Chemistry of transition elements

• 3d and 4s close, vacant d orbitals accessible: four properties. Count the ion's d electrons, count the dative bonds, and read colour as the complement of what ΔE absorbs.

• 28.1.1 I can define — define a transition element as a d-block element which forms one or more stable ions with incomplete d orbitals
• 28.1.2 I can sketch — sketch the shape of a 3d_xy orbital and 3d_z² orbital
• 28.1.3 I can understand — understand that transition elements have the following properties: (a) they have variable oxidation states (b) they behave as catalysts (c) they form complex ions (d) they form coloured compounds
• 28.1.4 I can explain — explain why transition elements have variable oxidation states in terms of the similarity in energy of the 3d and the 4s sub-shells
• 28.1.5 I can explain — explain why transition elements behave as catalysts in terms of having more than one stable oxidation state, and vacant d orbitals that are energetically accessible and can form dative bonds with ligands
• 28.1.6 I can explain — explain why transition elements form complex ions in terms of vacant d orbitals that are energetically accessible
• 28.2.1 I can describe — describe and explain the reactions of transition elements with ligands to form complexes, including the complexes of copper(II) and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions
• 28.2.2 I can define — define the term ligand as a species that contains a lone pair of electrons that forms a dative covalent bond to a central metal atom/ion
• 28.2.3 I can understand — understand and use the terms: (a) monodentate ligand including as examples H2O, NH3, Cl- and CN- (b) bidentate ligand including as examples 1,2-diaminoethane, en, H2NCH2CH2NH2, and the ethanedioate ion, C2O42- (c) polydentate ligand including as an example EDTA4-
• 28.2.4 I can define — define the term complex as a molecule or ion formed by a central metal atom/ion surrounded by one or more ligands
• 28.2.5 I can describe — describe the geometry (shape and bond angles) of transition element complexes which are linear, square planar, tetrahedral or octahedral
• 28.2.6 I can — (a) state what is meant by coordination number (b) predict the formula and charge of a complex ion, given the metal ion, its charge or oxidation state, the ligand and its coordination number or geometry
• 28.2.7 I can explain — explain qualitatively that ligand exchange can occur, including the complexes of copper(II) ions and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions
• 28.2.8 I can predict — predict, using E⦵ values, the feasibility of redox reactions involving transition elements and their ions
• 28.2.9 I can describe — describe the reactions of, and perform calculations involving: (a) MnO₄⁻/C₂O₄²⁻ in acid solution given suitable data (b) MnO₄⁻/Fe²⁺ in acid solution given suitable data (c) Cu²⁺/I⁻ given suitable data
• 28.2.10 I can — perform calculations involving other redox systems given suitable data
• 28.3.1 I can define — define and use the terms degenerate and non-degenerate d orbitals
• 28.3.2 I can describe — describe the splitting of degenerate d orbitals into two non-degenerate sets of d orbitals of higher energy, and use of ΔE in: (a) octahedral complexes, two higher and three lower d orbitals (b) tetrahedral complexes, three higher and two lower d orbitals
• 28.3.3 I can explain — explain why transition elements form coloured compounds in terms of the frequency of light absorbed as an electron is promoted between two non-degenerate d orbitals
• 28.3.4 I can describe — describe, in qualitative terms, the effects of different ligands on ΔE, frequency of light absorbed, and hence the complementary colour that is observed
• 28.3.5 I can use — use the complexes of copper(II) ions and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions as examples of ligand exchange affecting the colour observed
• 28.4.1 I can describe — describe the types of stereoisomerism shown by complexes, including those associated with bidentate ligands: (a) geometrical (cis/trans) isomerism, e.g. square planar such as [Pt(NH₃)₂Cl₂] and octahedral such as [Co(NH₃)₄(H₂O)₂]²⁺ and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ (b) optical isomerism, e.g. [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺
• 28.4.2 I can deduce — deduce the overall polarity of complexes such as those described in 28.4.1(a) and 28.4.1(b)
• 28.5.1 I can define — define the stability constant, Kstab, of a complex as the equilibrium constant for the formation of the complex ion in a solvent (from its constituent ions or molecules)
• 28.5.2 I can — write an expression for a K_stab of a complex ([H₂O] should not be included)
• 28.5.3 I can use — use Kstab expressions to perform calculations
• 28.5.4 I can describe — describe and explain ligand exchanges in terms of Kstab values and understand that a large Kstab is due to the formation of a stable complex ion

Contrast with the s block. Calcium, a Group 2 metal next to scandium, has only one oxidation state (+2). Its ion Ca²⁺ is colourless and forms few complexes. The d-block metals titanium to copper differ on all four counts, and the next three sections show that one electronic cause produces every difference.

