28.2 General Characteristics of Transition Elements
Learning outcomes
Candidates should be able to:
- 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
- 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
- understand and use the terms:
- (a) monodentate ligand, e.g. H₂O, NH₃, Cl⁻ and CN⁻
- (b) bidentate ligand, e.g. 1,2-diaminoethane (en), H₂NCH₂CH₂NH₂, and the ethanedioate ion, C₂O₄²⁻
- (c) polydentate ligand, e.g. EDTA⁴⁻
- define the term complex as a molecule or ion formed by a central metal atom/ion surrounded by one or more ligands
- describe the geometry (shape and bond angles) of transition element complexes which are linear, square planar, tetrahedral or octahedral
- (a) state what is meant by coordination number
- (b) predict the formula and charge of a complex ion, given the metal ion, its charge/oxidation state, the ligand, and its coordination number or geometry
- explain qualitatively that ligand exchange can occur, including the complexes of copper(II) and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions
- predict, using E⦵ values, the feasibility of redox reactions involving transition elements and their ions
- 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
- perform calculations involving other redox systems given suitable data
Ligands: types and bonding
LigandA species that contains a lone pair of electrons that forms a dative covalent bond to a central metal atom or ion.
The ligand donates the lone pair to the metal ion. Ligands behave as bases and also as nucleophiles.
Monodentate ligandUses one lone pair to form a single dative covalent bond to a metal atom/ion.
e.g. H₂O, NH₃, Cl⁻, CN⁻
e.g. H₂O, NH₃, Cl⁻, CN⁻
Bidentate ligandHas two lone pairs that each form a dative bond to a central transition metal.
e.g. 1,2-diaminoethane (en), H₂NCH₂CH₂NH₂, and the ethanedioate ion, C₂O₄²⁻
e.g. 1,2-diaminoethane (en), H₂NCH₂CH₂NH₂, and the ethanedioate ion, C₂O₄²⁻
Polydentate ligandDonates more than two lone pairs to form dative bonds to a metal atom/ion.
e.g. EDTA⁴⁻
e.g. EDTA⁴⁻
Complexes and coordination number
ComplexA molecule or ion in which a central metal atom/ion is surrounded by a number of ligands, attached by coordinate (dative covalent) bonds.
Coordination number = the number of coordinate bonds formed by ligands with a transition-metal ion in a complex.
RememberCoordination number counts coordinate bonds / donor atoms attached to the central ion — not necessarily the number of ligand molecules.
Example: [Co(en)₃]²⁺ has 3 ligand molecules, but each en forms two coordinate bonds.
Coordination number = 6
Example: [Co(en)₃]²⁺ has 3 ligand molecules, but each en forms two coordinate bonds.
Coordination number = 6
Geometry of complexes
The shape of a complex depends mainly on its coordination number.
LinearTwo coordinate bonds → bond angle 180°.
e.g. [Cu(NH₃)₂]⁺, [Ag(NH₃)₂]⁺
e.g. [Cu(NH₃)₂]⁺, [Ag(NH₃)₂]⁺
TetrahedralFour coordinate bonds often give a tetrahedral shape (angle 109.5°), especially with larger ligands — only four will fit around the metal ion.
e.g. [CoCl₄]²⁻
e.g. [CoCl₄]²⁻
Square planarSome four-coordinate complexes — particularly ions such as Ni²⁺ with strong-field ligands such as CN⁻ — are square planar (angle 90°).
e.g. cisplatin, [Pt(NH₃)₂Cl₂]
e.g. cisplatin, [Pt(NH₃)₂Cl₂]
Coming laterSquare-planar complexes can show cis/trans isomerism.
OctahedralSix coordinate bonds → coordination number 6, bond angles 90° and 180°.
e.g. [Co(en)₃]²⁺
e.g. [Co(en)₃]²⁺
Coordination number and shape summary
| Coordination number | Geometry | Bond angle | Example |
|---|---|---|---|
| 2 | Linear | 180° | [Ag(NH₃)₂]⁺ |
| 4 | Tetrahedral | 109.5° | [CoCl₄]²⁻ |
| 4 | Square planar | 90° | [Ni(CN)₄]²⁻ |
| 6 | Octahedral | 90°, 180° | [Cu(H₂O)₆]²⁺ |
Common ligands
| Ligand | Formula | Example complex | Coord. no. | Shape |
|---|---|---|---|---|
| Water | H₂O | [Fe(H₂O)₆]²⁺ | 6 | Octahedral |
| Ammonia | NH₃ | [Co(NH₃)₆]³⁺ | 6 | Octahedral |
| Chloride ion | Cl⁻ | [CoCl₄]²⁻ | 4 | Tetrahedral |
| Cyanide ion | CN⁻ | [Ni(CN)₄]²⁻ | 4 | Square planar |
| Hydroxide ion | OH⁻ | [Cr(OH)₆]³⁻ | 6 | Octahedral |
| Thiocyanate ion | SCN⁻ | [Fe(SCN)(H₂O)₅]²⁺ | 6 | Octahedral |
| Ethanedioate ion (‘ox’) | ⁻OOC–COO⁻ | [Mn(ox)₃]³⁻ | 6 | Octahedral |
| 1,2-diaminoethane (‘en’) | NH₂CH₂CH₂NH₂ | [Co(en)₃]³⁺ | 6 | Octahedral |
Ligand exchange: copper complexes
Ligands in a complex can be exchanged, wholly or partially, for other ligands — a type of substitution reaction. It happens if the new complex formed is more stable than the original.
