Ch 2 · General characteristic chemical properties of transition elements · AS & A Level Chemistry 9701

Topic 2 of 5 in Chemistry of transition elements — AS & A Level Chemistry 9701 — 59 lessons.

Introduction

Transition elements are defined by their readiness to form complex ions. This lesson builds the vocabulary and geometry needed to describe any complex, shows how one ligand can be exchanged for another, and applies standard electrode potentials to predict and quantify the redox reactions that transition metal ions undergo, including the titrations used to analyse them.

Key ideas — Ligands, complexes and coordination number

A ligand is a species with at least one lone pair of electrons that forms a dative covalent bond to a central metal atom or ion. A complex consists of a central metal atom or ion surrounded by ligands bonded to it in this way. The coordination number is the number of dative bonds formed to the central ion, not simply the number of ligand molecules or ions present. The overall charge of a complex is found by adding the charge on the metal ion to the total charge contributed by its ligands: in mathrm[Fe(H_2O)_6]^3+, mathrmFe^3+ plus six neutral H_2O ligands gives an overall charge of 3+; in mathrm[CuCl_4]^2-, mathrmCu^2+ plus four Cl^- ligands (4×(1-)) gives 2-.

🧑‍🏫 Teacher's explanation

Imagine a central magnet (the metal ion) that wants to surround itself with smaller, magnetic objects (the ligands). The ligands aren't just floating nearby; they are actively 'plugging in' to the metal using their extra electron pairs. This connection is a dative covalent bond, where the ligand provides both electrons for the bond, unlike a normal covalent bond where each atom provides one.

📖 Definitions

🔍 Worked example — Easy

Calculate the charge on the central metal ion in the complex [Co(NH_3)_6]^3+.

🔍 Worked example — Standard

A complex ion has the formula [Fe(CN)_6]^4-. Determine the charge of the Fe ion, given that the cyanide ligand (CN^-) has a charge of -1.

🔍 Worked example — Exam-style

A transition metal complex has a coordination number of 4 and consists of a central Cu^2+ ion bonded to four Cl^- ligands. Predict the formula and charge of the complex.

⚠️ Common errors

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🎯 Exam question

Key ideas — Monodentate, bidentate and polydentate ligands

A monodentate ligand forms one dative bond, for example H_2O, NH_3, Cl^- and CN^-. A bidentate ligand has two lone pairs positioned to form two dative bonds to the same metal ion, for example 1,2-diaminoethane, H_2NCH_2CH_2NH_2 (abbreviated "en"), and the ethanedioate (oxalate) ion, mathrmC_2O_4^2-. A polydentate ligand forms more than two dative bonds; mathrmEDTA^4- is hexadentate, forming six dative bonds to a single metal ion using two nitrogen and four oxygen lone pairs.

🧑‍🏫 Teacher's explanation

Imagine a central metal ion as a magnet and ligands as small magnets trying to stick to it. A monodentate ligand is like a single-hook magnet; it attaches at one point. A bidentate ligand is like a double-hook magnet; it has two points of contact, making it much harder to pull away. A polydentate ligand is like a multi-pronged claw that wraps around the metal ion.

📖 Definitions

🔍 Worked example — Easy

A copper(II) ion, Cu^2+, reacts with six water molecules to form an octahedral complex. Write the formula for this complex ion.

🔍 Worked example — Standard

Calculate the total charge of a complex ion consisting of one Fe^3+ ion and three bidentate oxalate ions, C_2O_4^2-.

🔍 Worked example — Exam-style

A transition metal complex has a coordination number of 4. If the ligands are bidentate, how many ligands are attached to the metal center?

⚠️ Common errors

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🎯 Exam question

(a) State the coordination number of the [CoCl_4]^2- ion. (b) Explain why the color of the solution changes when water is added, referring to ligand exchange. [3 marks]

Key ideas — Shapes of complexes

Four geometries are required: linear (coordination number 2, bond angle 180°, e.g. [Ag(NH_3)_2]^+); tetrahedral (coordination number 4, bond angle 109.5°, common with larger ligands such as Cl^-, e.g. mathrm[CuCl_4]^2-); square planar (coordination number 4, bond angle 90°, e.g. [Pt(NH_3)_2Cl_2]); and octahedral (coordination number 6, bond angle 90°, the most common geometry with small ligands such as H_2O and NH_3, e.g. mathrm[Cu(H_2O)_6]^2+).

🧑‍🏫 Teacher's explanation

Imagine a central metal ion as a busy celebrity at a party. The ligands are the fans trying to get close. The 'coordination number' is simply the number of fans that can physically fit around the celebrity at once. Because these fans (ligands) have lone pairs of electrons, they don't just stand near the celebrity; they reach out and form a dative covalent bond, sharing their electron pairs to create a stable 'complex'.

📖 Definitions

🔍 Worked example — Easy

A metal ion M^2+ reacts with 4 Cl^- ligands. If the geometry is tetrahedral, draw the bond angle and state the coordination number.

🔍 Worked example — Standard

Calculate the total charge of the complex ion Fe(CN)_6^4-.

🔍 Worked example — Exam-style

A complex ion has a coordination number of 2 and a bond angle of 180^°. If the central ion is Ag^+ and the ligands are NH_3, write the formula for the complex ion.