• “Scandium and zinc are transition elements because they are in the d block.” Correct The definition is about the ion. A transition element forms one or more stable ions with incomplete d orbitals. Sc³⁺ is 3d⁰ and Zn²⁺ is 3d¹⁰, so neither qualifies, even though both are d-block elements.
• “Excess ammonia replaces all six waters on both copper(II) and cobalt(II).” Correct Cu²⁺ takes four NH₃ and gives [Cu(NH₃)₄(H₂O)₂]²⁺, dark blue. Co²⁺ takes six and gives [Co(NH₃)₆]²⁺, straw-coloured, which darkens in air.
• “Cu²⁺ is [Ar] 3d⁸ 4s¹.” Correct The 4s electrons are lost first. Cu is [Ar] 3d¹⁰ 4s¹, so Cu²⁺ is [Ar] 3d⁹.
• “Octahedral splitting puts three orbitals up and two down.” Correct Octahedral is two higher, three lower. Tetrahedral is the reverse, three higher and two lower, with a smaller ΔE.
• “A solution is blue because it absorbs blue light.” Correct It absorbs the complementary colour and transmits the rest. [Cu(H₂O)₆]²⁺ absorbs orange-red light and so appears pale blue.
• “Any acid will do for a manganate(VII) titration.” Correct Use dilute sulfuric acid. MnO₄⁻ oxidises Cl⁻ (E⦵cell = +0.16 V), so HCl would use up extra manganate(VII) and make the titre too large.
• “[Ni(en)₃]²⁺ has coordination number 3.” Correct Coordination number counts dative bonds, not ligands. Each en is bidentate, so the coordination number is 6.
• “Kstab includes [H₂O] because water is displaced.” Correct [H₂O] is left out. Water is the solvent and its concentration is effectively constant.
• “A transition element is a d-block element.” Repair It must also form one or more stable ions with incomplete d orbitals. Sc (Sc³⁺ 3d⁰) and Zn (Zn²⁺ 3d¹⁰) are d-block elements but not transition elements.
• “Cu²⁺ is [Ar] 3d⁸ 4s¹.” Repair The 4s electron is lost first, then a 3d electron. Cu is [Ar] 3d¹⁰ 4s¹, so Cu²⁺ is [Ar] 3d⁹.
• “3dxy has lobes along the x and y axes.” Repair Its four lobes lie between the axes. 3dz² has two lobes along z and a ring in the xy plane.
• “Transition elements have variable oxidation states because they have many electrons.” Repair Because the 3d and 4s sub-shells are close in energy, so successive electrons can be removed without a large jump in ionisation energy.
• “A ligand donates an electron to the metal.” Repair A ligand donates a lone pair to form a dative covalent bond. The definition names both.
• “[Ni(en)₃]²⁺ has coordination number 3.” Repair en is bidentate. Three en form six dative bonds, so the coordination number is 6 and the complex is octahedral.
• “[CuCl₄]²⁻ is square planar, with bond angles of 90°.” Repair It is tetrahedral, 109.5°. Chloride is too large for six to fit, and the four sit tetrahedrally.
• “Excess ammonia gives [Cu(NH₃)₆]²⁺.” Repair With copper(II) only four waters are replaced, giving dark blue [Cu(NH₃)₄(H₂O)₂]²⁺. With cobalt(II) all six are replaced: [Co(NH₃)₆]²⁺.
• “Sodium hydroxide gives a copper hydroxo complex in solution.” Repair OH⁻ gives the insoluble hydroxide Cu(OH)₂, a pale blue precipitate that does not dissolve in excess.
• “The manganate(VII) titration was acidified with hydrochloric acid.” Repair MnO₄⁻ (+1.52 V) oxidises Cl⁻ (+1.36 V), so the titre would be too large. Use dilute sulfuric acid.
• “In the copper/iodide titration, n(Cu²⁺) = ½ n(S₂O₃²⁻).” Repair 2Cu²⁺ give 1 I₂, and 1 I₂ reacts with 2 S₂O₃²⁻, so n(Cu²⁺) = n(S₂O₃²⁻).
• “The compound is blue because it absorbs blue light.” Repair It absorbs another colour (energy ΔE = hf, promoting an electron between the split d orbitals), and we see the complementary colour that is transmitted.
• “Octahedral splitting is three orbitals up and two down.” Repair Octahedral is two higher, three lower. Tetrahedral is the reverse, three higher and two lower.
• “trans-[Pt(NH₃)₂Cl₂] is polar because it contains polar bonds.” Repair Its bond dipoles are opposite and cancel, so it is non-polar. The cis isomer is polar.
• “Kstab = [Cu(NH₃)₄(H₂O)₂²⁺][H₂O]⁴ / ([Cu(H₂O)₆²⁺][NH₃]⁴).” Repair [H₂O] is not included, because the solvent's concentration is constant. The units are then dm¹² mol⁻⁴.
• “A larger Kstab means the ligand binds faster.” Repair Kstab is an equilibrium constant. A larger value means the formation equilibrium lies further to the right and the complex is more stable. It says nothing about rate.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Drawing an optical isomer. Draw the complex with wedges and hashes, draw a dashed mirror line, and draw the mirror image: every ligand reflected straight across the line. Then check that no rotation of the second structure turns it into the first.
• Interleave with the chapters that use this one. When you revise topic 24, redo the feasibility drill in 28.2 H; when you revise topic 25, re-answer “why does EDTA⁴⁻ displace almost any other ligand?” and rewrite the Kstab expressions as Kc expressions. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 28 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• Paper 4 is structured questions only; there is no multiple-choice paper on A Level content. Topic 28 asks you to define a transition element, a ligand, a complex and Kstab in the syllabus's words. It asks you to explain the four properties, colour and ligand exchange, to describe the copper(II) and cobalt(II) reactions with observations and equations, to sketch two d orbitals, to predict the formula and charge of a complex, and to deduce isomers and polarity.
• E⦵ values come from the Data section, which is supplied in the examination; you combine them, you do not recall them. Titration calculations use the mole ratio from the ionic equation, then scale from aliquot to flask. Kstab values are not in the Data section, so a question that needs one supplies it; you derive their units from the expression. ΔE is never calculated in this topic.
• The syllabus lists manganate(VII) titrations (with iron(II), ethanedioate or hydrogen peroxide) and the iodine–thiosulfate titration among its quantitative procedures. A Paper 5 question may ask you to plan one, to choose the acid using E⦵ values, to process a titre table, or to name the largest error and suggest an improvement.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.