Whenever we write Cu²⁺(aq), we really mean the complex ion [Cu(H₂O)₆]²⁺, which gives copper sulfate solution its blue colour.
[Cu(H₂O)₆]²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(H₂O)₄(s) + 2H₂O(l)blue solution → pale blue precipitate (with NaOH(aq))
Adding concentrated ammonia dissolves this pale blue precipitate, giving a deep blue solution:
Cu(OH)₂(H₂O)₄(s) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2H₂O(l) + 2OH⁻(aq)pale blue precipitate → deep blue solution
The same pale blue precipitate can also be produced by adding concentrated ammonia drop by drop (or a dilute solution); it then dissolves as excess ammonia is added.
Water ligands can also be exchanged for chloride ligands by adding concentrated hydrochloric acid drop by drop:
[Cu(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) → [CuCl₄]²⁻(aq) + 6H₂O(l)blue solution → yellow solution
[CuCl₄]²⁻
yellow
⇌+ conc. HCl
[Cu(H₂O)₆]²⁺
blue
⇌+ conc. NH₃
[Cu(NH₃)₄(H₂O)₂]²⁺
deep blue
Real-world observationIn the reaction mixture, blue and yellow solutions mix, giving the equilibrium mixture a greenish colour.
Ligand exchange: cobalt complexes
Aqueous cobalt(II) compounds also form complex ions. Writing Co²⁺(aq) really means the complex ion [Co(H₂O)₆]²⁺, which gives cobalt(II) sulfate solution its pink colour.
[Co(H₂O)₆]²⁺(aq) + 2OH⁻(aq) → Co(OH)₂(H₂O)₄(s) + 2H₂O(l)pink solution → blue precipitate
[Co(H₂O)₆]²⁺(aq) + 6NH₃(aq) → [Co(NH₃)₆]²⁺(aq) + 6H₂O(l)pink solution → brown solution
[Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) → [CoCl₄]²⁻(aq) + 6H₂O(l)pink solution → blue solution
| Reagent | Observation | Product complex |
|---|---|---|
| NaOH(aq) | Blue precipitate; turns red-brown on warming if alkali is in excess | Co(OH)₂(H₂O)₄ |
| Conc. NH₃(aq), dropwise | Brown solution; oxidised by O₂ in air to a cobalt(III) complex | [Co(NH₃)₆]²⁺ → [Co(NH₃)₆]³⁺ |
| Conc. HCl, dropwise | Blue solution, tetrahedral | [CoCl₄]²⁻ |
RememberAqueous Co²⁺ ions usually form tetrahedral complexes with monodentate anionic ligands such as Cl⁻, SCN⁻ and OH⁻ — the larger ligands only allow four to fit around the ion.
Exam tips
- State ligand definitions precisely: a lone pair → dative covalent bond → central metal atom/ion. “Denticity” (mono-, bi-, polydentate) counts lone pairs donated, not molecules.
- Coordination number counts coordinate bonds, not ligand molecules — watch for bidentate/polydentate ligands like en and EDTA⁴⁻.
- Learn the four shapes with their bond angles: linear (180°), tetrahedral (109.5°), square planar (90°), octahedral (90°/180°).
- For ligand exchange, always show the correct number of each ligand, and quote the colour change alongside the equation.
Quick summary
- A ligand donates a lone pair to a metal ion via a dative covalent bond; mono-, bi- and polydentate ligands donate one, two, or more than two lone pairs.
- A complex is a central metal atom/ion surrounded by ligands; coordination number = number of coordinate bonds formed.
- Shapes: 2-coordinate linear, 4-coordinate tetrahedral or square planar, 6-coordinate octahedral.
- Ligand exchange is a substitution reaction that occurs when the new complex is more stable; Cu²⁺ and Co²⁺ complexes show characteristic colour changes with OH⁻, NH₃ and Cl⁻.