⚠️ Common errors

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🎯 Exam question

(a) State the coordination number of the complex. [1] (b) Calculate the charge of the complex ion. [1] [2 marks]

Key ideas — Ligand exchange

Complexes readily undergo ligand exchange, in which one ligand is replaced by another; this may be partial or complete, and is often accompanied by a colour change. Adding excess concentrated hydrochloric acid to pale blue mathrm[Cu(H_2O)_6]^2+ gives yellow, tetrahedral mathrm[CuCl_4]^2-; adding excess aqueous ammonia gives deep blue mathrm[Cu(NH_3)_4(H_2O)_2]^2+, in which only four of the six water ligands are exchanged. Similarly, pink mathrm[Co(H_2O)_6]^2+ forms blue mathrm[CoCl_4]^2- with excess concentrated hydrochloric acid, and straw-coloured mathrm[Co(NH_3)_6]^2+ with excess ammonia.

🧑‍🏫 Teacher's explanation

Imagine a central metal ion is like a host at a party, and ligands are the guests. The metal ion has 'open slots' (empty orbitals) where it can hold onto guests. Ligand exchange is simply a game of musical chairs: a new guest (ligand) arrives, pushes an old guest out, and takes their seat. Because different guests change the electronic environment around the host, the 'vibe' (the color of light absorbed) changes significantly.

📖 Definitions

🔍 Worked example — Easy

Calculate the number of water ligands replaced when 1 mole of Cu(H_2 extO)_6^2+ reacts completely with excess Cl^- to form CuCl_4^2-.

🔍 Worked example — Standard

A solution contains Co(H_2 extO)_6^2+ (pink). When excess Cl^- is added, it forms CoCl_4^2- (blue). If the initial concentration of Co^2+ is 0.10 mol dm^-3, calculate the concentration of Cl^- required for complete exchange, assuming the volume is 1.0 dm^3.

🔍 Worked example — Exam-style

Predict the geometry and coordination number of the complex Ni(en)_3^2+, where 'en' is ethylenediamine (NH_2CH_2CH_2\ extNH_2), a bidentate ligand.

⚠️ Common errors

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🎯 Exam question

Key ideas — Feasibility of redox reactions

Standard electrode potentials, E^ominus, predict whether a redox reaction is thermodynamically feasible. Writing both half-equations as reductions, the reaction is feasible if

🧑‍🏫 Teacher's explanation

Think of standard electrode potentials (E^ominus) as a measure of 'chemical hunger' or 'electron greed.' A species with a very high, positive E^ominus is extremely hungry for electrons; it wants to be reduced. Conversely, a species with a very negative E^ominus is 'generous' with its electrons and wants to be oxidised.

📖 Definitions

🔍 Worked example — Easy

Calculate the cell potential E_cell^ominus for the reaction between Fe^2+ and MnO_4^- given: E^ominus(MnO_4^-/Mn^2+) = +1.51 V and E^ominus(Fe^3+/Fe^2+) = +0.77 V.

🔍 Worked example — Standard

Predict whether the reaction Zn(s) + Cu^2+(aq) arrow Zn^2+(aq) + Cu(s) is feasible using E^ominus(Zn^2+/Zn) = -0.76 V and E^ominus(Cu^2+/Cu) = +0.34 V.

🔍 Worked example — Exam-style

A student wants to know if Fe^2+ can be oxidised by Cl_2(g). Given E^ominus(Cl_2/Cl^-) = 1.36 V and E^rho(Fe^3+/Fe^2+) = 0.77 V, determine the feasibility.

⚠️ Common errors

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🎯 Exam question

MnO_4^-(aq) + 8H^+(aq) + 5e^- arrow Mn^2+(aq) + 4H_2O(l) E^ominus = +1.51 V Fe^2+(aq) arrow Fe^3+(aq) + e^- E^ominus = +0.77 V \n(a) Write a balanced net ionic equation for the reaction between MnO_4^-(aq) and Fe^2+(aq). [3]\n(b) Calculate the standard cell potential E_cell^ominus for this reaction. [1]\n(c) Predict whether the reaction is feasible. [1] [5 marks]

Key ideas — Redox titrations of transition metal ions

Transition metal ions provide two standard titration methods. Manganate(VII) titrations use self-indicating purple MnO_4^-, which is reduced to colourless mathrmMn^2+ in acidic solution, for example oxidising mathrmFe^2+: MnO_4^- + 5mathrmFe^2+ + 8H^+ arrow mathrmMn^2+ + 5mathrmFe^3+ + 4H_2O. Iodine–thiosulfate titrations first generate iodine from mathrmCu^2+ and excess iodide, 2mathrmCu^2+ + 4I^- arrow 2CuI + I_2, then titrate the iodine against sodium thiosulfate, I_2 + 2mathrmS_2O_3^2- arrow 2I^- + mathrmS_4O_6^2-, to determine the original mathrmCu^2+ concentration.

Worked example

Question. A 25.0\ cm^3 sample of acidified iron(II) sulfate solution required 23.40\ cm^3 of 0.0200\ mathrmmol\ dm^-3 potassium manganate(VII) solution for complete oxidation. (a) Write the overall ionic equation for the reaction. [1] (b) Calculate the concentration of mathrmFe^2+, in mathrmmol\ dm^-3, in the original solution. [3] (c) Given E^ominus[mathrmFe^3+/Fe^2+] = +0.77\ V and E^ominus[mathrmMnO_4^-,H^+/Mn^2+] = +1.52\ V, show that this reaction is feasible. [2]

Ch 2 · General characteristic chemical properties of transition elements · AS & A Level Chemistry 9701

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