. A solution containing fluoride ions is added to the red solution. Would you expect to observe any changes? Explain your answer Yes; a colour change is likely / possible. F– has a higher value of Kstab than SCN–. So F substitutes for SCN (and for water) because the position of equilibrium is shifted to the right. Color of the complexes Theory ions with a d10 (full) or d0 (empty) configuration are colourless ions with partially filled d-orbitals tend to be coloured it is caused by electrons absorbing photons of certain frequencies match the energy needed for one electron jump into a higher energy level. the frequency of light is proportional to the energy difference ions with d10 (full) Cu+,Ag+ Zn2+ or d0 (empty) Sc3+ configuration are colourless e.g. titanium(IV) oxide TiO2 is white Color of the complexes The observed colour of a solution depends on the wavelengths absorbed blue and green not absorbed white light Energy corresponding colors is absorbed to these Copper sulphate solution appears blue because the energy absorbed corresponds to red and yellow wavelengths. Wavelengths remaining after the absorption gives the colour. Color of the complexes Cu2+ dyz degenerate (简并) orbital dxz dxy dx2- y2 dz2 The lone pairs of electrons donated by ligands repel electrons in dx2-y2 and dz2 orbital Color of the complexes Cu2+ degenerate orbital Non-degenerate orbitals The lone pairs of electrons donated by ligands repel electrons in dx2-y2 and dz2 orbital The orbitals are split into two energy levels, non-degenerate orbital exists Color of the complexes ΔE One place is available for an electron to be promoted from low-energy orbital to high-energy orbital once energy is inputted Energy difference between non-degenerate orbital is part of the visible spectrum of light Color of the complexes OCTAHEDRAL TETRAHEDRAL 3d 3d • In an square complex, [Ni(CN)4]2–, one (x2-y2) go higher and four go lower • In an octahedral complex, [Fe(H2O)6]2+, two (z2 and x2-y2) go higher and three go lower • In a tetrahedral complex, [CuCl4]2–, three (xy, xz and yz) go higher and two go lower Color of the complexes • ΔE between the non-degenerate orbitals is affected by many factors • Colorless complexes are either d0 or d10, as there are no d-orbital electrons to move up • Changing the ligands changes the amount of splitting, so changes the colors. Color of the complexes orbitals at the same energy level The ligands in a complex cause the d orbitals to split, forming two sets of nondegenerate orbitals. 1, The d orbitals are split into non-degenerated orbitals of different energy levels; 2, Color is due to absorption of light; 3, An electron absorbs energy from photon and is promoted into orbitals of higher energy level Color of the complexes Color of the complexes Sc3+ ions have electronic configuration [Ar]3d04s0. There would be no electrons in the three 3d orbitals of lower energy Visible light would not be absorbed in promoting an electron from a lower energy 3d orbital to a higher energy 3d orbital Zn2+ ions have electronic configuration [Ar]3d104s0. In a complex ion containing Zn2+, each of the 3d orbitals would be fully occupied. Visible light could not be absorbed in promoting an electron from a lower energy 3d orbital to a higher energy 3d orbital.
